Exhaust Pressure (back pressure)

[shelbyracing]

Okay, I've searched and can't find any concrete numbers on the subject. The most important thing in an exhaust (performance anyway) is velocity for the scavenging effect (negative pressure waves following behind the positive for those who didn't know). Also pressure is important cause if you build up pressure that's pressure combating the full fill of your cylinders and complete extraction of spent gases.
My question is how much is acceptable (I read 5-7) with anything greater than .3 (PSI is my unit of measure throughout article) is going to start hindering performance.
Also I am curious of the relationship between horse power % and exhaust pressure. Any numerical idea's?

[Marc]

I have one clue for you, although I don't how helpful it'll be: Back during the 1960s, I had a 1966 Cutlass with the 330 4bbl. It had a single exhaust. I switched to dual exhausts. Gas mileage went from
12 to 15 mpg on the road. 0-60 dropped from 11 seconds to 9 seconds.

[shelbyracing]

I'm thinking maybe we figuring pressure of post compression struck with piston all the way down. Then that would be the 0% power (wouldn't even run) then figuring possible power from that number. 0 or even .3 (psi) would be 100%. Any vacuum would be an addition of power (I do not think we could measure the negative wave nearly fast enough to figure it's properties though). We will just work off of static pressure.
Does anybody know the PSI of the wave expelling from the cylinders?
Just read somewhere 70-90. So if we say 80 psi and have factory 10 psi (assumption) that is a 1/8 loss of power or 12.5% loss. And that isn't even figuring scavenging velocity effects.
I should clarify I'm strictly talking WOT exhaust pressure

[shelbyracing]

"Horsepower ratings are still made with the engine on a test stand, but with stock ignition timing, carburetion, exhaust, and accessories."
So I'm guessing that our engine is actually about 285 with ideal exhaust and no other mods.

[wireless]

so you're expecting a 35bhp gain from exhaust alone? i thought they were 250hp?

[shelbyracing]

No I was just kinda using an instance # sorry bout that. I would like to know the factory pressure. Also that would be the absolutely best exhaust if it were to be 10 psi. I'm talking longtubes, X Pipe, and straight pipes to a tuned length.
I promise you my pressure is above .3 and do not even know what it is yet. It would just be nice to know what the pressure is with 250 hp to have something to go by.

[Kersh]

From what I have been able to figure out over my years of messing with cars (extensively) is that the scavaging effect really only has an effect at low RPM. For the Aurora that is all mid and upper-range power, I would think you would simply want the best flowing exhaust system that you can find. Not just the post-cat, but the exhaust ports in the heads, the manifold/header, the "Y" assembly and cat as well.

Unfortunately, I do not have any actual dyno numbers I can provide to back this up...unless you consider the "seat of the pants" dyno an accurate assessment.

I would be willing to bet, if you are able to free up the flow enough to have a negative effect at low RPM, you will more than make up for it in the mid and upper range of the powerband.

[shelbyracing]

Okay so I just checked my pressure and I'm really really happy. At first I was a little worried about dual 2 being enough but all I saw was 1-1.5 psi pulses and always going to 0 in between. The needle was bouncing very very rapidly it was hard to see.

[shelbyracing]

I think I need a vacuum gauge to do this adequately.

[shelbyracing]

Had a broken pipe due to no flex pipes this is the sound of my complete and non leaking exhaust ;D

[shelbyracing]

Got a video uploading right now but I bought a vacuum gauge and my exhaust is actually pulling between about .7-2" of vacuum. It isn't on the positive pressure side at all. I was super surprised and super happy at the same time. The faster then engine went the more vacuum it pulls. So my exhaust is definitely adequate for 7,200 rpms I would think.

[shelbyracing]

Are there any stock people out there that are curious? I am very curious about the stock exhaust pressure.

[Kersh]

What have you done to your exhaust to improve the flow?

[shelbyracing]

It's true dual 2" pipe, with an X pipe and 2.5" straight through Magnaflow's.

[Kersh]

Did you make any changes to the manifolds or cross-over?

[shelbyracing]

No, it looks like it stays consistently mandrel, it just has some sharp angles. It might flow 3-7% better or so if I did, and might consider it on the future.
Headers would be nice too, but can also wait, they would help with scavenging. The split cast Stainless isn't a bad flow design.

[shelbyracing]

I did a WOT load road pressure test so it was more accurate. Pressure fluctuated between 2 and 3 between the exhaust pulses leaving a static of 2.5 PSI. That makes me very happy ;D

[lanceslambos]

that's pretty cool so your exhaust has yielded you around a 3% gain?

[shelbyracing]

I read that a Buick Regal GS (240Hp) was used in a test by these guys and he stated that every inch of mercury was about 1 hp and the cars factory exhaust was 30" factory so netting a 30 hp loss. I imagine the Aurora exhaust is at least as quieted as a Regal, I actually think it is worse since we have a silent V8 stock (not bad for anybody stock though as the rated power was dynoed with that exhaust on it SAE NET Rating). My exhaust is only about 5" and I was on the Caddy forums where they are discussing exhaust as well, and Corsa showed a dyno comparison of gaining 22 Hp, my exhaust is far more free flowing than that.
My exhaust yielded about a 10%+ gain.

www.thrashercharged.com/tech_htm/exhaust.shtm

www.cadillacforums.com/forums/attachments/northstar-performance-technical-discussion/107359d1367339870-northstar-exhaust-sts-corsa-dyno.jpg