Chris 911

I had a primary muffler bypass fitted before leaving for the autobahn in Germany. After changing the primary muffler, I could only hear a slight difference, but after doing about 4000 km (about 2500 miles) on the autobahn it seemed that the exhaust opened up, and more sound and power could be felt.

After the return from the autobahn the secondary muffler started to crack on the outside shell. I therefore had to consider options of change. In long term I would probably change the system for a Fabspeed system, but since I had the primary muffler I thought I would try a G-pipe (secondary muffler by-pass) First with the primay muffler bypass, and secondary muffler bypass (this would be the same setup as the 964 Cup cars had, ref Fabspeed) The sound was great, but too loud. Tried it for a quick spin, and you could definately feel quicker acceleration, and a more responsive engine, so it might be something to mount for fun. (first gear was like it disappeared in no-time) Changed then to standard primary muffler, and secondary muffler bypass. The sound is deeper with the secondary muffler bypassed, and a little louder than with the primary muffler bypassed. I have not yet fitted a Cat-bypass but will probably have it fitted this spring. (I also have an round 10mm tip fitted.)

I´m thinking of the Fabspeed dual exhaust, anyone with experience from that system?

Marc Shaw

Do you mean a 10 cm round sport tip instead of the stock black one?

Marc

Chris 911

Yes that was 10 CM round tip, not 10mm that would probably give great sound ;-)

Marc Shaw

I think 10 mm would give a bit of a whistle (and a bit of back-pressure).

Marc

Dunasso

Hum, OK Bill D. you win. I am no, nor claim to be, a physics professor. So for me to try and explain theoretically what I know works in practice is beyong my abilities. Again you will get no argument from me. However, I will agree that enough backpressure will snuff out any engine. But, on the other hand it is the correct amount of back pressure that gives an engine some of its ability to gain power at low RPMs. When you elliminate back pressure it raises the usable low end power to a higher RPM range.

And thats all I can say about that. I am certain that there are more people on here that would be able to shed more light on this subject for you. But as for an explaination, this is the best that I can do.

Duncan

joey bagadonuts

You make a good point, Duncan, and I don't think Bill was actually challenging you. He just wanted to understand how it was possible. It's all good.

The one thing that I would add is that Bill and you are commenting on two different issues:

- The effects of exhaust restriction on usable torque (You)
- The effects of exhaust restriction on absolute hp or torque values (Bill)

I think it would take an engineer to explain the rest but my 0-60 comparisons (primary vs secondary bypass) seem to confirm what you've suggested. With the more restrictive g-pipe setup, my car appears to have more low-end torque as evidenced by the faster 0-60 time.

Dogmanus

Hi Guys!

I don't know anything about which muffler to bypass and so this thread has me intreeged(no) intrigged(no) oh heck, curious.

Regarding backpressure, doesn't a freeflowing exhaust, and even better, an engine with headers benefit from the scavenging which aids filling the cylinders with fuel?

Henry

Chris 911

Also find this strange. Went through the Fabspeed website where they have videos, and have done different tests with exhaust systems. This is a great site to see the different mufflers and the sounds they will give. Here they have the primary muffler bypass exampled. I am currently running the G-pipe,(secondary muffler by-pass) but used to have the primary by-passed. The sound is a bit louder and deeper with the secondary than just the primary removed. I plan to go for a Fabspeed sollution this spring as mentioned. Seems like a good product (although maybe not the best marketing to put competition in a bad light as they doo, althought they have a very good explanation on why their product is better than competitors)

Fabspeed concludes that no back-pressure is needed for turbo engines.... Will this apply to our engines as well ?

Most racing set-ups runs a free-flow exhaust (or as much as possible limited to noise restrictions on different tracks)

I am no expert, but find exhaust treads interesting, as I have been experimenting with this myself. And everybody loves the sound of a flat six. (summer time means windows open and sun-roof open to get the full effect while cruising)

Mike Andrews

A couple of years ago our region (RTR) was hosting a dyno day at AWE Tuning. They had just installed their four wheel chassis dyno and one of our members has a C4 and was curious what horsepower his engine was making. On a whim we made a pull to record the starting point and then rolled his car forward into their shop where they proceeded to show us what went into making a bypass pipe. When ever thing was completed the car was rolled back onto the dyno and another pull was made. The end result was a gain of 9.5 horsepower.

There's an on going discussion which muffler to "by-pass" and I think the general feeling is the secondary muffler is fairly efficient therefore the primary is the one most people choose.

FWIW, when I did my RSA I did the primary as I wanted to take weight off the back of the car and remove the heat sink from under the rear bumper.

craig001

This is an interesting thread. Especially Joey B's numbers. It makes me think about what I'm about to do. I have an Automotion Chip, a spare Airbox that I was planning to drill out ala 933 Motorsound, and a choice of two Primary By-pass Pipes. I have a Shubert (off eBay) that looks like it has a smaller muffler in it and I don't have the weight handy and I have a RUF Primary Bypass as well I want to do is lose some weight back there and maybe gain some HP.

This conversation is making me lean towards the Shubert since it probably won't decrease the back pressure as much. In turn, would this give me more pull at lower RPM's than the RUF and even less back pressure?

JAMES J

I went with a primary bypass an evo cone filter & chip + euro LW flywheel.....a porsche mechanic said "your's is as Quick as a 993 and it really sound's sweet".
J.J.

Doug H

I seemed to get a really huge torque curve when I bypassed the primary and went with GHL for the secondary. My hp was only around 245 at the wheels, but my torque was 242 at the wheels and came on really early in the rpms. I had other work done also so it is probably a combination of everything and just getting the right set up. I worried about loosing too much back pressure when bypassing the primary. I think my shop was of the opinion that bypassing the secondary would result in more hp and bypassing primary could lead to better torque.

GG Allin

Anyone recomend a brand GHL, Fabspeed, B&B??? My car came with the secondary allready bypassed by a previous owner. Not loud enough! Going to bypass the primary.

Chris 911

If you have a look on Fabspeeds website, they have videos of the different setup as I mentioned earlier. I currently have the G-pipe (secondary by-pass) Which has also lead to more sound after a bit of driving. Changed it from a cup-pipe (primary by-pass)
The G-pipe has more sound than the cup pipe, and as Doug pointed out, seems that the g-pipe gives more horsepower (increase in the high rpm) Whereas the cup-pipe gives more torque. (more low rpm power) It felt like both setups gave a slight increase in power, but not much.

If you want more sound than the secondary by-pass there are a few ways to go. I assume that another reason why I have more sound now is a 10 cm round tailpipe, which is more open than the original. You can also go for a Cat-bypass, for slight increase in sound and power.
If you by-pass with both the cup-pipe and the g-pipe, you would get "Carrera Cup" exhaust. This they also have a sample on at the Fabspeed website. This setup is LOUD. I tested it, it gave a noticable increase in power, and the sound is awesome. It can be used for fun, or if you only use your car for short trips. For long trips and daily driving it is just too much sound. You will get tired of the engine noise, and your neighbours will probably end up hating you after a while.

The Fabspeed exhaust seems to me like a good sollution. And I think I will upgrade to this. You can choose between single or dual outlet. To mee it seems that you would want the dual outlet sollution, which gives more sound than single. Both are sampled by videos where you can hear the sound on their website.

GG Allin

Just ordered a "Dansk" Primary bypass pipe. $299 from the Performance Products Catalog. If the term you get what you pay for applies to these pipes (fitting issues, quality of metal) I'll flip it on eBay and go with a B&B. Any noticeable increase in torque will be posted here.