You won't have any issue util 500+ whp.
I have a is38 full bolton with all apr tunes. I have apr catted downpipe and am running straightpipe from cat back witha 2.5 inch. I am going to do a 3 inch straightpipe from the catback. Is it worth it to do or should i just leave it?
You won't have any issue util 500+ whp.
This.
I made 502whp@8300 rpm with an AWD running a 2.5" elbow right off the turbo, which is the worst possible place. No signs of restriction at what was likely 600bhp.
Even Forced Performance told me not to bother with 3" at that power level.
Do you think a 2.75" downpipe would affect any power loss on IS38?
Its true bigger is better with turbo engines. Turbos cars don't need to worry about the back pressure like N/A cars do and prefer the least amount of back pressure to help the turbo spool faster and create more power sooner. That being said, you will see the biggest HP and TQ increases by having a larger downpipe then a larger cat back.
So if you have a 3 inch downpipe and a 2.5 inch cat back, not sure it will make much of a difference that far downstream.
Is it worth it? thats up to you to decide if a few hp is worth the money to upgrade. If you were going to replace your exhaust anyway, then yes go with the 3inch over the 2.5
I have a Stage 2 Uni GTI with a eurojet downpipe mated to the stock catback. I also have a QTP electronic exhaust cutout between the resonator and rear muffler. I dynoed the car at stage 1 and made 280whp with the cutout closed and 286whp with the cutout open. I just dynoed the car at stage 2 and made 321whp with the cutout closed and 334whp with it open. The stock catback might be a little restrictive.
I have a Stage 2 Uni GTI with a eurojet downpipe mated to the stock catback. I also have a QTP electronic exhaust cutout between the resonator and rear muffler. I dynoed the car at stage 1 and made 280whp with the cutout closed and 286whp with the cutout open. I just dynoed the car at stage 2 and made 321whp with the cutout closed and 334whp with it open. The stock catback might be a little restrictive.