stewardx

you need to raise the fuel 4-5 psi otherwise you lean and the gains will be small. 20 whp is my guess with fuel.

Wes

VWaddict

This is my intuitive thinking:

I picture it that at low-flow rate for the exhaust gases, the diameter wouldn't make much difference. think of exhaling gently into two different-diameter drinking straws. -Up to the point where a combination of friction and turbulence causes back-pressure to build, there is little difference. -Above that point however, I'd expect the pressure to build rapidly.

Based on the above intuition, I'd expect that the only noticeable gains would be higher up.

If anyone can see huge misconceptions in my above limited grasp of the problems, please point them out, since I don't claim to know what I'm spouting here, and I reckon my belief system should always be open to re-formultaion!

toddk911

I think accross the board as the more free flowing will allow gains in the higher rpms, but wit hthe reduced back pressure, you will probabaly also see boost come on a little quicker, thus showing gains in the lower/mid rpms as well.

Will you run the car without adjusting the boost? As we all know with less backpressure you will see a little more boost, and that would obviously show a total gain of power, but not in regards to just a more freely flowing exhaust.

So make sure it is run on the same boost as before. This will show only gainse due to flow and not regained boost that was lost from the backpressure.

Andial951

Do a search...there are plenty of people who have experienced nice gains by going with a 3" with no cat. I posted a thread a few weeks back on my gains which included the installation of a 38mm Tial. My gains were siginficant.

jerome951

Based on my butt-dyno experience of going from a stock exhaust w/ cat to a full 3" w/o cat, I'd guess you'll see 15 hp and torque gain at 3.5k rpm and about 8 at 6k rpm.

fast951

This subject takes me back 10+ years..
ASSUMPTIONS:
- YOU RUN THE SAME BOOST ON THE K26!!!
- Stock cat-back is not damaged or clogged

1- If the car has a cat converter - going to a test-pipe (even 2.5") will gain some power.
2 - If the car has a 2.5" test-pipe, going to a 3" cat-back will cause you to lose some mid-range, but recover up-top.
2 - If the car has a 2.5" test-pipe, going to a 3" system will not yield more than a few HP if any.
3 - With large turbos, the 3" makes a huge difference especially at higher boost levels.

Let's see if things have changed in the last 10 years.

PorscheDoc

Here was the baseline dyno. As far as we know, the car is stock with the exception of a gutted cat. Engine had a fresh rebuild about 2500 miles ago. I would imagine we will run the test around 5pm tonight unless something comes up.

ivai

Shouldn't it be getting higher numbers, especially with a fresh rebuild and gutted cat?

95Juan

i think those are pretty standard numbers for a stock car running stock boost, right? gutted cat shouldnt do much w/o more boost.

Chas

all I can say is the difference between a stock exhaust and a cutout right after the downpipe felt like about 45hp

My cat is pretty clogged tho...

pole position

It just shows how inaccurate chassis dynos are. Porsche rated the 951 at 217-220 CRANK hp on a calibrated TUV certified engine dyno back in the 80's, cat or no cat (which was optional at some point). Plus if you like rules and regulations Germany is the way to go because there is a law that a car manufacturer cannot go +/- 3 % over the claimed hp rating of said car or engine. So if it does not pass that requirement it goes right back to the engine assembly department until it falls into the acceptable range.
A drivetrain which consumes only 7-10 hp until it transfers the power to wheels (on a car) does not excist.
A chassis dyno will show you gains for mods after a baseline but the true accurrancy of those numbers need to be taken with a huge grain of salt.

PorscheDoc

Typical loss on a chassis dyno for 2wd is 10-15%. That is a pretty safe number....is it exact? Of course not, there is going to be some +/- in the numbers, but not huge numbers. A completely stock 951 (217hp) should put down roughly 185hp to the rear wheels, so we are looking at a gain of roughly 20hp to the rear wheels by gutting the cat. I wish I had a bone stock 951 to dyno, I would love to see the curve, then gut the cat and see the results, then go to a 3".....but alas I have to work with what I have at the moment. I think things are slowing down for the day, so we should be able to get after it here in an hour or 2.

Matt H

Most stock non S 951s I have seen on a dyno made something like 190ish, I dont think I have seen than 10HP in either direction different.
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PorscheDoc

Well the results are in, and interesting indeed. Seems that the 2.5" exhaust is not all that restrictive for a stock turbo as was mentioned before. Dyno with stock car and a gutted cat was 209hp/227tq. With no changes but the full lindsey 3" exhaust, we went to 213hp/234tq, with a very similar curve. A gain of 4hp/7tq over a gutted cat.

fast951

Doc, thank you for sharing this info. This is about what I expected!