LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums - Tech/FAQ: General Tech
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Common Acronyms
! - using this before an acronym means removal of that item from the vehicle (example: !EGR)
1/4 - Quarter Mile (drag racing)
1LE - Suspension Handling Package Option
2WD - Two Wheel Drive
4L60E - actual part name for the Automatic 4-speed transmission
4WD - Four Wheel Drive
700R4 - older generation 4-speed GM automatic transmission
A3 - 3-speed automatic transmission
A4 - 4-speed automatic transmission
A/F - Air Fuel Ratio
AIR - Air Injection Recirculation (part of emissions system.)
APHR - Adjustable Panhard Rod (Suspension)
AFR - Air Flow Research (Vendor)
ASP - Automotive Specialties Performance (Vendor)
ASR - Camaro version of traction control
ATF - Automatic Transmission Fluid
ATI - Supercharger Company (Vendor)
AWD - All Wheel Drive
B4C - Police Package Option
BBC - Big Block Chevy engine
BBDC - Before Bottom Dead Center
BDC - Bottom Dead Center
BHP - Brake Horsepower
BOV - Blow Off Valve (turbo related)
B/O - Bolt-On Modifications
BSM - Body Side Moulding - door ding guards on your doors.
BTDC - Before Top Dead Center
BTW - By The Way
C4 - Fourth Generation Corvette (1992-1996)
CAGS - Computer Assisted Gear Selection (the "skip-shift" on M6 cars.)
CATS - Catalytic Converters (emissions/exhaust system)
CAI - Cold Air Induction
CI - Cubic Inches (engine displacement)
CKP - Crankshaft Position (sensor)
CMP - Camshaft Position (sensor)
CONV/'vert - Convertible
CR - Compression Ratio
DEG - Degrees (timing or temperature)
DRL - Daytime Running Lights
DR's - Drag Radial Tires
DSL - Driveshaft Safety Loop
DTC - Diagnostic Trouble Code
ECM - Engine Control Module
ECL - Exhaust Lobe Center Line (camshaft)
ECT - Engine Coolant Temperature (sensor)
EGR - Exhaust Gas Recirculation (emissions system)
EGT - Exhaust Gas Temperature
E/T - Elapsed Time (drag racing time measurement)
FAST - Fuel Air Spark Technology (Vendor)
FBODY - GM chassis designation for Camaro/Firebird
FI - Forced Induction (turbos, superchargers)
FIPK - Fuel Injection Performance Kit (See K&N)
FMIC - Front Mounted Intercooler (turbo)
FMS - Futral Motorsports (Vendor)
FMU - Fuel Management Unit
FPR - Fuel Pressure Regulator
FPSS - Fuel Pressure Safety Switch
FRA - "Free Ram Air" modification
FRC - Fuel Rail Covers
FLP - Finish Line Performance (Vendor)
FTRA - Fast Toys Ram Air (product)
FWD - Front Wheel Drive
FWIW - For What Its Worth
GMMG - Vendor known for producing modified GM backed F-bodies
H/C - Heads and Camshaft
HO2S - Heated Oxygen Sensor
HP - Horsepower
HPE - Horsepower Engineering (Vendor)
HPP3 - Hypertech Power Programmer 3 (product)
IAC - Idle Air Control (solenoid)
IAT - Intake Air Temperature (sensor)
IC - Ignition Control Circuit
ICL - Intake Lobe Center Line (camshaft)
IGN - Ignition
IMHO - In My Humble Opinion
IRS - Independent Rear Suspension
K&N - Brand name of air filter
KR - Knock Retard (tuning)

LCA - Lower Control Arm (suspension)
LGM - Lou Gigliotti Motorsports (Vendor)
LM - Loud Mouth (exhaust system by SLP)
LMAO - Laughing My Ass Off
LOL - Laughing Out Loud
LPE - Lingenfelter Performance Engineering (Vendor)
LT1 - Generation 2 GM smallblock V8 engine (introduced 1992)
LT4 - Generation 2 GM smallblock V8 engine (introduced 1996)
LT5 - GM/Lotus/Mercury Marine joint collaboration project, DOHC 5.7L V8 engine found in the Corvette ZR-1 (introduced 1990)
LSA - Lobe Seperation Angle (camshaft)
LSS - Lou's Short Stick (shifter handle)
LT - Long Tube Headers
LTFT - Long Term Fuel Trim (tuning)
LTRIM - Long Term Fuel Trim (tuning)
M6 - 6-speed transmission
MAF - Mass Air Flow sensor (measures amount of air entering intake)
MAP - Manifold Absolute Pressure (sensor)
MAFT - Mass Air Flow Translator (product)
MN6 - Option code for factory 6-speed transimission.
MTI - Motorsport Technologies, Inc. (Vendor)
N/A - Naturally Aspirated (all-motor)
N20 - Nitrous Oxide
NBM - Navy Blue Metallic (color)
NOS - Nitrous Oxide Systems (brand name)
NWS - Not Work Safe (do not click link if you are at work, unsuitable material inside)
O2 - Oxygen Sensor
ORY - Off Road Y-pipe
ORP - Off Road Pipe
PCM - Powertrain Control Module (your vehicles engine/tranny computer)
PCV - Positive Crankcase Ventilation (emissions system)
PE - Power Enrichment (tuning)
PHR - Panhard Rod (suspension)
PITA - Pain In The Ass
POS - Piece Of ****
P/P - Ported And Polished
QTEC - Quick Time Electric Cutout (product)
QTP - Quick Time Performance (Vendor)
ROFLMAO - Rolling On Floor Laughing My Ass Off
RPM - Revolutions Per Minute (engine speed)
RWD - Rear Wheel Drive
RWHP - Rear Wheel Horsepower
RWTQ - Rear Wheel Torque
SAE - Society of Automotive Engineers
SBC - Small Block Chevy engine
SC - Super Charger
SES - Service Engine Soon lamp (on dash)
SFC - Subframe Connectors
SLP - Street Legal Performance (Vendor)
SOM - Sunset Orange Metallic (color)
SOTP - Seat Of The Pants
SS - Super Sport
SSRA - Super Sucker Ram Air
STB - Strut Tower Brace
STFT - Short Term Fuel Trim (tuning)
STRIM - Short Term Fuel Trim (tuning)
T56 - actual part name for the M6 transmission
TA - Trans Am
TB - Throttle Body
TBODY - Throttle Body
TBRAKE - Transmission Brake
TEA - Total Engine Airflow (Vendor)
TC - Torque Converter
TCS - Firebird version of traction control
TDC - Top Dead Center
TFP - Transmission Fluid Pressure (switch/valve)
TH350 - Turbo Hydromatic 3-speed automatic transmission
TH400 - Heavy Duty Turbo Hydromatic 3-speed automatic transmission
TNT - Texas Nitrous Technologies (Vendor)
TP - Throttle Position
TPiS - Tuned Port Induction Specialties (Vendor)
TPS - Throttle Postion Sensor
TQ - Torque
TR - Thunder Racing (Vendor)
TSP - Texas Speed & Performance (Vendor)
TT - Twin Turbo
TT2 - Torque Thrust 2 (type of wheels by American Racing)
TTOP - T-Top roof
TTT - "To The Top"
UCA - Upper Control Arm (front suspension)
VATS - Vehicle Anti-Theft System (the coded chip in your ignition key)
VERT - Convertible
VSS - Vehicle Speed Sensor
WB O2 - Wide Band Oxygen Sensor
WOT - Wide Open Throttle
WS6 - Firebird Performance "Ram Air" Package (RPO code)
WTB - Want To Buy
WTF - What The ****?
WTT - Want To Trade
WU8 - Camaro Performance SS Package (RPO code)
Y2Y - Exhaust Upgrade Option Code for SS Camaro (Dual-Dual system)
YBODY - GM chassis designation for Corvette
Z28 - Camaro equipped with V8 engine (RPO code)
ZR1 - Corvette with LT5 motor, GM and Lotus joint project in the early 90s

Firebird/TA - My headlights make this horrible grinding noise!
In almost all cases this is the nylon gear in the headlight motor. Brent Franker, a fbody enthusiast has made a site on how to deal with this issue, and how to order a metal gear which in most cases is a permanent fix. Brent Franker's Headlight Gear Fix

Camshaft timing and specs for dummies
By Chris 96 WS6 ([email protected])

The camshaft is the single most important part in an overhead valve engine in terms of how much horsepower and torque an engine will make and at what RPM it will make it. There is a lot of confusion out there about camshafts and how to choose one. In this article I will explain the basics of cam function and the definitions of the various specifications, which should help readers when choosing a cam.

To keep this article short and to the point, I'm not going to get in to detail on lifter types and rocker arms, springs, etc. This can be left for a future article or for further reading on your own. Since most of us drive late model vehicles I will make the assumption that we're dealing with hydraulic roller cams that are typical in the LS1, LT1, and 87+ TPI engines.


Lift and duration are the two cam specs that get the most attention. Lift is simply how far the cam lobe will open the valve at its peak. More lift generally equals more flow, but too much lift is very hard on valve springs and lifters and if your heads have a definite flow peak then lifting the valve beyond that point usually does not give increased power. Lift is generally expressed as a decimal value of less than an inch, and a distinction must be made between lobe lift and valve lift. Lobe lift is the physical measurement of the lift of the actual cam lobe. Valve lift will always be higher because the rocker arm multiplies the lobe lift through the use of the fulcrum. Valve lift can be calculated by multiplying the lobe lift by the rocker arm ratio (e.g. .300 lobe lift x 1.6 rocker = .480 valve lift). Valve lift in excess of .600" is usually not worth any extra flow on a street Small Block Chevy head.


Duration can have a huge effect on how well the engine runs and where it likes to make power. Duration can be described as how long the valve is open, and is expressed in crankshaft degrees. (the camshaft turns ? of crank speed, but the valve opening is relevant to piston position so it is best to think of it in terms of crank degrees).

Duration specs are given in two measurements, Advertised (also called seat-to-seat) timing, and duration at .050" lift. Seat-to-seat duration is simply measuring the duration from the time the valve first comes off the valve seat in the head to the time it closes again. There is some variation to the way different cam companies measure this spec, so the industry came up with Duration at .050" lift. By measuring the duration from the time the valve opens .05" to the time it closes back down to .05" open, the specs can be standardized so you can better compare two cams. Most "old school" hot rodders still talk in terms of advertised duration, which is why you will hear them talk about 280/290 duration cams as compared to a .050" spec cam that might be described as 220/230 duration.

Intake and exhaust lobes can have different duration specs (split-duration) or both can have the same duration (single-pattern). Whether you will need a split-duration or single-pattern is usually determined by how well your heads flow, specifically the exhaust flow compared to the intake flow. It is also important to note that two cams with the same Advertised duration can have different .050" specs, and vice versa. This is accomplished by changing the ramp rate of the lobes.

In general, the greater duration a cam has, the better it will breath and therefore the more power it will make at higher RPM. Opening the valves earlier and holding them open longer gives the cylinder more time to fill with air/fuel mix at higher RPM. The faster you spin the engine, the less time the valves are open, so the added duration increases that time. The other positive effect is that a larger duration lobe can also hold the valve near peak lift longer, which enhances cylinder filling even more.

However, there are downsides to increased duration. Going too large in duration will result in poor idle quality and bad street manners. The reason for this is that the valves are now open too long at low RPM and the cylinder is bleeding off pressure it uses to build torque. A typical large duration cam is going to hold the intake valve open longer to ensure adequate filling, but at low RPM this is a detriment to making good power because the piston is on its way back up in the bore and therefore not able to build as much pressure prior to ignition.

The key to determining a good duration choice for your cam is to decide what the vehicle is going to be used for and then choose a cam that is suited to that use. Obviously a weekend warrior is going to need a compromise in terms of streetability and all out power, whereas a track-only car will allow the use of much larger duration figures.

Lobe Separation Angle

Lobe separation angle (sometimes referred to as Lobe Center Angle) is a topic of much confusion. Simply put, Lobe Separation Angle (LSA) is the distance, in crank degrees, of the centers (point of peak lift) of the intake and exhaust lobes. What this tells you is how far apart the two lobes are spread. Typical street LSAs for aftermarket cams are in the 112-114 range. The LSA is important for determining the amount of valve overlap a cam provides, which is the amount of time between the exhaust stroke and the intake stroke that both valves are open at the same time.

Overlap can be an aid to producing good top end power and torque, because the exhaust gases, hot as they are, are making their way out the exhaust port and through the headers at a high rate of speed, and their exit helps pull in the air/fuel mixture from the intake valve, aiding in cylinder filling (known as scavenging). At low speeds this causes the lope that everybody loves, but it is accompanied with lower intake manifold vacuum, poor gas mileage, and overall reduced streetability.

It is important to note that a 112 LSA on a 210/220 cam does not equal the same amount of overlap on a 220/230 cam with a 112 LSA. Even though the angle between the lobes is the same, the larger cam has fatter lobes, meaning that it will have more overlap even at the same LSA. To achieve equal overlap between the two, the bigger cam would need a wider LSA.

LSA is calculated by adding the centerlines of the intake and exhaust lobes, adding them together, and then dividing by two. For example, a cam with a 108 intake centerline and a 116 exhaust centerline would have an LSA of 112. The LSA is ground into the cam and cannot be changed without grinding a whole new cam, as it necessitates spreading the lobes or tightening the lobes.

Intake Centerline
The intake centerline (ICL) of a cam is the most overlooked spec. The ICL can have nearly as much of an effect on peak power and the RPM at which peak power occurs as duration. The intake centerline is the position, in crank degrees, that the intake lobe hits peak lift (the center of the lobe). Most small block Chevrolet cams seem to come ground on a 108 ICL, meaning the intake valve reaches peak lift at 108 degrees. ICL is inter-related with LSA, in that the LSA of a given cam is typically considered the "straight up" point for that cam. For example, a cam with a 114 LSA, if ground with an ICL of 114, would be considered "straight up". However, the same cam ground on a 110 ICL is considered to be 4 degrees advanced.

This is a good time to discuss retarding/advancing and the effects of both. Retarding or advancing a cam can be achieved in two ways. The most common is to install the cam with offset bushings in the cam gear to adjust the cam timing, but it can also done by grinding the advance/retard into the cam core itself. You might have a cam ground with a 112 LSA and a 108 ICL...that means that if installed straight up it would be 4 degrees advanced. You could chose to retard or advance the cam from that point using an adjustable timing set as well, but the changes would be + or - from the 108 degree starting point. A cam that has a 112 LSA and 112 ICL would be ground straight up, and to advance it four degrees (resulting in a 108 ICL "as installed") would require an adjustable timing set. For an LT1, any advance or retard must be ground into the cam, as using offset cam bushings would alter ignition timing since the Optispark is driven off the cam gear.

Advancing a cam means you are moving the valve events earlier in the cycle. If done when installing the cam you will be moving both the intake and exhaust events earlier in the cycle. This generally boosts low speed torque at the expense of peak HP. Retarding a cam does the opposite, it will improve peak power by delaying valve events, which will increase peak HP and move the peak HP rpm up, but at the expense of low-end power.

Grinding the advance/retard into the cam offers a couple of advantages. You can advance the intake valve while holding the exhaust valve where it is, which would increase overlap, or you could retard the intake, which would increase the LSA and reduce overlap, but it would also delay the intake valve closing which would improve top end power. Or you can move both lobes and split the difference. This is where the benefits of custom cams come in: you can tailor the cam to your specific needs.

Valve Events

The discussion of intake centerline and exhaust and intake events necessitates discussing the most basic of cam specs: the intake opening and closing events and the exhaust opening and closing events. With some math (or a program like Desktop Dyno) and only these four numbers you can calculate all the above specs.

It is best to begin with the exhaust valve, because in the four-stroke cycle the exhaust valve does it job before the intake valve. After combustion, the piston moves down on the power stroke (this is where the power comes to spin the crankshaft and thus move your car down the road). At the end of the power stroke, when the cylinder is approaching bottom dead center (BDC), the exhaust valve opens to allow the burned gases to exit the cylinder. This is the exhaust valve opening point (EVO). The exhaust valve stays open while the piston is on its way back up, pushing the exhaust gases out past the valve. To allow complete scavenging and removal of the exhaust gases, the exhaust valve is held open for a short time after the piston reaches top dead center (TDC)--this is true for even stock cams. So, the exhaust valve opening will occur before BDC (BBDC) and closing will occur after top dead center (ATDC).

The intake valve actually opens right before the exhaust valve closes. As discussed earlier this takes advantage, to a varying degree depending on the cam, of overlap and exhaust scavenging. Opening the intake valve before TDC (BTDC) allows the incoming air charge to begin filling the cylinder before the piston begins its travel downward. Even though the piston is not yet dropping, the velocity of the air stack in the intake manifold will ram air/fuel mix into the cylinder (this effect is greater at higher RPM). The intake valve stays open as the piston moves all the way down and will typically stay open for a short time after the piston has reached the bottom, because the piston is dwelling near the bottom and for a moment does not have any significant speed coming back up. Keeping the valve open after BDC (ABDC) gives the air stack a little bit more time to get the cylinder completely full. Once the valve closes and the piston is on its way up, the compression cycle begins and the spark plug fires right before the piston reaches top dead center, then the power stroke begins and we start all over again.

Effects of valve events on power

Exhaust opening -- Opening the exhaust early gives the cylinder a head start on evacuating the spent gasses, but open it too early and it will eat into the power stroke. Generally a higher RPM engine will appreciate the extra open time more. By contrast, waiting to open the exhaust valve later will help extract every last once of power out of the hot exhaust gasses as they push the piston down, but waiting too long will cost top end power because the valve will not be open long enough at high RPM to vent all the gases.

Exhaust Closing -- Closing the exhaust valve earlier will reduce valve overlap and improve low end torque but will have negative effects on high RPM power by not completely venting the cylinder of gasses. However, delaying exhaust closing will increase overlap, improving peak power and torque, but at the price of reduced streetability.

Intake Opening -- Related to exhaust closing point, opening the intake valve earlier increases overlap and also allows the cylinder to begin filling earlier, but open it too early and you can cause reversion, which is when exhaust gases are actually pushed out into the intake because the piston is still on its way up with the exhaust valve open. Reversion has very nasty effects on idle and low speed driveability, but luckily is not typically an issue with the range of durations seen for typical street/strip LT1/LS1/TPI (or 3800 V6 for that matter) engines. Delaying the intake opening will reduce the chances for reversion and limit the effects of excessive overlap, but will obviously eat into the time allowed to get air/fuel mix into the cylinder.

Intake Closing -- This is the most important of the four valve events. Intake closing will--to a large extent--determine at what RPM peak power occurs. By manipulating intake closing you can make small cams make good power. Closing the intake valve early helps trap the air/fuel mix and build good cylinder pressure, which is good for low speed torque. However, at high RPM the cylinder doesn't get as full. Delaying the intake closing makes sure every available bit of air/fuel gets in to the cylinder, which greatly benefits high end power, but at low piston speeds you can lose some of the mixture as intake manifold and cylinder pressure begins to equalize and the velocity of air into the cylinder slows.

Conclusions & Additional Considerations

This should be a good beginning overview of cam function. An important thing to come away with is that lift and duration are not all there is to a cam. Smaller cams can make good top end power by delaying intake closing and/or retarding the cam (Example is the ZZ3 cam. At 208/221 it is considered small, but because it is ground on a 112 intake centerline it will make similar power--with stock heads--to the 108 centerline LT4 Hotcam, but with much less duration and overlap, meaning better streetability). Last but not least, it is important to realize there is no single magic cam that works well for all intended uses, and because each engine is different, the best solution isn't always an off-the-shelf grind.

Further reading\

How to Build & Modify Chevrolet Small-Block V-8 Camshafts & Valvetrains (Motorbooks International Powerpro Series) by David Vizard (Paperback - September 1992)

Basic Header Tech
Courtesy of our friends at

A: Header Basics by Loren Barnes, President, S&S Headers, Inc.

You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.

The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.

The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.

The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.

The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.

The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.

Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.

In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.

Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.

The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes.

In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.

The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.

The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.

This sounds so easy, you are probably asking why wasn't this done from the start?

If you have ever seen a set of 180 degree headers you would understand.

On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".

Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.

The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.

Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.

You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.

These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.

PCM Tuning for Dummies: Part 1
By Chris 96 WS6 ([email protected])

(NOTE: This article first appeared in the December 2003 edition of the Middle Tennessee F-Body Association newsletter

The cries were heard from across the land: ?We want an article on tuning!? And so, the MTFBA Newsletter staff has delivered. For your consumption, here, now, we present Part 1 of an article on tuning your GM EFI vehicle.

I must address some caveats before I delve into the meat of the article. First, all of my tuning experience is with a ?96 OBDII LT1 car using LT1 Edit. I am not familiar with other software or other engine management systems. LS1 PCMs function very similarly although some of the variables and tables in LT1 Edit work differently or are measured differently?but the point is the general concepts are the same. I have never tuned a car with a replaceable EEPROM chip, like the L98 cars.

But rest assured, the general principles in the way the ECM operates are similar. So, no matter what you drive, if it?s GM, you should be able to get something useful from this article. It is important to point out that tuning your own vehicle is a complex process, and if you are not comfortable with it you should seek out an expert to help you. If done wrong, you can seriously damage your engine if you don?t know what you are doing. Further, I must note that modifying the factory tune should be for ?off road use only.?

I am going to assume the reader has a basic understanding of how modern GM EFI works. However, I will provide a brief overview. In general you have three unique modes of operation, open loop, closed loop, and power enrichment. Open loop is used for startup running and the PCM will remain in open loop until a specific operating temperature is reached. In open loop, the PCM takes the reading from the Mass Airflow Sensor (referred to as MAF or sometimes MAS), the Manifold Absolute Pressure sensor (MAP) and an RPM reading. Based on these factors plus air temperature, the PCM makes an educated guess about the correct amount of fuel needed for engine demands. In open loop, the primary function of the PCM is to keep the engine operating despite the fact that temps are not optimal yet. The PCM must also keep emissions as low as possible, because startup is the time of greatest emissions production. As a side bar, this is why the Air Injection Reaction pump runs only for a few minutes upon startup?its job is to pump unburned oxygen into the exhaust, which will then combine with fuel remnants in the catalytic converter, further reducing emissions and getting the converter up to operating temperature much more quickly. In Open Loop mode, the oxygen sensors (we?ll call them O2s) are not yet up to full operating temperature either, and are therefore not used by the PCM for fuel corrections. Thus the term ?open loop.?

Once an appropriate operating temperature is reached, the PCM then brings the O2 sensors on line and uses them for feedback about the fuel adjustments it is making. The O2s complete the feedback loop, thus it is called Closed Loop operation. In Closed Loop, the PCM takes dozens of readings from the O2 sensors each second and uses that data to fine-tune the amount of open time the injectors are on. In this system, the MAF, MAP and RPM are used to make a base fuel guess, then the O2 sensors give the PCM feedback on how well it has guessed, and it adjusts. During Closed Loop, the PCM is constantly trying to maintain 14.7:1 air to fuel mix. 14.7:1 is important because that is what is called the stoichiometric ratio for air and gasoline. In other words, it is the most complete burning (i.e. cleanest from an emissions standpoint) mixture ratio. If a car was equipped with a wide band O2 during closed loop operation, you could see the air/fuel ratio swapping back and forth between 14.4:1 to 15:1 as the PCM adjusts back and forth constantly.

The adjustments from the PCM are not just thrown away however. The adjustments are fed into a storage system that is used for future reference. For OBDI cars these are called Integrators (INT) and Block Learn Multipliers (BLM). In OBDII, these are referred to as Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). For our purposes I am going to refer to them under their OBDII names, because they are more meaningful. Each ?bank?, or side of the engine has one oxygen sensor (in OBDII there are two per side, on in front of the catalytic converter which is used for fuel adjustments, and one behind the converter, which is simply used to verify the cat is there and is working). Each bank also, therefore, has its own STFT and LTFT values and the PCM can adjust injector pulse width (simply the amount of time the injector is open each engine revolution) independently on each side.

The STFT is simply a correction value for the fuel requirements which changes several times per second, which then average out into the LTFT value, which is recalculated every 20 seconds or so. These values are fed into a grid of 16 cells, with each cell corresponding to a different range of RPM X MAP reading for varying driving conditions. By using this system of cells and LTFT values, the PCM can be more accurate in its fuel adjustments. All of this fuel trim data is deleted when power is removed from the battery, which is why you often hear about the PCM having to ?relearn? a mod. The PCM can make permanent adjustments to overcome changes in fuel or airflow caused by engine modifications?.until the battery cable is removed. Then the learning process begins again.

OBDI cars operate on a system of INT and BLM values from 108 to 160, with 128 equaling a perfect 14.7:1 mixture, with 108 being max rich. In OBDII, the system is percent based in terms of read out. The PCM is still using the 128 based system for calculations, but you and I see readouts in the form of percentages. 0 is 14.7:1, -10% would mean the engine is running rich and the PCM is removing 10% from the fuel requirements. Next time the PCM encountered that same RPM/MAP situation, it would look in the table for the appropriate cell and see that ?10% fuel is required.

The process of tuning a PCM is the attempt to eliminate this learning curve so that engine performance is not poor until the PCM re-learns the modifications. Also, if the modifications are significant enough, as in a camshaft change, the PCM may not be able to adjust enough to overcome the modification. Camshaft changes, more than any other modification, alter the efficiency at which the engine converts air and fuel to power. The optimum RPM ranges for max efficiency will change because of that, and the PCM can sometimes run out of room to adjust.

Now, you might say at this point, ?If my car can adjust within a certain range, why would I need tuning?? The answer is not so simple. Yes the PCM is designed to compensate and learn, but even with the learning the car can run rich or lean. I have never gotten a complete answer from anyone but it seems as if even the learning capability is not complete. For example, after my GTP Stage 1 heads and CC304 cam install in 2001, I did not tune any part throttle variables. When it came time for emissions testing, the car failed?miserably rich. When scanned it showed Bank 1 ?9% and Bank 2 ?5%. Looking at those values, you would think the PCM was pulling fuel and thus compensating?but the car was well past the hydrocarbon limits. The car only passed after I tuned it to near 0% LTFT?s. This is just one incident but it shows that even the PCM?s learning capability is limited in its effectiveness. The best way to achieve optimum driveability at part throttle and to ensure emissions compliance is to tune for 0% fuel trims. In my personal opinion if you are expecting the PCM to compensate more than 3% lean or rich you are outside of its range off effectiveness and you need to get the PCM tuned.

By now you should have a good understanding of how the PCM governs fueling for your engine. In Part 2 we will discuss what parameters are commonly ?tuned? and the methods used to tune for maximum power. NEXT: On to Part 2.

PCM Tuning for Dummies: Part 2
By Chris 96 WS6 ([email protected])

In part one (see last issue) I discussed how the PCM manages fueling for the engine, and the various sensors and software schemes employed to do so. In Part 2, I?m going to give you the ?meat? of tuning a GM fuel-injected engine.

At a minimum, this article should give you the knowledge to understand what a tuner is modifying in your PCM to give you that max power you are looking for. And, after all, a better understanding of how stuffworks is half the reason we enjoy these cars so much in the first place.

There are many, many variables that can be tuned in the PCM to govern things from fueling, spark curve and idle speed to things like emissions controls, fault codes and even transmission shift variables for automatic equipped cars (GM vehicles prior to 1994 did not put transmission control in the engine computer, hence the distinction between a 94 + Powertrain Control Module and a 93 & earlier Engine Control Module). There is no way I can address the various aspects of tuning each variable, or even most of the common ones. Instead, Part 2 covers how to tune your car for maximum power at full throttle, whether on the dyno or at the track.

Most obviously, to tune the PCM, you must have some kind of scanning software to read sensor outputs such as O2 sensor readings, knock counts, Fuel Trims, etc. Assuming you have modified your car enough that you think you need tuning, sourcing some kind of scanning equipment (whether laptop based or stand alone handheld device) is the next step. If you are going to tune the car yourself, you?ll also need software like LT1/LS1 Edit or Tunercat, or take it to a reputable performance shop with a chassis dyno and tuning capabilities.

When you stomp on the gas, most of you know the PCM commands WOT, or wide open throttle, operation. This condition is governed by throttle position and varies based on RPM. When you go WOT, the fueling scheme changes somewhat, and becomes a kind of mix between the closed loop and open loop modes. The new mode is called PE, or Power Enrichment.

When you enter PE, the PCM takes the last known Long Term Fuel Trims and uses them for the base fueling, then looks in the Power Enrichment table for a percent value to add or subtract fuel from that baseline. The most important thing is to have the LTFT?s near zero or slightly rich before entering PE. If the trims are zero or rich, the PCM defaults back to zero for the baseline before adding the PE fuel. If the LTFTs are lean, the PCM will continue to add the extra fuel it was using to compensate for the lean condition, in addition to the PE fuel, to prevent you from blowing up your motor. The point here is that the car should have a good part throttle tune first, otherwise future part throttle tuning changes will throw off your PE fueling.

Again, it is important that part throttle fueling be tuned for near-zero LTFT?s before beginning to tune for power. The simplest way to tune the fuel trims is to add or subtract from the Injector Constant variable, which is a value telling the PCM how big your injectors are. If you are running rich, and you increase the injector constant from 24.8 lbs/hr (stock setting) to 25.3 lbs/hr, the PCM now thinks the injectors are about 2% larger and will dial back the injector open time in order to provide the correct amount of fuel. Likewise, if the car is lean across the board, decreasing the constant can richen it up.

We?ll assume, from this point on, that you are dyno tuning, but the same principles will apply if you are track-tuning. Once the Fuel Trims are where they need to be, you need to get a baseline dyno pull to evaluate where the current tune is and where you need to tune. I strongly recommend utilizing a dyno that has a wide band O2 sensor. The reason for this is the wide band is much more accurate than the O2 sensors on your car. OEM O2?s are only accurate around .450 mV, which is roughly the 14.7:1 air fuel range. Many people will tell you that .880-.900 mV is where you want your O2 readings to in order to reach 12.8:1 or thereabouts (12.8:1 to 13.3:1 is optimal for power in a naturally aspirated car, for forced induction, you will want to be closer to 12.5:1), and while this is generally true, the O2 sensors on your car are not consistent enough to be trusted ultimately. I If track tuning, always try to start with a dyno tune first to get the fueling as close as possible, making only incremental adjustments at the track based on the on-board O2s.

The dyno?s wide band will give you an air-fuel ratio readout. Depending on whether you are rich or lean on the first pull, fuel will need to be added or removed via the Power Enrichment table in the PCM. There are multiple cells in the table, each one representing about 400 rpm from 400 all the way up to 7000 rpm. An ?8? in a given cell would represent +8% fuel in PE mode. A ?-5: would represent ?5% from the base fueling in PE mode. I recommend gradual changes, 3% increments is a good rule of thumb, until you get a consistent 12.8:1 reading on the dyno graph all the way through the RPM range. 12.8:1 is the preferred air-fuel number to shoot for on the dyno. Since a dynamometer does not load the engine the same as does the car moving down the track, it is a good idea to tune richer on the dyno, because the greater real-world loads will tend to lean the car out more.

Your scanning software is also going to come in handy during this process. During the initial run, you will want to monitor knock counts and knock retard. Knock retard is the PCM?s method for dealing with detonation and pre-ignition, or knock. Knock occurs because either the chamber is too hot and the air/fuel mix ignites prematurely, or the spark is timed too early (too much advance). Both conditions can be damaging to your engine and you want to tune this out of the car while on the dyno. Knock retard is a number representing degrees of ignition timing (not valve timing) the PCM is removing to prevent the knock. You will want to identify the source of the knock and eliminate it so the PCM can command full spark advance for best power.

Timing in the PCM is based on RPM versus MAP reading. For each RPM/load combination, there is a value of spark advance (in degrees) commanded for that situation. When tuning for power, you only need to be concerned with the values at 90, 95 and 100 kPa MAP values (these values are only ever seen when the throttle is near to or completely wide open) at all RPM points from, say 2500 or 3000 up to just past your red-line RPM. If the car is particularly lean, more fuel may remedy any knock readings you are getting. If the fuel is right on or rich, try pulling 2 degrees of timing in the PCM.

If you do not have any knock counts or knock retard readings, and you have your air/fuel ratio where it needs to be, try adding some timing. To stave off knock, add 1 degree at a time, 2 if you are bold, and measure the effects on power. Odds are you won?t see a tremendous increase in power but torque could pick up decently. If you have put in too much timing, you will get increased knock counts, knock retard and therefore will make less power. Most LT1s don?t like more than 36-38* of total advance, while LS1s seem to make best power with 28-32*. As with the fueling, it may take a little more or a little less timing advance at a particular RPM to maximize power at that RPM point relative to the other cells in the spark table, but generally the differences from cell to cell should be minimal. If you have 36* all the way across on your table but in 1 cell you need 25* to stave off knock, then there is a problem.

A very efficient combustion chamber will not need as much advance to make good power. It is therefore a myth that more timing is always better. The best rule is use only as much timing as needed for max power. Don?t add in more even though it doesn?t improve power just because ?more is better.?

Once you?ve achieved the proper fueling and spark, you should have maximized your horsepower and torque output in terms what can be tuned with the PCM. Your particular car may respond better a bit leaner or richer than average, but in general 12.8:1 to 13.3:1 air/fuel ratio produces best power. Again, if you are not comfortable tuning yourself, have a professional do it. If you don?t know what you are doing, you can seriously damage your engine.

At a minimum, this article should give you the knowledge to understand what a tuner is modifying in your PCM to give you that max power you are looking for. And, after all, a better understanding of how stuffworks is half the reason we enjoy these cars so much in the first place.

F-Body Rear End & Gearing FAQ
By Keliente

1. I have a leak, where is it coming from?

The two most common places for a rear end to leak are the pinion seal (the front of the housing) and the rear end cover. Pinion seal seepage is very normal for an f-body, as long as you check your fluid and it doesn?t leave drips on the ground, it is usually okay. Many times people take their cars to be fixed at the dealership only to have it leak again. The rear cover will leak because of a bad gasket/poor use of RTV sealant. You will notice a few drips hanging from the very bottom of the housing, and the area around it may be damp as well. Fixing a rear cover leak is very simple ? remove the cover, scrape the old gasket/RTV off of the cover & the housing, lay down fresh RTV sealant on the cover, bolt it back on and add the appropriate fluid (75w90 & GM rear end additive). The rear end is full when fluid begins to spill out of the drain plug on side of the housing.

One should not attempt to fix a pinion seal leak unless they are familiar with the way a rear end goes together, because pinion depth & crush sleeves are vital to correct setup. Fixing a pinion seal includes removing the driveshaft, pinion nut, washer, yoke, and old seal. The housing should be clean from nicks, and the yoke should be cleaned with a scotchbrite pad to get rid of any unsmooth areas before the new seal is installed. When retightening the pinion nut, it is important not to overcrush the crush sleeve. Pinion bearing preload needs 24-32 inch/lbs tq with a new crush sleeve, or 8-12 inch/lbs if reusing your old sleeve.

Another area that leaks, although somewhat uncommon, is an axle seal. If there is fluid dripping/seeping out of the axle seal, it must be replaced. In order to look at the seal, you will need to remove the brake caliper & rotor. The seal is the piece surrounding the axle itself.

2. What gears are best for my car?

To find what gears are best for you, it would be wise to use the search function of this website to look at cars that are similar to yours in modification. In general, most people with 6 speed cars choose 4.10s, and most people with automatic cars choose 3.73s. However once you start adding modifications like a new camshaft or a forced induction system, you should spend time researching before you decide. Automatic LS1 cars come with 3.23s or 2.73s from the factory, and six speed LS1 cars have 3.42 gears.

3. What is a gear ratio, and how do I know what I have?

A gear ratio is the number of teeth on one gear compared to another. To find the ratio, you divide the number of teeth on the driven gear by the number of teeth on the driving gear. For instance, the number of teeth on a ring (driven) gear is 41. The number of teeth on a pinion (driving) gear is 10. 41 divided by 10 = 4.10.

4. What can I expect to pay for a gear upgrade?

A set of gears themselves will cost $150-$250 depending on what brand you purchase. Most shops will charge $250-$400 in labor, and may add more for supplies. Overall, you will spend $400-$700 on a gear change depending on your location. Don?t skip out and go with the cheapest one ? be certain that a trained professional is handling your installation!

5. Can I set up my own rear end?

You may or may not be able to, depending on your level experience. If you perform all of your own installations and are patient, it is possible to perform it yourself, but it would really help to have someone who has done it before to help you through your first time. Setting up pinion depth & backlash are vital to a well-working rear end. Gears that are not set up correctly will whine or howl, and fail prematurely.

If you want to read more about installing gears, visit the site below for a complete how-to.

Is a 10 bolt even worth putting money into?

Theoretically, a 10 bolt is going to be weaker than a 12 bolt, or Ford 9 inch rear end due to the size, but that doesn?t make it useless. Some people have more luck with 10 bolts than others, obviously. The life of your rear end will depend on how you drive and maintain your car. If you are running slicks at the drag strip and launching from 6,000 rpms, something is bound to break eventually. If you don?t keep up on fluid and let it run low, the bearings will fail prematurely. If your gears are not set up correctly, it?s more bad news. However there ARE people out there who have ridden their 10 bolt right into 10 second passes in the quarter mile. Two of our personal cars, a bolt-on 2000 Z28 M6 with Motive 4.10s and a supercharged 1993 Firebird Formula with Richmond 3.73s are taken to the dragstrip weekly with drag radials, launched from at least 4,000 rpms, and have no issues to speak of thus far?

Whether you will need a ?better? rear end is up to you. If you are replacing parts of your 10 bolt every other week, yes, it?s time for an upgrade. However if it is your daily driver/occasional track car, there?s no sense in upgrading until something breaks. A new 12 bolt runs at least $2,000+, and that is more money that can be put into a nice heads & cam package, or wheels, or whatever your fancy. People do encounter problems with 12 bolts and 9 inches as well; they are not the ultimate solution!

7. Are some gears noisier than others?

Yes, some gears (such as Richmond) will be noisier than others. However, the noise should not be overpowering. If it is unbearable, the gears may have been set up incorrectly.

8. What is backlash?

Backlash is the play between the ring and the pinion, i.e., how much space there is exactly before the pinion actually contacts the ring. It is measured with a dial indicator gauge (either magnetic or clip on). Too little backlash will make for a bad whine, and too much makes for a sloppy setup. In general, different manufacturers call for .006? - .010? backlash (some may even allow for more).

9. What is a ?paddle?, and a paddle kit for that matter?

A paddle takes the place of an axle pin in a Torsen differential car. Instead of a thin cylinder, it is shaped like a block and is held in with an 8mm bolt. The purpose of a paddle or axle pin is keep the axles pushed outward, and thus retained with the c-clips. Sometimes when installing different gears, a paddle kit may be needed for re-installation. Depending on the brand & gear ratio of the selected gears, the ring gear may be too wide after installation to insert the paddle. SLP?s paddle kit is a two piece paddle that allows it to fit. To decide if you will need one or not, be sure to contact the sponsor from whom you are purchasing the gears.

10. Besides gears, what do I need for an install?

It is good practice to replace all of the bearings when installing new gears?if you have the rear apart anyway; it only takes extra minutes to replace the bearings. A good master installation kit will include everything you need, but specifically if you choose to replace everything you will need: carrier bearings (2), front pinion bearing, rear pinion bearing, crush sleeve, pinion nut, axle bearings (2), axle seals (2), pinion seal, a set of shims, gear paint, loctite, RTV gasket maker, 75w90 (2), GM rear end additive (for LSDs). It is also a good idea to replace the ring gear bolts, which may stretch over time. They are left hand thread, fyi.

11. I can?t get my axles out, they won?t slide in far enough to remove the c-clips!

Got a car with traction control? The sensor is prohibiting the axle to move in far enough. Remove the sensor from the backing plate and it will allow you more room to slide it inward to drop the c-clips.

12. What is a series 2 or series 3 carrier, and what do I have?

A 2 series carrier is a car that came with 3.08s or lower from the factory (i.e. all of your 2.73 cars). A 3 series carrier is a car that came with 3.23s or higher. This is very important when you order new gears, as they will differ from 2 to 3 series carrier!

Some helpful definitions of common rear-end terms:

? the play between the ring and pinion gear

? a c-shaped piece of metal used to retain an axle shaft

Carrier- the piece that is containing the limited slip clutches, spider gears, and the piece to which the ring gear bolts

Coast ? a load condition in which the vehicle is driving the engine, as during deceleration

Differential ? a gear arrangement that allows the drive wheels to be driven at different speeds

? a load condition where the engine is applying power to the drive wheels

? the outer end of a gear tooth

Limited slip differential (LSD) ? a differential that uses internal clutches to limit the speed difference between the axles.

Pinion depth ? how deep the pinion gear is in the housing, where it contacts the ring gear

Race ? a hardened surface for the bearing rollers/balls to roll on (kind of ?cups? the bearing)

Toe ? the inner end of a ring gear tooth

Ten bolt
? the rear end under an fbody, characterized by 10 bolts on the ring gear (or rear cover )

How do I calculate my compression ratio?
Compression ratio is a term that all of us are familiar with, but very few of us really understand. Whether we are milling down stock heads, swapping for some aftermarket ones, changing the stroke, the pistons or just simply using a thinner head gasket, changes to you engine combo have a profound affect your compression ratio and, consequently, on overall performance and drivability. With that in mind, I developed and easy to use formula to calculate compression ratio.

Compression ratio is defined as "the volume of the cylinder at BDC (bottom dead center) divided by the volume of the cylinder at TDC (top dead center).

CR = (Vbdc/Vtdc)

Without getting into too much detail, we can expand this to the following equation:

CR = [{(B/2)^2 * S*Pi}+Vcc+Vp+{(B/2)^2*Pi*(DH+TG)}] [{(B/2)^2*Pi*(DH+TG)}+Vcc+Vp]


CR= Compression Ratio
B = Cylinder bore (Stock LT1 = 4.00in)
S = Stroke (Stock LT1 = 3.48in)
Vcc= Combustion Chamber Volume (Stock LT1 = 58cc = 3.539 in3)
Vp= Piston Volume (Stock LT1 = 4.5cc = 0.274606 in3)
DH = Deck Height (Stock LT1 = 0.015 in)
TG = Head Gasket Crush Thickness (Stock LT1 = 0.05 in)
Pi = 3.1415

As long as all other variables remain constant, you can change a single variable (for example larger/smaller combustion chamber size) to determine your new Compression Ratio.

*Convert Cubic Cenimeters (ccs) to Cubic Inches (in^3) using the following formula

in^3 = (CCs/16.387064)

Big thanks to [email protected] for submitting this formula.

What is a cutout?
Courtesy of JRP from

A: What they are: Flowtech QTP electric cutout

What they do:
What to look for: An electric cutout is the best bet, you can be loud when you need/want it to be and quite when you need/want it to be all at the flip of a switch. A standard cut you you?ll need to get under the car to cap or uncap it. A cutout is a great mod for cheap horsepower and sound.

How/Where to install: For an electric cutout follow the wiring guide instructions. For both type of cutouts you'll need to have them welded in. You have a few options of placement. The easiest is the I-pipe as there is plenty of room. You can also run dual cutouts in place of where the cats would be (on a LT' setup). Dual cats and cutouts can be done but the fitment will be very close and you'll need to run some small cats.

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