951juden

I have an '86 951, and I would like to replace the catalytic converter with a test pipe. Has anyone done this? Have you noticed positive or negative effects? (ie- more or less power, longevity, etc..)

I am concerned about backpressure and the turbo being able to operate as effectively. Should I worry about that?

Thanks, Dave

John.

Any amount of back pressure in the exhaust system will delay the onset of boost. I noticed this with my turbo 928, the car is less "snappy" with a cat installed. Removing the cat should yield more power, but I don't recommend it for street driving. My guess would be that you will see the turbo spool up a bit faster, but the car will also sound raspier and rougher than with the cat. Remember, the size, length and shape of the pipes determines the harmonics of the exhaust system. The manufacturers spend lots of time to design the system with harmonics in mind.

fletch.

Save your money, No improvement what so ever when I installed a test pipe. When I smog my car i am removing it and selling it, spend the 250.00+ on a wastegate, it will net you tons more punch.

TurboTim

Fletch,


If you had no improvement when you installed a testpipe then there was something wrong with your car.I have seen gains as high as 30 horsepower on a dyno by installing the testpipe itself. We were actually going to dyno our full 3" exhaust system(cat-back and testpipe) and the stock system last Friday but unfortunately the test car never showed up.This is the only way to provide conclusive proof that it does work.

Tim
86 951 http://www.speedforceracing.com

Danno

These harmonics are only in effect between the exhaust valves, through the header to the turbo's turbine. Once the exhaust hits that, it's all chopped up and there's no resonance-tuning effects afterward. The section that Dave's talking about is after the turbo's turbine. In that respect, you want as free-flowing an exhaust system from after the turbo as possible.

The turbo works off of the pressure differential before and after the turbine. The higher the pressure you can make before the turbo (note 4-2-1 headers) and the lowest pressure possible behind the turbo will give you the most and fastest boost.

But it's easy to OVERDO the exhaust as well. A 4" cat-bypass and exhaust may be overkill because you'd have to push over twice as much air out the tailpipe at any given time (due to larger volume). So the best exhaust size, depending upon mods of course, is probably 3.0 to 3.5". Now the absolute best by far, would be a 3.0" exhaust that dumps out the side under the driver's door.
Check out this recent thread on the exact same discussions: http://forums.rennlist.com/scripts/rennforums/ultimatebb.cgi?ubb=get_topic&f=15&t=0020 18

I'm going to have dyno chart comparisons of stock vs. cat-bypass vs. cat-back exhaust vs. both upgrades sometime soon (like next two-three weeks). Will post them for your review...

slevy951

Dave-

I have a test pipe on my '86. Slight improvement in lag. Noticeably better sound. Car sounds like a performance car now! I'm still running stock muffler. Worth the change just for the sound IMO.

Scott
'86 951

OZ951

Danno,

I wouldn't go so far as to say there are no resonance effects after the turbine. The catalytic converter assembly does include a resonator section for that reason. In fact the reactive material in the cat is only an inch or so thick and the rest is a tuned cavity (resonator) designed to eliminate certain frequencies from the exhuast note using backscatter/destructive cancellation.

I think that is why you end up with greater resonance from the cat if/when you gut it becuase knocking out the cat material alters the dimensions of the tuned cavity to the point where it is no longer tuned to eliminate the expected frequencies in the exhaust note.

Wayne.

fletch.

Your right on the dyno proof Tim, I don't have access to one, and would love to think I was getting even half what your seeing.

Seat of the pants and overall comparison to cars I race with have shown no noticable gain, could have been my turbo at the time, It was spent. I since have a new one and have attributed the massive improvements on that and the wastegate.

Glad to hear I did not spend the money needlessly.

Danno

Yes, there are frequency-tuning effects in the cat & muffler sections, but these are strictly for noise-cancellation. They aren't used to coincide pressure-vacumn pulses to extract extra exhaust from the cylinders like on NA cars.

No matter whatever kind of resonance-tuning you do with the cat & muffler, it will never be as beneficial as running a straight pipe.