Bypass Pipe HP Increase?

[vesnyder]

I burnt out my catalytic converter and had to replace it with a bypass pipe. Had plans on doing it anyway, this just expedited the process! Can I expect any performance increase?

[dickster]

lots of threads already on this so you might be well to do a search.

i think you may as the cat is restrictive but not much

richard

[chuckr]

It is hard to say with just the by-pass pipe, you should get a couple of extra ponies.With my 83SC I got about 15HP with a cat by-pass, Bursch muffler and a K&N filter.I dyno'd the car so this number is accurate.

[89911]

A bypass pipe and a free flowing exhaust is not only an easy upgrade for performance but also for sound! Before I switched to SSI's, I had a bypass pipe and a Borla exhaust. The car sounded sweet, especially at tracks, (You could tell what car it was before you saw it). This was mostly evident at high rpms's. Around town, it was just a bit deeper but not offensive. Since switching to the SSI's and a Dansk 2in 1 out muffler, the sound actually is quieter. I am told the muffler will get louder with age. What is that saying? Speak softly, but carry a big stick?

[vesnyder]

I want to eventually upgrade the muffler and heat exchangers, but thay will have to wait.

[carnut169]

Expect no gains from the cat bypass pipe over a working cat. If your cat was shot you may feel an increase in performance similiar to what you would feel if you replaced it w/ a new cat. It will be louder- and it will run cooler. Do not install an aftermarket muffler w/ this set-up- you will feel a DECREASE in low end torque as you will no longer have enough backpressure (personal experence). Good luck, Sean.

[Steve W]

89911,

When you went from the cat pipe and Borla muffler, to the SSIs and Danske muffler, did you notice any increase in performance or power? I am curious because I have heard several claims that a cat replacement + sport muffler almost equals the gains of SSIs + sport muffler.

[DANNOV]

I have been considering a similar modification for my 1985 Carrera. There have been a lot of comments about such a conversion on this BBS and I have to admit that if you sum it all up, the opinions are all over the lot. I decided to do my own research. I am a pack rat who never throws away anything and I also have a fair amount of free time. Here is what I have found, quotes from the "MAN", Bruce Anderson.
From: Best of Excellence Technical Notes published in 1994, page 46. Anderson discusses a test of a 1988 3.2 Carrera engine. He concludes that replacing the cat with a Euro-style premuffer (not a straight buypass) coupled with a modified dual stock muffler (second exhaust pipe added to the right side), results in a HP increase at peak power of 5.3%. On my 1985 that increases my power from about 217 to 228.5 HP. He further explores a full exhaust modification (SSI + twin pipe sport muffler) to the same engine. At peak power the increase is 5.7%, or an increase on my engine from 217 to 229.4 HP. He does comment that the SSI system had a power increase earlier in the rev range. He said: "The SSI system had a 10.5% power increase back up to 3500 rpm where you really appreciate a nice powe increase" There is a chart (unfortunately not very legible in the article) which appears to show the SSI and the Euro premuffler/ modified exhaust showing an equal power increase at about 4800 rpm and almost equal thereafter to 6000 rpm. From about 2500 to 3500 rpm the SSI generates a bit more HP.
From: Excellence May, 2000 pages 24 and 28. An Anderson quote: "As I have pointed out in the past, the exhaust systems on 911s from 1975 through 1994 are horrid, but there isn't much you can do without running afoul of the smog police. One thing that does work is replacing the cat with the Euro premuffler, but we are not talking about huge gains. Some of the low backpressure mufflers will help a bit, but, again, no huge gains will be found. I have seen a gain of around 8-12 HP when you combine the premuffler and a dual-outlet muffler".
From: Porsche Panorama November, 1987 page 86-87. A similar discussion where he estimates a 6-8 HP gain from just replacing the cat with a premuffler.
What have I decided to do? I am going with a FabSpeed Euro-premuffler cat bypass. It has a 02 sensor fitting and it looks like the stock cat. I will keep my old cat for smog tests and the street (ha ha). I am also having my stock muffler modified to a dual exhaust. The cat bypass is $225 and the muffler conversion about $250. The result is about 10-12 HP for $475. It may not be as good a gain as complete SSI and new sport muffler, but the Anderson analysis seems to indicate that you only gain a bit more HP with the SSI. In checking out the price of an SSI with new sport muffler, I doubt if it is an economic modification compared to the Euro-premuffler/ muffler modification alternative.

[N11Porsche]

Hey ChuckR, if you got an extra 15 h.p. with the cat by-pass, bursch muffler and K&N filter: than put the stock filter back on and your gain will be 16 h.p. Bruce Anderson did dyno test of differant filter combo's on a Porsche 3.0 engine and found the K&N lost one hp. over the stock filter. Or if you want I will sell you my K&N for 10 bucks (almost new) and you can loose 2 hp.

[chuckr]

I have the greatest respect for Bruce Anderson and I have his book and treat it like the Porsche Bible..... However, I work for one of the top Porsche shops in Austin, we build Porsche track cars as well as do maintaince on all model Porsches, I also have spent time at the dyno, the dyno tests for the Austin Porsche Club are posted on our web site www.hcrpca.org.Each Porsche engine is different, my 3.0 will give different readings than somebody elses 3.0.Mileage or other upgrades will effect how a peticular engine preforms.What was the milage on the engine Bruce Anderson used for his test ? did it have new injectors ?
what exhaust?.His was a general statement based on testing of that engine, not very scientific.I have seen K&N filters that are not benifiting the car and in my case I did not have the time to do a with and without the K&N test, one or two HP verience on a run at a dyno is not uncommon.I posted my dyno results 6 or 7 months ago and I have yet to see anybody else with a 3.0 with similar upgrades post better numbers, to sum all this up...
till I see better numbers I'll stick with what I got, 15HP for under
$400.00 is a good deal for me. I will be going to the dyno in two months, I'll try the with and without the K&N and post the results
I am curious.

[89911]

No offense to B. Anderson, but I have seen that quote verbatim from him in many different publications with no change. In fact I emailed him about this very question and I got the exact statement as above. As the above post remarked, it seems as little is done to follow up the changes or specify the testing procedure. This is such a common upgrade, you would think someone would produce a fresher test rather then reprint the results of testing done over 10 years ago. One test on one engine, to me, is not a large enough scientific sample.

[DANNOV]

Originally posted by 89911
No offense to B. Anderson, but I have seen that quote verbatim from him in many different publications with no change. In fact I emailed him about this very question and I got the exact statement as above. As the above post remarked, it seems as little is done to follow up the changes or specify the testing procedure. This is such a common upgrade, you would think someone would produce a fresher test rather then reprint the results of testing done over 10 years ago. One test on one engine, to me, is not a large enough scientific sample.

If your are looking for a scientific sample for assurance then you should believe nothing on this BBS. As far as I know the only thing we can depend upon regarding Porsche performance mods is random subjective experience or isolated tests, rather than scientific studies. I wish it were different but that is just the way it is. The test in question was done on "a fairly new, 1988 Carrera 3.2 liter engine" sometime before 2/1994. As for the validity of a test done at least 8 years ago, I can only say that I too would like to see something more recent. I have searched my archives and that is the best I can do. Its not perfect but at least is more than a "seat of the pants" judgment. If someone has something more recent please post it. Maybe the reason we haven't seen anything else is because the test is accurate, fairly well known, and no one has anything to add.