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Discussion Starter #1 (Edited)
Correct me if I'm wrong. I'm just trying to get the right information out there.

O.k, so I've noticed the comment of "I'll lose low end torque because of the lack of back pressure" is all to frequent; not just on this forum either. So I thought I'd start a thread to help identify why exhaust over sizing is bad and why reducing pipe size increase power and not because of increased back pressure.

Back pressure in any exhaust system is bad. This is why many people increase exhaust size.

The larger the exhaust diameter doesn't equal more flow though. This is where people think that a smaller diameter equals back pressure equals more power. This is wrong.

There are many members here and other forums that understand this concept with heads, bigger ports don't always equal more horsepower. The same applies to exhaust. And intake, and any other part of the engine that plays a role in getting the air in or out.

If the pipes are too small they create back pressure and will choke the engine depending on how and what the engine was built to do (operating range, displacement, and VE), which leads to a loss in power.

Alternatively too large of exhaust reduces exhaust velocity which in turn reduces scavenging which leads to a loss in power.

Air, like any other matter bears mass. Mass moving in a certain direction will continue to do so. This is how we can use the exhaust to pull the old burned gases out and pull cool new air and fuel in. Hello valve over lap.

Smaller pipes naturally have a lower maximum flow potential than larger ones in equal conditions. But they also create higher velocity of the gases when moving the same volume of air. Simply put if you're moving 300CFM of exhaust through a 2" pipe it will flow faster than 300CFM through a 3" pipe. But they are both flowing the same CFM. The advantage of the 2" is higher exhaust velocity and more scavenging, better emissions, cooler engine, and more power. The benefit of the 3" is less back pressure, which reduces pumping losses and increases power.

The challenge is finding the best combination of velocity and LACK of back pressure to optimize the exhaust flow for more air out.

Basically, and this I think we all know, is that a smaller pipe on the exhaust CAN make more power than a bigger one. But it is NOT because of the "increased back pressure" it's because the back pressure wasn't increased based on that particular engines flow potential but scavenging was increased.

On a some what of a side note, the longer the pipe, the higher the velocity also (it will also hold more air, which equals mass, and be able to pull harder). A main reason LT headers out perform shorty's particularly in the low volume air movement of lower RPM's.

Timing of the collector scavenging can be done optimally on shorties or long tube headers equally, but the LT's will always have more velocity and air mass at lower RPM's at the collector and will have more PULL (read: vacuum not pressure) than shorties because of the longer primary's.

This doesn't apply to just exhaust either. It applies to every aspect of engine operation when in comes to moving air.

Also, if you have a single 3" pipe and drop it down to a single 2.5" it is worse than running 2.5" the whole way. Example, if you have a muffler with a 2.5" inlet, but a 3" I pipe, replace the 3" with a 2.5" the entire length of it; or get a 3" inlet muffler. You never want to reduce exhaust sizes as it's traveling away from the engine. It creates a choke point. And while the power loss might be negligible, it's still there. Same thing applies to a performance Catalytic converter. If you're I pipe is 3" don't buy a 3.5" cat, get the 3" (assuming collectors are 3" or smaller, if not then DAMN! that's some seriously huge motor you're building there and cats wouldn't even be in consideration).

NOTE: Moderators feel free to add, change, or delete any of this if this information is incorrect or misleading. I'm not an expert in exhaust, this comes from what I have learned through personal research.

Here's a good read done by Popular Hot Rodding that goes into much more detail and explains everything.

Another link that just repeats what I said.

Another forum with the math behind the physics.
 

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Discussion Starter #3

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Air flow in a reciprocating engine is not constant. This is the one fact that most "hot rodders" overlook when putting a system together.

Wave theory comes into play here. When the flow is pulsed (which is a pressure pulse in the exhaust tube), there is a reflective action that takes place, even at the end of an open tube. It is this fact that makes shorties function different than long tubes. The frequency of the reflected wave is the key. It is affected by the length of the flow path, and the density of the gas media. The density of the gas affects the frequency because the frequency is determined by the speed of sound - higher density = higher speed of sound, lower = slower - if the pulse travels faster, the frequency will be higher. You may have noticed that not all of the tubes in the typical header are the same length - this means they will all have a different pulse frequency. And, of course, engines don't operate at a constant RPM, so the frequency of the exhaust pulses will change. The exhaust is "tuned" when the reflected pressure pulse goes back to the closed exhaust valve, and is just starting to travel back down the header tube, creating a low pressure condition, when the exhaust valve starts to open. THAT is what "scavenging" is all about. And, why the higher the RPM range, the shorter the header primary length. And, why tuning is more to get a better overall average than to optimize the headers.

The collector also plays a part here, because the pressure pulse from one tube is reflected up the others. And why Doug Thorley came up with the "tri-Y" collector concept - to use Y-joints to tune cylinders to each other. It also plays into why the 4-7 swap cams work.

Similar things happen in the intake tract. Except you want the reflected pressure pulse to arrive at the intake valve as it starts to open, to increase the pressure forcing the air/fuel into the combustion chamber. This has as much to do with why one intake manifold will produce more power than another, and why intakes have a specific power band, as does flow area; and why the intake needs to match the cam.

Sources? Eh, it's just basic physics.

FWIW, if you've ever wondered around the pits when the top fuel cars are there, you may have noticed that the exhaust pipes of the dragsters are the same diameter down the whole length, but the funny cars start out with a smaller straight section and transitions to a larger diameter before curving up. Now, why do you think that would be, hmmm?
 

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Discussion Starter #5
Air flow in a reciprocating engine is not constant. This is the one fact that most "hot rodders" overlook when putting a system together.

Wave theory comes into play here. When the flow is pulsed (which is a pressure pulse in the exhaust tube), there is a reflective action that takes place, even at the end of an open tube. It is this fact that makes shorties function different than long tubes. The frequency of the reflected wave is the key. It is affected by the length of the flow path, and the density of the gas media. The density of the gas affects the frequency because the frequency is determined by the speed of sound - higher density = higher speed of sound, lower = slower - if the pulse travels faster, the frequency will be higher. You may have noticed that not all of the tubes in the typical header are the same length - this means they will all have a different pulse frequency. And, of course, engines don't operate at a constant RPM, so the frequency of the exhaust pulses will change. The exhaust is "tuned" when the reflected pressure pulse goes back to the closed exhaust valve, and is just starting to travel back down the header tube, creating a low pressure condition, when the exhaust valve starts to open. THAT is what "scavenging" is all about. And, why the higher the RPM range, the shorter the header primary length. And, why tuning is more to get a better overall average than to optimize the headers.

The collector also plays a part here, because the pressure pulse from one tube is reflected up the others. And why Doug Thorley came up with the "tri-Y" collector concept - to use Y-joints to tune cylinders to each other. It also plays into why the 4-7 swap cams work.

Similar things happen in the intake tract. Except you want the reflected pressure pulse to arrive at the intake valve as it starts to open, to increase the pressure forcing the air/fuel into the combustion chamber. This has as much to do with why one intake manifold will produce more power than another, and why intakes have a specific power band, as does flow area; and why the intake needs to match the cam.

Sources? Eh, it's just basic physics.

FWIW, if you've ever wondered around the pits when the top fuel cars are there, you may have noticed that the exhaust pipes of the dragsters are the same diameter down the whole length, but the funny cars start out with a smaller straight section and transitions to a larger diameter before curving up. Now, why do you think that would be, hmmm?
Thanks!

I know what you are talking about and wasn't about to get into the sound reverb of the discussion, but you are correct.

Shorties can have the sound reverb bounced in the right direction and timing just as well as a long tube. The long tubes benefit comes from the added velocity plus they will be easier to tune because of their length which gives more milliseconds between bouncing between each end of the pipe and therefore more rpm range. The downside is the added mass of air in each long tube is more restriction at higher RPM when flow is at it's greatest.

I was going to mention researching fluid dynamics, but thought that may be too complicated and possibly information over load for many people.

I was just addressing the back pressure myth.

Any and all information added is welcomed! Maybe we can make a thread that covers this topic in detail to help people pick out the right exhaust for their vehicle.
 

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Oh, that Vizard article is pretty good. If you don't want to read them all, read that one.
 

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How dare you debunk a popular hot rodding myth! Honda's are known for the mountains of torque they produce with their tiny exhausts. :D
 

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exhaust tuning is very complex. that's for sure.. people focus a lot on their headers, but not enough on the rest of their system. pipe sizing can definitely shift torque vs rpm just like header length

we're lucky with these gigantic engines, you could chuck a restrictor anywhere in your exhaust and barely feel it. i run stock axle back still and its just fine.

try messing with motorcycle exhaust some day... on some race bikes, you put a slightly larger slip-on muffler on the thing, and suddenly you lose any hint of torque for the first 4000 rpm, no joke.
 

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Discussion Starter #10

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Magnaflow study did show a loss of mpg in around town driving but performance gain usually 8-10Hp.

91 Mazda B2200Punched the pre cat and ran a pipe through the inside of it when I polished and ported the cast iron headers and added custom exhaust with turbo muffler shaped like a glasspack. Runs better than similar truck with aftermarket headers but I did lose 1mpg without the pre cat regular cat still in place. The back pressure allowed me to tune to higher vacume at cruise rpm. Still have lower emmissions and better mpg than before I started modifying the truck.


This link is interesting.

How many horses do you get out of removing Catalytic Converter?? - Corvette Forum
 

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Any power or fuel efficiency "gain'" from a more restrictive exhaust will be due to tune or because the set up likes the lower power band. It won't be due to backpressure.

My 4th gen had true duals. I could run it dumped at the axle or 2otl. By the buttdyno it felt stronger with the full exhaust. At the track it ran a .2 quicker without the back section. I backed that up on 2 other outings in different temps.
 

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Magnaflow study did show a loss of mpg in around town driving but performance gain usually 8-10Hp.

91 Mazda B2200Punched the pre cat and ran a pipe through the inside of it when I polished and ported the cast iron headers and added custom exhaust with turbo muffler shaped like a glasspack. Runs better than similar truck with aftermarket headers but I did lose 1mpg without the pre cat regular cat still in place. The back pressure allowed me to tune to higher vacume at cruise rpm. Still have lower emmissions and better mpg than before I started modifying the truck.


This link is interesting.

How many horses do you get out of removing Catalytic Converter?? - Corvette Forum

Does that truck have a carb... ? Bet it does, and does any LT1/LS1 run a carb and care about mph? No.

My truck picked up a solid 2mpg from deleting the cat and running a 5 inch exhaust.



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Discussion Starter #14
Magnaflow study did show a loss of mpg in around town driving but performance gain usually 8-10Hp.

91 Mazda B2200Punched the pre cat and ran a pipe through the inside of it when I polished and ported the cast iron headers and added custom exhaust with turbo muffler shaped like a glasspack. Runs better than similar truck with aftermarket headers but I did lose 1mpg without the pre cat regular cat still in place. The back pressure allowed me to tune to higher vacume at cruise rpm. Still have lower emmissions and better mpg than before I started modifying the truck.


This link is interesting.

How many horses do you get out of removing Catalytic Converter?? - Corvette Forum
A loss it F/E (fuel economy) can be attributed to the increased flow of air in and out of the engine because of the higher flowing exhaust.

There are only a few ways to increase power output of a cars engine. 1. increase thermal efficiency, 2. increase air and fuel input/output, 3. reduce anything that causes drag or resistance.

When you reduce back pressure in the exhaust it allows more air to flow at any given RPM. This also increases fuel consumption, more air equals more fuel, generically speaking. Increasing back pressure has the opposite effect.

This however is negligible at best when you are talking about 5-10 measly horsepower. Unless you actually use that horsepower F/E will not change. And even if you do use that extra 5-10 horsepower the difference in F/E will be in the tenths of MPGs.

What people are noticing isn't the added power reducing F/E, it's the extra right pedal input reducing F/E because suddenly you have more power and you want to use it.

It takes X amount of power to move Y amount of resistance Z speed every single time (E=MC^2).

Things get more complicated in a car though, because the BSFC and V/E that are best for engine power to F/E ratio don't necessarily (and rarely if ever) work out in the vehicle it's self. A Corvette with an LS1 gets fantastic F/E at 75mph in 6th on the highway putting along at 1800rpms. But the BSFC of the LS1 is closer to the 2500-3500rpm range (not 1800). So the LS1 pushes out more power per drop of fuel at double it's best F/E rpm when it's actually in the car. But the Vette doesn't need the 200+ hp it's making at 3000 rpm to maintain 75mph, it only needs about 25hp which at 1800rpm it's more than quadrupling anyway.

The way this can most easily be illustrated is to compare two identical cars with different engines. Like a V-6 Camaro vs. a V-8 Camaro. The V-6 will get better gas mileage because the extra power the V-8 makes isn't there. So less fuel is consumed because less is used because less power is made. If you had a 300 horsepower V-6 Camaro and a 300 horsepower V-8 Camaro that were just as fast in every metric of each other, the deciding factor would be the BSFC at operating RPMs in F/E (usually the V-8 actually fares better on the highway because it's massive lowend torque and tall gearing, while the V-6 takes the city F/E because it's BSFC range and power match that type of driving better).

GM recently built a C7 Vette with a TTV6, they are not pursing production because it got worse F/E than the more powerful LT1.

Long post short. Just because you increase airflow through an engine doesn't mean you'll lose mpg. On the contrary, you should actually see an increase in F/E if the driving habits remain constant.
 

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Discussion Starter #15
Any power or fuel efficiency "gain'" from a more restrictive exhaust will be due to tune or because the set up likes the lower power band. It won't be due to backpressure.

My 4th gen had true duals. I could run it dumped at the axle or 2otl. By the buttdyno it felt stronger with the full exhaust. At the track it ran a .2 quicker without the back section. I backed that up on 2 other outings in different temps.
Thanks. I agree completely. People tend to think smaller = more restrictive. In a generic sense that is true. But, in working with fluid dynamics this isn't always the case. Sometimes smaller isn't more restrictive, but does create more velocity. This makes the smaller pipe actually flow MORE than the bigger one in that application.
 

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Discussion Starter #16
Does that truck have a carb... ? Bet it does, and does any LT1/LS1 run a carb and care about mph? No.

My truck picked up a solid 2mpg from deleting the cat and running a 5 inch exhaust.
Thanks. I'me sure your driving habits remained the same before and after your mods. Most people get that little extra power in the seat of their pants and end up using a lot more throttle after wards. Especially after they make their engines louder.
 

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Efficiency is the key, not backpressure. If you put a plate with a 1/2" hole in the exhaust you've made more back pressure, but I doubt you'll gain anything from it.

Just because the exhaust is flowing more freely doesn't mean you need more fuel. It just means your engine isn't working as hard to breath. My 4th gen went a tad rich with headers on the stock tune.
 
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