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 dyno needed), I'm thinking it's currently around 375 HP (on street gas) with the large intercooler Whipple blower on the 3.6 motor (I currently have a single pipe 3" exhaust system throughout with stock manifolds, a cat bypass pipe, and a single Bassani muffler).

Due to 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" 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 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...

Beetos

only thing I would offer is that exhaust performance difference is directly propotional to the output of the motor. For example, if you had a stock motor and changed the exhaust the difference might be small, but on a blown 3.6, the stock exahust becomes limiting. I have a supercharged 3.6 C2 and run a eaton 90 blower, with cat bypass, priamry bypass and mass air. I can tell you I can feel the difference big time when I put my cat and primary muffler back on for inspection. I would definately not recommend going any smaller with your current set up. A dyno is the only hard and fast way to tell.

P.S. what size injectors are you running? what engine management are you running, show us a picture! sounds like a great set up.
Cheers.

Joe@Fabspeed

you must stay with 3.0 single outlet if you putting out over 325HP or "you'll choke it to death". Or run duals in 2.25" minimum or 2.5" duals. Let me know what type of headers you are running. I can supply you with something that will attenuate the sound, not break, and works.
joe fab fabspeed motorsport USAinfo@ fabspeed.com

SuperCarrera

I am currently running 30# injectors and have an EFI Systems programmable engine management set-up. I do plan to install larger injectors when I add headers and the upgraded mass flow unit. Unfortunately, I don't have an "internet-friendly" photo of the motor (and the large intercooler blocks a lot of the "view" anyway).

Joe -- I bought my cat bypass pipe from you and the guy who did the supercharger install commented on what a nice piece of "hardware" it was!

I am currently running the stock 964 exhaust manifolds -- this was part of my inquiry in regard to the restriction that already exists with these stock pieces in place. Although I think the motor may be currently putting out a maximum of about 375 HP, with these manifolds in place, will it still restrict the airflow enough to affect the vehicle's performance if I go down to a 2.5" muffler size in my 3" system?

Thanks,

Tim

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.

???

Beetos

Sorry for this off topic post, but I am struggling to understand how you are getting 370hp with 30# injectors? Even with a duty cycle of 100% you would need a fuel pressure of about 70psi. If you are running a more normal 80-90% duty cycle, then fuel pressure would need to be in the 90psi range.

I have 6 x 36# injectors and 1 supplemental 36# injector and still have to run 50psi fuel pressure to stay rich at WOT. Even then my duty cycle is reading about 90+%......Just curious, cheers.

SuperCarrera

Beetos,

Sorry for the delay in response -- I (personally) do not really understand the performnce equation you are referring to.

As far as my (reported) HP figure, that is what I've been told my motor should be producing with the stock manifolds, no upgraded mass air sensor, and these injectors. I do have an upgraded fuel pump to help push the liquid part of the equation and have honestly really struggled trying to figure out exactly what size my injectors are (they are not Porsche components). I know they are in the "30's" somewhere but could be as high as 36# units (I may get a definitive answer tomorrow in my planned discussion with someone who knows this system well).

Anyhow, I guess the short answer is that I don't know how to properly answer your question (I'm most definitely a Porsche performance "newbie") -- but I do know that I don't have a problem with lack of fuel at WOT (and won't unless I get on a track with an extremely long straightaway).

I really hope to find an AWD dyno somewhere nearby so I can check things out, for real; and I do look forward to putting in larger injectors at some point in the future.

I'd like to hear more about your car -- I recently met with Cecil (from the 993 Forum) and checked out his TPC-equipped 993 as he lives about an hour from me.