Lack of backpressure or O2 sensor?

[carnut169]

Hello all. I just replaced my cat w/ a cat test pipe and although the sound is fantastic it seems to have LESS power! I am not sure if this is because I have a bad O2 sensor or because the system does not have enough back pressure... what do you think? I unplugged the O2 sensor for the drive to work this AM (25 miles) and it did not seem to make any difference. I want to keep the cat test pipe but not at the expense of hp. It just does not seem to make sense that the car would have less power. Do you think I should replace the sensor? The car does have 59k, and I remember reading that Porsche recommends replacing it at 60K. What else can I do to determine the problem? Thanks, Sean.

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87 Carrera Cabriolet

[This message has been edited by carnut169 (edited 08-10-2001).]

[carnut169]

No one has any experience with this???? I guess I'll order a O2 sensor and see what happens!

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87 Carrera Cabriolet

[nhromyak]

I recently replaced my CAT with a straightpipe too.

I only noticed a slight increase in sound, no power gains, although I did not notice a power loss either.

Originally, I did not have the O2 sensor installed. I only got worse gas mileage. I guess around 20MPG on freeway.

I then had a muffler shop install the bung and reinstalled the O2 sensor. Now I get my mileage back to normal around 25MPG on freeway.

Perhaps there are other issues?
Have you changed the cap, rotor and plugs lately? This seems to have helped me quite a bit after I first got my car.

Good Luck

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Nick Hromyak
'85 Carrera 7 & 9 Fuchs
Havin' Fun in Sacramento

[carnut169]

Nick- just had the 15K service done two weeks ago, after which the car ran great. My mechanic said the b/c I am running the B&B and the test pipe I may not have enough backpressure and could be losing my "umph" there. He said I may be better off w/ one or the other, but not both. I have ordered the new O2 sensor, and plan on replacing my cat on Tuesday unless someone here comes up w/ something better. I imagine I will have a test pipe for sale shortly- it is too bad b/c the car has never sounded better.

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87 Carrera Cabriolet

[movin]

You could try a performance chip that takes into account the changes you made such as available from Autothority. My opinion is the DME has to be remapped (reprogrammed) to optimize the new parameters.

[carnut169]

Just trying to keep this fresh- picking up the new O2 sensor today... Tuesday I will be putting my cat back on.... any other suggestions would be great! Can Autothority make a special chip for my car? How would that help if I don't have enough backpressure? Seems like I would need to replace the cam if anything. Lack of backpressure means that the exhaust is not able to "suck" out the remaining exhaust in the cyl- correct?

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87 Carrera Cabriolet

[nhromyak]

Knowing about Chevy V8s if you have more back pressure, you gain low-end torque.

Not really sure, but here's my theory, the carb/FI see less vacuum and low rpm and part throttle, thereby the carb/FI adds more fuel (thinking throttle is open more) hence, more low-end torque.

Now when you go to the higher RPMs you have TOO much back pressure and not enough fuel/air can flow into the engine and you stifle the engine because the exhaust can only let out so much air/fuel.

I would think you upper RPMS have gained some HP, while suffering low-end torque.

For a daily driver, we might be better off with one or the other, or if $ permits time to get SSIs.

Generally speaking, these cars have had MANY MANY years/man hours into the engineering on them, I think Ferdinand and his team of engineers have done a much better job of tuning the car than we (or aftermarket) can accomplish.

That being said, there are some EPA CARB, and DOT requirements which have stifled the cars over the years. Hence, removing or upgrading these items seem to have made a difference for me.

I have a CHIP and the bypass tube in place. I am thinking of finding a Bursch exhaust simply to hear the sound and "see" if there is a HP difference. I am sure, I will eventually get the SSIs when $ permits.

Have FUN! (I know I am)

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Nick Hromyak
'85 Carrera 7 & 9 Fuchs
Havin' Fun in Sacramento

[KTL]

When you have little or no backpressure, the motor can experience scavenging. I believe this draws the exhaust out very quickly along with some of the incoming air fuel mixture. Hence, a power loss. I don't quite understand how that could happen because it seems like that occurrence would be due to valve train timing, not exhaust flow. Hey, that's just what i've read. Obviously, i'm no expert motor builder.

I don't think your mech. is entirely correct. Whether my backpressure description is correctly described or not, is not necessarily important here. From what I understand, the 3.2 exhaust system develops plenty of backpressure with it's unequal length manifolds and siamesed twisty turn crossover tube. The removal of the cat and factory muffler surely don't eliminate all of the backpressure. With the addition of a B&B muffler, you're gaining back some backpressure. The B&B is not a straight through design. There are perforated baffles inside which slow down and smooth the exhaust pulses to give you the rumbly sound.

As far as the custom programming is concerned, Autothority (and others like Rennsport Systems for one) can custom program your ECU PROM to account for your modifications. A friend had Autothority map him a PROM to maintain the factory rev limit and richen the fuel delivery for a B&B full header system-single outlet on his 86 Carrera. It scoots pretty good with the free flowing headers. No power loss whatsoever. The bone-stock 87 in my garage wouldn't stand a chance against his 86 (in any sort of race).



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Kevin
87 Carrera coupe

[carnut169]

Thanks for the replies- you guys have been helpful as usual. I spoke with Ali (Autothority) who said the chip would help- but I would most likely feel a flat spot between 3-4,000 rpm. He also said that I would be gaining 16-19 hp at higher rpms- but I spend most of my time between 3-4,000 so for me, that does not make any sense.


What I am going to do is re-install my cat with the new O2 sensor I picked up yesterday. FWIW the O2 sensor is not the problem. The system ignores this sensor under WOT and since my car feels flat at WOT it could not be the O2.

I will report back with the results.

P.S. Autothority's mass flow kit adds 38hp! Thats not to shabby!

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87 Carrera Cabriolet

[Superman]

I also do not believe that the test pipe can cause poor power. I think it's something else.

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'83 SC

[carnut169]

Perhaps it is something else... but replacing the cat fixed it. It is back to the smooth accumulation of power with revs. Guess it was too little backpressure.


I wonder if anyone has figured out what the various mods cost per hp. I may start on that.

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87 Carrera Cabriolet

[This message has been edited by carnut169 (edited 08-14-2001).]