Pre-muffler vs Cat bypass
 

1984 Carrera

I am thinking of trying to get a bit more power and a better exhaust note - on the cheap. I read the whole SSI vs. OEM vs headers debate, and I am definitely not going to change out my HEs. I will probably upgrade to a SW chip at a later date.

I have a line on a used "Schnell cat bypass" that I think I want. Is is not touted as a premuffler, but is not a straightthrough pipe either.

I have a few questions- what is the difference between the test pipe and premuffler in terms of construction, power increase and noise??

Do they typically make the same power? Will a stock 1-in/1-out sound good bad or obnoxious with either?

Finally, - same questions but different car - 80's 3.0 SC.

Thanks!



On a stock 80's Carrera or SC

A pre-muffler will be a bit quieter than a bypass. Here's mine with a factory pre-muffler with a Flowmaster type muffler. 1in-1out.
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I would suggest checking out the ultimate exhaust thread here:
http://forums.pelicanparts.com/porsche-911-technical-forum/332660-ultimate-exhaust-sound-clip-thread.html?highlight=Ultimate+exhaust

Wayne- You exhaust sounds great! Thanks for the link. I went through all 9 pages, but no one is out there bragging about a stock setup with just a bypass or premuffler. They are mostly non-stock mufflers.

What about the performance side. Do you think there would be a meaningful benefit in HP with the cat bypass or premuffler?

We installed a hollowed out cat on a buddy's 86 Carrera and the loudness factor really didn't change much although he said the butt-o-meter felt a difference. He has an aftermarket muffler setup waiting to be installed in the spring.


I really didn't notice any HP gain but the sound is better and I think it breathes a bit better in the upper RPM range.

I think the consensus here is that unless you go to headers/SSI's AND a performance muffler, the gains are small, if any.

What, exactly, is the difference between a pre-muffler and a catbypass?

The short:

A premuffler has an expansion chamber, a bypass does not, it's just a straight pipe.

GBG1000:
I have a Dansk premuffler with the original muffler, but a SW chip installed. The performance is there, but you really don't feel it. The sound is a little louder and deeper. Not much to brag about. The Dynolicious app for the iPhone says that I am getting about 224 crank HP (at least I think it's at the crank). Either way, successive runs come in about the same.

Think of a cat bypass as a "test" pipe. Simple, straight through design.

A premuffler is more like a cherry bomb as it has some material in it for muffling.

A pre-muffler is just a small muffler in place of the cat which is what the euro cars came stock with. I have a stock 3.2 with a stock muffler one in one out and stock heat exchangers. I have run the stock cat as well as a hollowed out cat. The hollowed out cat is essentially the same as a "cat bypass" or "test pipe". My car has a louder but not obnoxious exhaust note with the hollowed out cat vs. the stock cat. Performance wise, it seems to pull harder from midrange to top end and rev quicker without the cat but this is based on seat-of-pants not dyno results. I have read that the pre-muffler actually produces more power than the bypass due to the fact that these motors like a small amount of back pressure.

There have been a couple of posts where it is proposed that a factory Pre Muffler may make more HP than a cat bypass.

I have not seen any dyno tests to verify this or that any other pre muffler is equal or any better. I am surprised that someone has not dynoed the two options to see if there is a difference. It is just not that expensive to do so.

A used cat bypass or just gutting ones cat are the cheapest way to HP on a 3.2. Next is adding a second tail pipe to the right side muffler (Per Bruce Anderson's book). This matches most sport mufflers.

A 911chips.com chip ties it all together and makes for about a total gain of very close to 26hp (per SW data).

I defer to the experts. :)

Thanks for the explanation.

Have any of you in CA had trouble getting through smog with either a cat bypass or a pre-muffler (or do you just put the cat back on?).

Put the cat on. Cat bypass will not pass visual, premuff can, but I doubt you'll pass emissions.

My .02 worth....most of the "sound" is derived from your choice of muffler. I already had a GHL 1/1 on my car, and when I installed my Fabspeed premuffler, I could not tell one bit of difference. IMO, my car does sound great however, and many have told me so...including the LEO who was handing me a ticket a few years ago :)

Thank you one and all.

Very interesting and informative. I am sort of intrigued by the idea gutting the CC. 140K miles- maybe it is a significant obstruction. I will probably do one or the other as soon as I get a little breathing space with work.

You might want to consider saving your cat, if it's still good. Florida may be changing their emissions laws in the near future.

Old thread...

Question... I'm still not entirely clear on this.

Both the premufflers and cat bypasses both eliminate the cats, right?

It's just that the premuffler has some sound deadening material that the cat bypass does not?

That is correct.

Years ago was lucky enough to have a nice long conversation w/ Bruce Anderson about 3.0 exhausts. During that conversation he mentioned that the 3.0 Euro pre-muffler was worth some hp/torque over a cat or straight pipe cat bypass.

A pre-muffler is a muffler.
A cat bypass pipe is a pipe.
That's why they call them that. ;)

Both add power compared to the stock catalytic convert that they replace.
The M&K PreMuffler and Bypass Pipe both have the same ID.
For the Carrera, a premuffler/muffler/chip combo yields similar gains as SSI/dual inlet muffler/chip combo.

I have one of Rarlyl8 pre mufflers on my car and I like it. After listening to a lot of exhaust sounds online, I realized that a cat upas seemed higher pitched and rattly sounding. The pre muffler is a lower pitched, growly sound. It's hard to describe, but I personally didn't like the cat bypass sound. The pre muffler is not too loud either. A bit louder than the stock muffler, but most of the change is in the quality of the sound, not the volume.

Yes. A pre-muffler is a larger diameter pipe with some sound deadening material (fiberglass) in its periphery that mitigates the noise. I believe the straight pipe may produce more horsepower because the gasses flow through it more freely than with a pre-muffler. A pre-muffler has no physical restriction, but I think the larger diameter creates some turbulence that slows down the exit of the gasses. There is a picture of the inside of an M&K pre-muffler in this thread. The inside of a cat bypass just looks like the inside of a pipe.

http://forums.pelicanparts.com/porsche-911-technical-forum/515525-criminal-behavior.html