I've got the 3" downpipe. 2.5" straight through or 3" straight through? May silence depending. Thanks

Have a look at Jason's solution. IMO 2.5" is plenty if done right and fit a single silencer.

2.5" on mine has been fine.

 

There's been a big debate in the past about whether a 3" is needed at just the point the exhaust gases leave the turbo. The Longlife dealer who built my exhaust is convinced that 2.5" is fine though.

 

Definitely fit a silencer. It will be VERY loud on boost otherwise.

Mine is 3" straight through although where it bends for the side exit, the compression needed to make it oval probably makes it 2.5" roughly.  Either way, it rocks!

 

Oh, and I have a single fairly big longlife silencer in it (3" in, 3" out) to take the edge off the note on normal driving.  On full power it almost becomes a straight through anyway

3" is good for 500 BHP, 2.5" is big enough for 350 BHP, your call.

I've heard people have had problems with the 2.5 being restrictive on the bigger GTB's....

Not had any personally but I've had people ask me to supply them 3" with the flexi because of that reason.

I've heard people have had problems with the 2.5 being restrictive on the bigger GTB's....

Not had any personally but I've had people ask me to supply them 3" with the flexi because of that reason.

 

Most people wouldn't know a restriction if it jumped up and punched them in the face, however there is a difference between exhaust requirements for VNT and FGT, with a FGT spool is critical because they're really slow to spin up, however with a VNT this isn't the case and oversized exhaust systems can actually slow spool due to the sudden drop in gas speed as it exits the turbine.

If this were a FGT then I'd say 3", since it's not I'm saying 2.5" and there won't be any 'restriction' anyway.

Ask yourself what internal diameter the exit from the turbo is and then make a case as to why the exhaust needs to be bigger than that, remember that a 2.5" pipe has ~70% of the CSA of a 3" pipe.