I have a 69 camaro with a Lt1 procharged and soon to be intercooled.I will be putting out some where around 12lbs of boost.I have dual 3 inch exhaust with 1 5/8 long tubes now.Do you think 1 7/8 headers would be to big?

 

Nope. Those should help you gain optimum HP compared to smaller headers.

 

with blowers, its better to have bigger primaries

 

I disagree. In my experience, exhaust improvements don't benefit a supercharged car as much as they do NA or turbo.

With an NA engine, exhaust backpressure can cause reversion (exhaust going upstream into the intake port) during camshaft overlap, thus reducing power. This is not possible on a supercharged motor since there is a higher pressure in the intake port.

With a turbo engine, you already have way more exhaust pressure than intake pressure. My engine has 34 psi exhaust pressure at 17 psi boost. Any improvements I make to the exhaust will help with that situation. With a supercharged car, exhaust pressure is almost always lower than boost, so directionally the exhaust system is less critical.

 

Does anyone else agree with him? So you are saying that f i go from 1 5/8 headers to 1 7/8 headers there might not be a gain?

 

That's what I'm saying. Or maybe a small gain.

 

I agree with engineermike.

 

Eaven if its a blown engine i think you will see some gains if you can optimize scavange effects and get moore exaust out of the combustionchamber beafore next intake cykel starts.
here you have som simplified info http://www.superchevy.com/technical/...t/0505phr_exh/
Theres way more about choosing headers , Piston speed, valve throat diameter, exhaust flow numbers, header lenght etc. You want to get the right exhaust pulse speed to optimize the scavange effect and thats a primarydiameter vs lenghtquestion.
So its hard to say yes or no on youre question.

 

I agree with Lisa33. Like I originally posted, you will see optimal gains by going with the larger headers. True, you would probably see more gains if it were a turbo car, but there gets to a point on the power curb at which too much back pressure can become a restriction, and therefore a potential power weakening point. This is not the case in some N/A cars as for example with a bolt on LT1, going with headers that large would give you less power than say a set of mid length headers would. However on an FI car, you will need less restrictive exhaust more than backpressure for optimal power gains.

Long story short here, if you had a Turbo instead of a Supercharger, you would see greater gains, however you will still see gains by going with the bigger headers. The 1 7/8 headers will give you more HP than the 1 5/8.

 

That's just the problem, though. Supercharged engines have the possibility of overscavenging. That is, during overlap, you totally clear the exhaust out of the cylinder and start losing intake air/fuel out the exhaust port. Get the exhaust too good, and this can really be an issue.

Mike

 

If not enough back pressure would lead to a HP loss, I think it would be more obvious on guys running with cutouts and their dyno numbers as proof.

 

I would think losing charge out the exhaust port is maximizing the efficiency of the charge in the cylinder during filling. Not all the exhaust (old charge = exhaust) gets pushed out of the cylinder when the exhaust valve is open.

Now, IF losing some intake charge helps to evacuate the cylinder than its a good thing. Howver, if losing intake charge doesnt help evacuate the old charge than its worthless. I tend to think it helps evacuate the old harge, so it may be ok to lose some fresh charge.

 

I strongly belive that its worth to waist a small amount of the intakecharge out of the exhaust port to flush out the old exhaust and cool the combustionchambers.

If it isent that way oure car shouldnt make the power it does, Oure cam has so much overlap that you wouldn belive it if i told you (will not give out the numbers) Hope Bret comes in and give his opinion.
We have also tried to designed oure headers for max scavange and the primary are 2" outer diameter. Mufflers are 2 big resonans boxes widh a short muffler at the end (all built in one pice) for so low pressure drop as possible.
http://www.nonameracing.se/photos/de...ASCONA0703.jpg
http://www.nonameracing.se/photos/de...ASCONA0705.jpg
The ideas are mostly coming from http://www.amazon.com/Scientific-Exh.../dp/0837603099 and its built by http://www.ferrita.se

If you worrie aboute pass emissiontests, forget my ideas

 

I had a 3" single Aerochamber on my old 13 psi T-trim setup. I removed the entire cat-back from the y-pipe and it gained only 5 rwhp (<1%). Later on, with the turbo setup, I made huge gains by removing the Aerochamber.

 

Yea thats proberbly beacuse turbocars are sensivity to backpressure, turbos take out a lot of enrgy from the exhaust wich gives lower temperatur, gas speed and higher density gases, that creates higher back pressure in the exhaust in a turbocar compered to a N/A or supercharged, A free flowing downpipe from a turbo also gives shorter spole up / better respons.