2.5" vs. 3" downpipe

[pboglio]

Anybody have any experience running a 2.5" downpipe/3" catback setup and then switched to a 3" downpipe. My turbo is flowing 35 lb/min on 19 psi, dropping to 17 psi at redline on pump gas. Airflow is flatlining in part because of the boost dropoff, so a little boost creep would actually help me out. Any comments on SOTP gains, boost behaviour, etc. is welcome.

 

[LA97GST]

Update your mods please. It is my general understanding that going 3 inch DP will result in bad boost behavior for any smaller sized turbo. Unless you are getting into 375-400 hp I would stay with 2.5 to 3.0. I keep an open mind about these things so, so please inform me if this is not correct.

 

[TSIfreek]

You are most likely getting knock and having your timing pulled causing the boost drop. Check your logger and see whats going on. 3" dP really isnt needed unlesss you are running 30 psi. Its all in the tune.

 

[blcknspo0ln]

agreed. my 2.5" --> 3" exhaust didn't feel any different when I had a tubular 2.5" o2 and 3" DP..

so, yes, I went backwards.. and it didn't do anything different but make things a TAD bit quieter

 

[definitiveno]

I don't really know why people mess with 2.5 inch, I imagine a bucnh of people will try to tell me but..........buy once is what I say.



If it's too noisy put a silencer in your muffler.

 

[blcknspo0ln]

If it's too noisy put a silencer in your muffler.

and defeat the purpose of having a 3" exhaust?

 

[definitiveno]

and defeat the purpose of having a 3" exhaust?

Yeah well, so does having a 2.5 inch exhaust.



One's alot cheaper to reverse though.

 

[pboglio]

No knock on my last recorded datalog back in July, 18* up to 7000 rpm or so.

I've already logged 35 lb/min, 291 w.h.p. (via DSMLINK V1) on a mild 93 octane tune in 65*F temps. With the HKS VPC installed & 2g MAF removed, on a cold 22*F day I laid done a repeatable 310-312 w.h.p. with a few runs at 320-325 w.h.p. Still on 20 psi and 93 octane.

I expect on a full race tune running 23 psi & 100 octane in winter temps I'd be well beyond 325 w.h.p. on my current T28. My goal is 350 w.h.p., which is the reason I wanna get rid of some backpressure.

Thanks for the replies, I think I'll try the 3" downpipe once I get my tranny back in.

Short list of mods are: FP T28/28, 23"x10"x3" FMIC, 2.5" intake pipes, HKS 264/264 cams, 2.5" downpipe, 2.5" testpipe, 3" Thermal catback, ported 2g O2 housing, ported turbine housing & manifold, DSMLINK, HKS VPC (2g MAF removed) HKS EVC @ 19-20 psi, 660cc Denso inj, 150 LPH fuel pump rewired, 1g throttlebody, K&N FIPK, 4" cold air intake, 2.25" turbo intake pipe, etc. etc.

 

[TSIfreek]

Check for boost leaks and also check to make sure the WG door on the turbo is fully seating against the housing.

 

[pboglio]

Check for boost leaks and also check to make sure the WG door on the turbo is fully seating against the housing.

Boost leak test good to +30 psi with zero leakage. WG is clamped tighter than a drum using FP's 17 psi adjustable wastegate, 19 psi by 3100-3200 rpm in 3rd gear. I've got a suspicion the wastegate is blowing open above 6000 rpm, thats why I was considering the 3" downpipe. Cheers.

 

[TSIfreek]

Let us know if the DP solves your problem. Intersting

 

[pneumo]

I'm not sure if a bigger DP would help. Less backpressure in the exhaust would...(EDIT: ack! what was I thinking!?!?)

Since you have an adjustable WG actuator, why not try tightening it first? I was suprised how tight I had to adjust mine to get it to hold 25psi on a 25mm flapper. It has eight full turns worth of preload.

 

[definitiveno]

Less backpressure in the exhaust would suck the wastegate open even more.

WTF

 

[blcknspo0ln]

Yeah well, so does having a 2.5 inch exhaust.

I don't get it.

besides, do you REALLY think with the setup he has, a 3" exhaust is going to make any difference?

 

[blcknspo0ln]

. I've got a suspicion the wastegate is blowing open above 6000 rpm, thats why I was considering the 3" downpipe. Cheers.

maybe it's too late for me to be posting, but it's escaping me about how the downpipe has ANYTHING to do with your wastegate opening..

 

[definitiveno]

I don't get it.

besides, do you REALLY think with the setup he has, a 3" exhaust is going to make any difference?

I'm not a pro by any means so it doesn't really matter what I think. For shits and giggles though I think 3 inch will help even a lowly t25. I could be wrong in thinking I suppose, cooling gases crowding the path and all....

Unless a t25's been dyno tested with turbo back 2.5 and TB 3 inch to prove a 2.5 was better I subscribe to the bigger is better theory. I think the community has accepted that no exhaust is best and 3 inch runs a close second.

Why not 3.5 or 4 inch? Parts hardly available and clearance problems for street car....

I think there IS a point of deminishing return but with the prices being close to identical and the performance potential measurably gapped. I think it's a good idea to buy once and forget about exhaust for as long as you own the car.

IMHO

 

[pboglio]

I'm not sure if a bigger DP would help. Less backpressure in the exhaust would suck the wastegate open even more.

Since you have an adjustable WG actuator, why not try tightening it first? I was suprised how tight I had to adjust mine to get it to hold 25psi on a 25mm flapper. It has eight full turns worth of preload.

Pneumo,

Tried it, still drops off. I can run 25 psi to 6000 rpm, then down to 21 psi at 7000 rpm. If I set it to 20 psi, I'm down to 17 psi by redline. Obviously I have enough spring preload, but my boost controller can't deal with the way the exhaust backpressure "shoves" on the valve at higher rpm's, see what I mean? I'm looking at that E-boost which can dial in boost via rpms and specifically has a function for high rpm boost fade, I like to experiment

Don't forget manifold pressure is MULTIPLIED by post turbine exhaust pressure. Every 1 psi drop in exhaust backpressure is worth well over 2 psi or more in the manifold under high boost, right where the wastegate senses it. So say you have 10 psi in the exhaust and 20 psi in the manifold, then there is a 10 psi delta acting on the wastegate. Now drop to 8 psi in the exhaust (i.e. bigger pipes, etc.) then your manifold pressure goes to 16 psi, for a delta of 8 psi. That delta decrease is clamping the wastegate tighter and tighter.

For the rest of the geniuses, your right, 3" downpipe is too much for my setup

 

[pneumo]

Don't forget manifold pressure is MULTIPLIED by post turbine exhaust pressure. Every 1 psi drop in exhaust backpressure is worth well over 2 psi or more in the manifold under high boost, right where the wastegate senses it. So say you have 10 psi in the exhaust and 20 psi in the manifold, then there is a 10 psi delta acting on the wastegate. Now drop to 8 psi in the exhaust (i.e. bigger pipes, etc.) then your manifold pressure goes to 16 psi, for a delta of 8 psi. That delta decrease is clamping the wastegate tighter and tighter.

Ah! Good info! I didn't know that, thanks.