Pizza

After doing research and talking with Woody & Louie Ott I have come to the conclusion that adding headers and 2 1/2" exhaust to my Twin Screwed 86.5 would probably yield no additional HP due to overlap and back pressure. Has anyone installed headers and exhaust systems to a SC'd 928 with good results?

Thanks

SwayBar

You must have misunderstood them since that's simply not the case.

Headers and exhaust are worth 20 rwhp on a NA GT; any gains made while NA will be reflected moreso on a supercharged engine. There are literally hundreds of dyno's from other cars supporting this principle.

andy-gts

boy I hope it works cause that is my plan...I have the devek level2 headers and am waiting for the exhaust. then by late summer the dr supercharger should be here and hear comes an easy 460 rwhp...oh baby!

Imo000

My past experience with superchaged engines makes me beleive that a supercharged engine needs a very free flowing exhaust, the freer the better. With a N/A engine there is a limit of how much free flow is good and anyting above it is too much.

Pizza

Swaybar,
I don't think I misunderstood. I was on the phone with Louis Ott for an hour. (Thanks Louie) His website is www.performance928.com Woody lost HP when he installed his headers and 3" exhaust. He had to install restrictors to try and regain what he lost. I think the problem is the compressed air from the SC is blowing through the heads too fast due to intake and exhaust overlap with not enough backpressure. With a NA engine the lack of backpressure is not as much of a factor because the air is not being forced into the heads.

Imo000

OK, so the real problem is the overlap of the stock cams? Are they not boost friendly?

T_MaX

I can see their point, bigger is not alway better!

I just wonder how far away from optimum valve overlap (for a SC set-up) till you end up blowing too much of your fuel out of the exhaust valves. I guess a person could always just throw on bigger injectors till the lean condition went away, but at what cost, HP/TQ?

Louie928

I must admit that I also figured what was good for a NA engine exhaust should be even more good for a SC engine. I accidently ran across a post on the Innovate forum where one of the tuner experts made a comment on a thread about why a certain turbo engine was showing lean. While turbo engines and SC engines react differently to cam overlap, here is his comments regarding blow through during the cam overlap period, and its effect on measured AFR. Here is the whole thread. http://www.innovatemotorsports.com/f...?t=5526&page=2

That got me thinking. About the same time I was talking with Tim Murphy about what exhaust to use on SC 928 engines. He mentioned that modelling using Engine Analyzer Pro didn't show any advantage with headers over the stock exhaust manifolds. I was about to start on some work to fit a better exhaust on a SC 928 and I figured headers and a 3" dual exhaust would be the ticket. I spent some time with Engine Analyzer Pro and found that there was no particular gain with headers and 3" exhaust over the stock manifolds and 2.5" exhaust. It did show about 3% improvement at 5000 rpm, but about the same below 4000 and above 5500. This was with 10 psi boost at the intake. Was that small gain worth the effort? The stock manifold/2.5" exhaust showed about 5 psi back pressure while the headers and 3" duals only showed 0.7 psi back pressure. The engine needed that 5 psi backpressure to keep more mixture in the cylinders. Without it, a lot simply blew out. Since it would have been unburned oxygen going out, the mixture would have shown leaner than it actually would have been too so more fuel would be added. I think I'd have created a fuel hungry car with not much more power than it had originally. In the end, I decided to stay with the stock manifolds, but go to 2.5" duals all the way back since that modeled pretty good in EAP and was a bit better than the stock rear exhaust.

Possibly on low boost SC engines freeing up the exhaust flow would help similar to the way it does on NA engines. And I'm sure that on higher boost engines having a too restricted exhaust isn't good either. There is probably an optimum point between exhaust back pressure and boost, and cam overlap. Right now, I don't know what it is. Bigger may not always better, for exhaust at least.

pmotts

Nope, Charles didn't misunderstand. Louis was ready to build a 3" system for my SC'd GT and his engine analyzer program doodad showed minimal gain for a lot of work. I have the Devek headers and we are going to install them on my NA GT, there just wasn't enough payback to do the work on the SC'd GT.
Jim

SwayBar

Interesting information to say the least.

Let me think about this.

Vlocity

Louie:

Would there be any advantage that you can see to retarding the timing at the cam?

Regards,

Ken

hacker-pschorr

Just curious - does anyone have the flow characteristics of the stock exhaust manifolds? Wouldn't this be needed for an accurate "before" number? Or is entering in the pipe size / length etc..... good enough for an accurate comparison? Again, I'm not trying to question Louis's data, just curious how this process works.

Pizza

Looks like I might have to be the Guinea pig on this one. This is my new plan of attack suggested by Loius Ott. I already know I have 372 RWHP & 354 RWTQ with 6 psi boost, no intercooler, stock headers, Ott X-pipe, stock 2 1/4" exhaust & 928 Spec RMB. I plan on installing my Devek Level II 1 3/4" primaries to 3" collector headers. I'm going to reduce the 3" to 2 1/2" and custom fab a Burns stainless 2 1/2" X-pipe and install my stock 2 1/4" exhaust with the RMB. I'll put it on a Dyno and see what I get. While at the dyno I'll see if they will let me unbolt the stock exhaust after the X to see if I lose power or gain power without the 2 1/4" restriction. Any suggestions are appreciated. Any donations for this test are welcome too! Ha! Ha!

AO

Fascinating read! I wonder if this is why more than a few of us with boosted engines have commented that a catless X-pipe combined with an RMB produces less power (measured via 0-60 & 1/4 miles times) than with the stock rear muffler!

Good question. It makes sense, I think.

Louie928

There isn't a way to accurately know the characteristics of the stock exhaust. The Engine Analyzer Pro program has entries for the general exhaust configuration, (straight , tapered, or stepped primaries) primary pipe diameter, primary pipe length, runner flow coefficient, open headers or full exhaust, type and length of collector, and flow in CFM. If you know the flow, you can enter it. Otherwise there are choices for the engine hp and type of exhaust (Full race, Aftermarket, Sport production, quiet production) which then calculates a flow depending on your selections.

It may not be dead on accurate, but change a system parameter and a trend will show. On a NA engine you can change primary diameter and length, collector diameter and length and see changes in the output. For the stock exhaust you can use primary dia of 1.625", length as 8", and collector length as 24" which is about average until the pipes from both sides merge. Then change primary length to 29" (keep the dia at 1.625") and collector length to 10". Leave the rest of the exhaust as it was. You'll see an increase in power. Do the same with a SC engine and there won't be much of a change.