your timing map is bone stock where it matters (heavy load, wot.)
for that kind of cam, WOT timing should look way different than a stock cam ...
This is what the build liked. As you can see this is the 16th Tune file for my car, and each file has been adjusted multiple times. I've done less timing and more timing. This is where it ended up. It is close to stock, but it's where it ended up. My dyno tuner isn't what I would call well versed on LT1's. LOL.
Originally Posted by steveo
max power on fast burning aluminum heads like this is found more high 12s to low 13s. you should head a bit leaner and see how it behaves.
It could use a little more leaning out. But, at 12.9:1 AFR at max VE and a little richer down low I'm o.k with it. Here's my last dyno chart. The AFR line is at the bottom and peaks at 12.9:1 at about 5400RPMs.
I did find a place 5 minutes from my house that will rent me their Mustang Dyno for $100/hr. But, I want to get some other mods (like the exhaust) done before I dyno tune it again. I can play with AFR and Spark timing (I want to play with injector timing too) more then.
Originally Posted by steveo
why is that? you're only 1/4" under a 3" exhaust. a single 2.75" isn't actually that bad for this kind of car, in fact it can promote velocity of exhaust gasses within the operating range of the engine to create more useable power in normal driving ranges. it might choke it out at very high rpm, but not much.
I understand exhaust gas velocities, but at my power level a 2.75" pipe (and a single 3") is a pretty good restriction to power. Optimally I should be at about 3.5" single at my dyno powered. But, that will go up as I open the exhaust. And it'll rise more with future mods too.
The further away from the engine the smaller the pipes need to be due to cooling, but with my engine VE exceeding 100% and pushing 6600 rpm (going to be 7K in the future) the math shows the 2.75" diameter over axle pipe is too small. And to that you should never reduce exhaust size down stream as it introduces a restriction. Even if it's set up to go from 3" to 3.5" to 3" again it's losing power. Once the exhaust pipe goes to a certain size it should stay there or get bigger only, never smaller.
Here the math. A straight pipe will flow ~115 CFM per square inch of cross-sectional flow area
At 6100 my VE table is maxed out in the tune (tuner pro RT won't let me go any higher than 99.61%) and my AFR is ~12.5:1. Extrapolating the data from my PE AFR correction tables and the data log AFR on the dyno chart I can conclude that at 6100 RPMs my engine is at 106% VE.
At 6100 RPMs and 106% VE the engine is pulling in 664CFM of cool air, but pushing out about 970CFM of burned air and fuel. I need 115CFM per square inch of cross sectional straight pipe to match my 970CFM of exhaust gases. Of course the pipes are not straight, but it doesn't have to be perfect just close. It is a DD after all. My engine at 6100RPMs needs a 3.40" diameter single pipe exhaust, straight (using .060" for wall thickness). At 7K (assuming 100% VE, a 6% drop from 6100RPMs) I need a 3.53" diameter exhaust, straight piped. In either case they are both about 3.5".
At lower RPMs I will lose some scavenging, but it actually won't be too noticeable from where I am at now due to my LT headers and Y-Pipe and VE chart. To that point, my VE tables at my cruising speeds are ~50%, that would mean that optimally a single straight 1.5" exhaust pipe is optimal. I'm well over that, going even larger will have nil an effect at those engine speeds from where I currently sit.
This is all taking into account my engine as it sits. The TB will get done along with the Intake in time. The exhaust right now is about 65% the size it should be, so it's a restriction and a pretty good one at that. My current exhaust flows a max of 624CFM before it represents a restriction (and that's straight with no bends), I need 970CFM or 55% more cross-sectional area.
Most guys go with 2.2CFM per HP and 115CFM per sq inch of exhaust pipe cross section. That's 2.2CFM per flywheel horsepower. That puts me at about 1071CFM needed or just over 3.5" diameter single straight pipe as I sit.
No matter how I cut the math it points to a single 3.5" pipe straight (or 3/4" larger than where I am now). Remember that area grows at double the rate than diameter when it comes to a circles. Doubling the diameter will quadruple the area. The 3.5" exhaust is 27% larger in diameter, but 65% larger in cross-sectional area. A 3" diameter exhaust is only 1/4" (9%) larger than 2.75", but it's 20% larger in cross-sectional area. These numbers seem slightly off because I'm accounting for the .060" wall thickness.
Taking all that into account I've decided to shoot for a 4" Single exhaust with dual bullet mufflers (in series one on the I pipe and one right before the exhaust tip). That should support about 525rwhp without any losses in power, keep the exhaust tone low and at a decent dbl and like running Open headers.
That lets me upgrade the TB and intake manifold and still have room for some juice down the road with out ever having to change my exhaust again. And in the interim it wont effect driveability noticeably either.
1995 Camaro Z28 LT1 LE2 355 w/ forged pistons/rods, 230/238 .565/.565" 110+6, 1.6:1 SA RR, LT Headers with ORY, T-56 (rebuilt w/ some upgrades), McLeod Street Twin, Lowered 1.25" (Stagg Shocks and Summit Springs) LCA Relocation brackets, Adj Panhard Bar, Poly suspension bushings, STB , CAI, !emissions crap, Magna Flow Cat Back. 146,xxx miles. Purchased Feb 7, 2012 (168K) 414rwhp
2014 Ford Focus SE Hatchback- Stock (Wifes Ride)