BGLeduc

Subject says it all...

Lost of posts about the pros and cons of doing primary or secondary muffler bypass, as well as cat converter bypass, but pretty much always in combination.

My interest is the sound of a stock exhaust with JUST a cat bypass pipe installed, leaving the pri/sec mufflers in place.

On previous vehicles, I have done cat-back upgrades, and usually end up taking them off in favor of the stock muffler. While the newer mufflers sound great at full throttle, there always seems to be a boomy resonance at freeway speeds that just give me a headache.

So, hoping to avoid that with the P-Car, my thoughts are that a plain cat bypass will give my whatever performance gains are out there, without getting radical, and perhaps NOT cause me to regret the decision on my next long distance blast.

So, if you have tried this, your opinions would be most appreciated.

BGL

Doug H

I use Fab speed cat bypass, GHL primary and no secondary. The sound is awesome with no resonnance. The head service manager at my Porsche dealer has told me me at least two or three times and maybe more that he just loves the sound of my car. My neighbor the other day said the same thing.

I have a cabriolet with a god bit of the soundproofing pulled out and it is really not that loud or unbearable. If it was too loud or had a bad resonnance, I can assure you that neither my wife nor my 7 and 8 year old daughters would like it and this is my daughters favorite car I/we have.

BTW, it seems as if the engine has much more low end torque set up this way and my torque was about 250 pounds at the wheels and very flat. This didn't perhaps answer your question, but I am recommending that perhaps the GHL muffler sounds more restrained and with less resonance when used with a by pass.

joey bagadonuts

I have a similar setup to Doug's and recently did the 8-hour roundtrip to Mid Ohio Sports Car Course--piece of cake. I also averaged 22 mpg which is only 1 mpg less than the Factory estimate for hwy mileage. Finally, I sadly have one functioning stero speaker (right rear) but had no problem enjoying tunes during my roadtrip. I think your sound system, Brian, will make exhaust noise a non-issue.

Sorry I didn't answer your question, either, but I was aiming for max low-end toque for autocrossing and feel this setup is the ticket without being annoying. The fellows who may know how a cat-less exhaust sounds, though, are the UK Rennlisters. It seems many run their 964's without a cat.

Bill Gregory

As long as you leave the stock airbox in place, playing with the mufflers/cat shouldn't produce any obnoxious sounds. I have a primary bypass, which didn't do much for exhaust sound, however, adding holes to the airbox made a significant difference, and from my view, improvement. However, that doesn't sound like the type of improvement you're looking for. With my setup, the sound is most noticeable at full throttle.

BGLeduc

Thanks for all comments thus far.

I guess I can say that I want my cake, and want to be able to eat it too. Or, more clearly, I want my car to sound like a 993 with motor sound package, but normally only when driven in anger.

And of course, I would not be opposed to any performance gains I could get as well, so long as it pretty much on the cheap (no full up header/exhaust systems for me at present)

With an all stock arrangement right now, it is just too damn quiet. My previous non P-Cars with high-po mufflers and/or intakes (4 banger rice cars) sounded great at idle, and fabulous full throttle, but, highway cruising was just brain damage.

I guess its high time I get on with it and drill some holes in the airbox. I have been threatening to do it since I got the car.

And I could still always bypass the cat later if I felt that I would like a wee bit more performance out of the motor.

BGL

bikeaddict

Hey,

I've got the Fabspeed bypass/muffler with the stock primary and secondary in-place. It sounds very similiar to stock. I'm only keeping it on for the reduced weight and lower heat around the LHS lower head.

Bike Addict

91 911 C2

joey bagadonuts

Correction to my earlier post. FWIW, my roundtrip journey to Mid Ohio was actually 12 hours, not 8 as previously posted. Think of it, 12 hours and one tiny little speaker on its last leg. If it was really bad, I guarantee you I'd be posting for trailer and tow vehicle recommendations.

BTW, Brian, are you still planning to make the trip to Ft Worth for the Parade?

Ruairidh

Joey - are you coming to Fort Worth for the Parade?

Colin 90 C2

I used to run that setup. The cat bypass gives the exhaust a throaty growl on acceleration. At idle it is slightly louder than stock. It almost makes for a slightly metallic sound.

Here is what I've found through experimantation. Your results may vary.

Cat and secondary muffler replaced with bypass pipes- great sound at idle and very loud during acceleration. Sounds like a v-8 engine with flowmasters. too loud for my wife. Maybe a slight increase in low to mid range torque, but really too loud to make a decision.

Just cat bypass. Just the right sound. slightly louder at idle, but a little louder than stock on acceleration. From the resonance coming through the tailpipe, I suspected that this setup was on its way to blowing a hole in the primary muffler. The low to mid torque seemed to dramatically increase.

Replaced primary muffler with a Gemballa muffler. Cat still bypassed and original secondary muffler in place. Nice all around sound. The Gemballa muffler seems to open up under full throttle and provides a great sound... but the engine seems to be down on low to mid range torque with a feeling of a slight increase on the top end.

All these changes were done with a drilled airbox and a Cyntex chip that was tuned specifically for a cat bypass.

I think the best all-around mods are: cat bypass, drilled airbox and Cyntex chip tuned for cat bypass. This setup seems to sound best at all times and the increase in drivability is nice.

joey bagadonuts

Nice info, Colin.

No, Ruairidh, I won't be attending this year's Parade but thanks for asking. It's just a bit too much of a drive.

BGLeduc

I decided against it. There were some scheduling/organization issues that did not work out to my liking.

They scheduled the kiddie Autocross the same time I would be running in the P-Car, which meant I would not be able to watch my daughter run, nor she I. And I really do not like that the Autocross banquet and the road race banquet were combined, AND they require formal attire. Whats up with that? Rally has their own banquet. Why not Autocross.

Since the only thing at Parade I care about is the Autocross, all that stuff really screwed the pooch for me going.

So, instead, I am going to the SCCA Solo II Nationals (Topeka, w/o 9/13). There, I will have to run in ASP (due to the lowering springs) and will be getting severly waxed by psuedo race cars!

But it should be fun! Two days of competiton, at very low altitude, on very grippy concrete. My only goal will be to NOT come in last!

I checked the entry list, and in addition to the usual Boxters, there are some 996 and 993's entered in the various classes. That's a change from years past when the Boxter was pretty much it. I have not kept up with class alignments the last few years, but I know the drivers that are entering those cars, and they would not do so if they were not expecting them to be competitive. Could be a good year at Nationals for Porsche.

In case you are interested,. here is a link to the list:

http://www.myautoevents.com/pls/scca...sevent_id=1149

BGL