Hi Rennlisters,

I am getting a custom valved exhaust built for my 996 Gt3 but am having some questions about exhaust diameter. I am looking at two options: 2.5" or 2.75" with sport cars and custom high-flowing mufflers.

My plan was to simply go with 2.75" as I have heard that a better flowing exhaust cannot hurt a NA engine; however, the exhaust valves in that diameter are currently unavailable. It's unlikely that they will become available until the end of this year. The specs of my car are the following:
- Engine upgraded to 3.9l from Manthey making ~370hp at the wheels
- Original exhaust system was 70mm (2.75") but this thing was obnoxiously loud which is why I am getting a new one built

What is your experience with exhaust diameters?
Am I likely to lose any power by going with the 2.5" option? I have seen 991 Gt3 RS exhausts are with that diameter and make 500hp.
Should I simply go with the 2.5" version or opt for a 2.75" one with a 2.5" valve that I can then replace at a later date?

Ivan

 

I dont think there will be any different unless u r incressing the diameter of the primarys and collector of the header too. R u installing mufflers and cats? The exhaust needs ro be looked at as a system not just as a tube diameter. Our engines r relatively low displacent so efficiancy is better then size.

I did find with my larger fvd exhaust i needed a tone to get a better power curve. Not sure how a 3.9 manthy motor would respond

 

I'm not sure that the 2.75 is worth THAT much power, even on a 3.9 like you have, but If I'm building something from scratch (which I have in the past) I'd go for the large one all the time, just add more mufflers in there to decrease the sound to be acceptable. My 2.5 ad 2.75 exhaust in basically the same design have been equally loud and obnoxious.

 

@AudiOn19s Tried to send you a PM but mailbox full. Ping me when you get a chance.

 

here is my kline inconel. buddy was driving my car, there is a stock gt3 a few cars later, you can hear how quiet it is.