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Is38 2.5 inch exhaust vs 3 inch Is there really a difference?

ML1234

Ready to race!
Location
New York
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?
 

PacDawg

Drag Race Newbie
Location
Gilroy, CA
I'm APR IS38 and trapped 119 mph on the stock catback.
 

king12sky

Ready to race!
Location
FL
I was wondering the same thing as well. Will i get the same Hp out of a 2.75" downpipe even. Like Remus downpipes are 70mm/2.75".

Im thinking that might not be as effective as a full 3" downpipe.
 

Venumous

Ready to race!
Location
NE
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?

There are 800whp Coyotes running 2.5" catbacks. I think you'll be ok.
 

Hoon

Autocross Champion
Location
Rhode Island
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.
 

king12sky

Ready to race!
Location
FL
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?
 

Hoon

Autocross Champion
Location
Rhode Island
Do you think a 2.75" downpipe would affect any power loss on IS38?

Bigger is always better with post-turbo exhaust flow, but that said, I highly doubt you would see a measurable difference.

I don't know what an is38 flows, but it's probably under 40lbs/min.

You can get almost 100lbs/min through a 3" pipe before it becomes a real problem. That's enough air to make 1000whp. An acquaintance of mine ran well into the bottom 8s @ 17xmph trap speeds before he actually had to replace his 3" with a 4".

I'm sure he would have benefited sooner, but he didn't actually have restriction problems with a 3" pipe until 1000whp...so I don't see a little IS38 noticing the difference between 2.75" and 3".
 

2slowvw

Moderator
Location
VA
Car(s)
2022 Tesla Model 3
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
 

king12sky

Ready to race!
Location
FL
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

Yea i have 3inch downpipes on now with the stock exhaust. Its my second set up till day. What i was wondering if running the whole remus system will affect the flow/gains. Remus systems are 2.75".
 

dcgtimk7

Go Kart Newbie
Location
DC
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. :eek:
 

Hoon

Autocross Champion
Location
Rhode Island
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. :eek:

Good info, thanks
 

normcaldwell

Go Kart Newbie
Location
Denver, Colorado
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. :eek:

The downpipe change from stock to eurojet alone probably accounts for a good portion of the gains.
 
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