Running a 2.5 inch down pipe and 3inch catback is this bad?

[TheRealist]

Is this bad? can it couase back pressure major boost leaks? i have a 2.5inch downpipe already installed but looking for a 3inch exhuast system

 

[elfroggo]

Our DPs have a 2 1/2" stainless steel first bend with a 2 1/2" stainless steel flex section. They then expand to 3" right after the flex section right under the oil pan area. Why start out at 2 1/2"? If you are running on the stock O2 sensor housing, even ported it is 2 1/2" inside diameter. When the exhaust gasses are hot, they flow better, 2 1/2" is plenty for the first 12". As the gasses cool , bigger helps. By going to 3" after the flex section, we allow this expansion and also allow for better clearance at the front where room is tight.

http://roadraceengineering.com/eclexhaust.htm

 

[TheRealist]

you talking about that site dp or the stock gst DP?? STOCK DP IS 2.5???

 

[elfroggo]

No, I don't think the stock DP is 2.5"

But it is commonly believed that since the exhaust gases are still fairly hot exiting the O2 housing that it is fine for the diameter to be 2.5" as opposed to 3". You will not see large gains by expanding the piping diameter at that point.

Actually now that I read your question, you are asking about a 2.5" downpipe all the way through. I was thinking of a downpipe that was 2.5" before the flex and 3" afterwards.

You should see noticeable gains if you switched to a larger diameter (3") downpipe. Either choice of 2.5" before the flex section or 3" throughout will be fine.

Also, a downpipe properly installed will not cause a boost leak. Actually that is not possible as if it were leaking it would be called an exhaust leak since it is after the turbo. What you might see is boost creep from the increased exhaust flow.

 

[t0ny007]

no this is not bad at all. you could get a little more power with a 3" dp but the setup you have is not bad. with the 2.5" dp could even save you from getting boost creep.

 

[Dream On]

It's fine. I'd leave it alone.

 

[1997turbodsm]

Since i have this setup on my car let me tell you a little of what you want to know. With my Stock Exhaust running an EVO III 16G i had no boost Creep. Now i've got a Megan Racing 2.5" to 3" DP and a Greddy 3" Catback. I'm Creeping like it aint nothing. I've got my boost controller turned off and i'm running 12PSI until i hit around 4500rpm. then my boost goes to 13....14.....15.....16.....17....18......19 around 7000RPM. I've got a stock Fuel Components so i'm kinda limited to what i do right now.

 

[donmagicjuan]

Since i have this setup on my car let me tell you a little of what you want to know. With my Stock Exhaust running an EVO III 16G i had no boost Creep. Now i've got a Megan Racing 2.5" to 3" DP and a Greddy 3" Catback. I'm Creeping like it aint nothing. I've got my boost controller turned off and i'm running 12PSI until i hit around 4500rpm. then my boost goes to 13....14.....15.....16.....17....18......19 around 7000RPM. I've got a stock Fuel Components so i'm kinda limited to what i do right now.

Boost creep gets worse as you lower your boost controller setting, meaning the difference in peak value and your controller's setting will be larger. EvoIII's really like to run above ~16 psi if you're running an internal gate. Upgrade your fuel system so you can turn up the boost and you'll most likely see the problem disappear.

 

[NewTurboTuner]

I wouldn't worry about it. Your 2.5" down pipe is fine for now. If you switched to 3" right now, you would hardly notice a difference if any.

For engines making less than 300hp. A smaller down pipe & exhaust could be beneficial, because it is good to keep exhaust gas velocity high.

When you hit over 300hp, then switch to a full 3" and you'll gain another 10-12hp.