Exhaust size vs. HP equation

[SuperCarrera]

I could really use some guidance with an exhaust issue I am currently dealing with.

On my supercharged 964 C4, although I've never dynoed it (AWD dynos are few and far between!), I'm thinking it's currently around 375 HP (on street gas) with the large intercooler Whipple blower on the 3.6 liter motor (I currently have a single pipe 3" exhaust system throughout with stock manifolds, a cat bypass pipe, and a single Bassani muffler).

To alleviate the slightly excessive noise (for street use) with this set-up, I'm considering an alternate performance muffler but, due to the constraints of my pipe configuration and larger size of this new muffler, I've been told that only a 2.5" (pipe opening) muffler size will work.

My question is this -- with my current (overall) set-up, will this 1/2" reduction make any difference in my vehicle's performance?

If not, as a follow-up question, when I go to headers, larger injectors, and a mass flow air sensor upgrade, should I then (definitely) go to dual exhaust or will the future 400-425 HP output still be OK with this one reduction to 2.5"?

Thanks...

--------------------

Tim O'Neill
1989 964 Super-Carrera 4
Sarasota, Florida USA

[SuperCarrera]

One other item of "confusion" regarding this issue -- I've heard both sides of the "best exhaust flow for my car" theory from several different people.

One "group" has said that, by restricting the airflow a little more (such as with this 2.5" muffler), I will gain low end torque and acceleration but may lose some top end speed/power.

The other "group" says that, because my motor is supercharged, it does best in all RPM ranges with the minimum airflow restriction at all times (so header tuning does not apply) and I should use the largest size, most free-flowing components through-out.

???

[RarlyL8]

Blown engines of any type (supercharged or turbocharged) force air through the engine. The more free flowing the exhaust the better.
Low gain or low boost systems will perform better with a tuned exhaust, high power systems perform better with no exhaust at all.
Large bore short pipe headers and a large bore muffler are a must if you wish to optimize the power output. Any restriction in the system will increase heat output while reducing power.

This is a generalization. Many, many other internal engine issues play a part. You should probably consult the manufacturer of your kit for specific information.

[SuperCarrera]

Thanks for the information -- I'm definitely now leaning away from using a 2.5" muffler in place of the current 3" unit as I do not want any heat or back-pressure issues.

I'm thinking I'll take my car to another recommended exhaust specialsit here locally and see if he can get this new muffler in a 3" size to fit in my current pipe configuration. Then, at least I haven't gone "backwards" and will re-evaluate my entire set-up when I install headers at a future date.

Does anyone on this forum have experience with the Powertone exhaust tips? Supposedly, they have a decibel reduction rating of about 10 units and do not create any back-pressure -- ?