3" dump pipe into 2.5 system is it OK?

[chrisGT4]

Hello Toy Modders,
I have a 2.5 inch with high flow cat system and recently acquired a 3" dump pipe. Is it OK to use this dump pipe into the 2.5 inch system?
Any information is much appreciated

Chris
(ST185)

[Draven]

yes.

generally speaking, the most gains are to be made closest to your engine, so the 3" dump into a high flow cat then down to 2.5" will be just about perfect

[LeeRoy]

How long is your dump?

Keep in mind that as the exhaust reaches the bottom of the car it has cooled down a fair bit and is therefore denser meaning it takes up less space. As such the step down should be fine. Just keep it smooth.

- LeeRoy

[Andrew162]

I used a similar setup on my RAV with the 205 half-cut in it, except that went from a 4" dump into a 3" system. Still damn good.

[chrisGT4]

Thanks for the info DRAVEN , LEEROY.
Now I have the green light to go with the installation. I very much appreciate your time to answer my question,.
HAVE A GREAT DAY.

[chrisGT4]

Thanks Andrew. WOW 4" dump to 3" coooool!!!!!

[TurboRA28]

I was reading Corky Bell the other day about exhaust sizes.. It was amazing how much HP a 2.5" system can flow wihtout hampering performance.. Something like 400 - 500hp was fine with a 2.5" exhaust.

3" was like 800hp or something crazy.

Discuss?

[Draven]

I'm curious about that, 'cause think it was peewee mentioned gains to be had going from 3" - 3.5", and I'm sure that wasn't at more than 700bhp

[TurboRA28]

I'll try and remember to dig out maximum boost tonight and find the chapter again..

[JustCallMeOrlando]

Hmm, i'm at a dilemma with this. I have a 3" dump, followed by a 3" high-flow cat, followed by a 2.5" rest of the exhaust, topped with a 3" muffler. The 3" muffler just sorta came up, so that's why it's there.

Now, my car after tuning, with highflowed turbos @ 17-18psi, a China Racing 450mm cooler, my car made 167.5rwkw in Shootout mode. A little while earlier, with the factory ECU it made 164rwkw @ 16-17psi with a BHG on another dyno in Shootout mode. Given both were Shootout accredited workshops, i'd expect them to be fairly similar. It's quite eery the figures being so close, and given the $ spent to have a new ECU, install it and get it tuned, quite frustrating. As such, i'm looking for problems, the 2.5" section is a candidate.

Now, given my inlet temp sensor is right in my inlet manifold, and my inlet temps are quite satisfactory considering, I'm ruling out my IC and IC piping as being the problem.

I know of another person with my turbos and a programmable ECU and they made 195rwkw at 15-16psi. The only difference (aside from tuning) is that they have a full 3" system.

Therein lies my dilemma. My AFR's are pretty damn good, fairly consistently just under 12:1.

Without being able to scrounge up the coin for getting it retuned, I can't check whether it's the timing. This is a suspect, because i've actually lost mid-range over the factory ECU and it seems to be a little laggier than before.

It could be the 2.5" exhaust section, but as you say, the exhaust gas cools down pretty quickly, and as Corky says (thanks Joel) a 2.5" pipe can flow a shiteload of air.

I know this is a little of a sidetrack, but given I have this exact same setup in question, and my power seems to have plateaued, i'd like to push it to find out if it's a problem!

[YLD-16L]

I run a 3" dump into a 2.5" system and that is currently good for about 170rwkw. I know that the 2.5" is not my limiting factor for peak power at this point, my tiny little turbo is

JCMO run on the strip/gtech/dyno with your current system then remove the 2.5" section and run again. Should give you some indication of whether the 2.5" section is causing any restriction and inhibiting power.

[JustCallMeOrlando]

Hmm fair enough. I'll have a think about it next time I'm at the drags. The next dyno day coming up is a group event, so they wont wanna put it on twice. It's going to be enough of a struggle convincing him to play with my TVIS switching point between the two runs

[82MKII]

Nathan - maybe you can unbolt the bit after the cat before you stick it on the dyno next Saturday - providing it is flanged. Then you'd know if that section is part of the problem.

Regards

Peter

[JustCallMeOrlando]

It's flanged, but it still takes a while to remove it from all the hangers and get it out of the way. I'll do it at the drags one day instead when we're not so pushed for time.

[brett_celicacoupe]

It's flanged, but it still takes a while to remove it from all the hangers and get it out of the way. I'll do it at the drags one day instead when we're not so pushed for time.

remove your muffler and cat... or at least gut it.

i was running a holden 3.8L cat converter for a long time and removed it about 2 months ago. the cat was about 2.5" and probly not the best in the flow department.

removed it and suddenly the 1g spools HEAPS earlier, has a better sound but isnt excessively loud.

it really isnt loud until full boost (wastegates open?). although most of the loudness comes from the turbos

at the moment i got NO muffler and a gutted cat converter. you should at least try the same