2.5" opening to a 3" downpipe a waste?

[inspector-111]

Greetings!

Hmmmmm. . . . . .3" down pipe, with a 2.5" inlet. . . . . Is this even going to work as a 3" down pipe, or is it a waste of space, bottle necked and work as a 2.5" down pipe?

example - http://cgi.ebay.ca/Downpipe-Down-Pi...ryZ33633QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

I understand that the O2 housing will be part of the equation, but would still like to know how this would work.

Thanks for your time!

Chris.

 

[kromwell]

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. From there they are a full 3" back to the Apex 3" flange with 14 gauge aluminized steel piping. We had some Apexi flanges laser cut out of thick 1/2" steel for a good seal and fit.

Here is the perfect explanation quoted from Road Race Engineering's website.

http://www.roadraceengineering.com/eclexhaust.htm

 

[JDMHVN]

Absolutely it will work. Anytime you increase the air-flow of your engine your going to make more power. Plain and simple. I would look at getting an o2 housing though, or at least porting your stock one down the road. Also, one of the biggest power adders I did to my car in terms of basic bolt ons was my DP. Keep in mind that your engine is just a big air pump. Get it in, and get it out!

 

[inspector-111]

O.K., thanks for the info!


Will the 2.5" -> 3" also help to reduce the possibility of boost creep? (as apposed to the full 3")

 

[JDMHVN]

Its plausible. I'm pretty sure the main issue with boost creep is with the o2 housing. I know there are a hundred-bazillion threads about boost creep so just use the search feature for more positive information on that subject.

 

[chrish1440]

Its plausible. I'm pretty sure the main issue with boost creep is with the o2 housing. I know there are a hundred-bazillion threads about boost creep so just use the search feature for more positive information on that subject.

It helped with my boost creep issue with my 20G

 

[DSMunknown]

You can’t easily bolt up a 3” pipe to DSM 02 housing, so most GOOD pipes neck down to the 2.5” at the O2 housing. Believe me, you do not want a true 3” downpipe!!!

Frankly, 2.5” is fine for most people. Unless you want to run some 10’s

^ Consider this quote.

 

[want_a_gsx]

I bought an ATR o2 housing eliminating full 3" downpipe. Still need other exhaust parts, so it's not bolted up yet...

Why don't you want a true 3" downpipe?

 

[inspector-111]

I bought an ATR o2 housing eliminating full 3" downpipe. Still need other exhaust parts, so it's not bolted up yet...

Why don't you want a true 3" downpipe?

I might like one, just trying to deside if its worth going that route (full 3" or 2.5"->3"). Or if its even necessary for my goals. I'm trying to keep things as simple as possible (for now anyway) and not get into changing or messing with the fuel system. (except for a new 190lph fuel pump, and of course a rewiring)

 

[oldman]

If you're using a stock or 2.5" tubular O2 housing, there is little to no difference between a 2.5" - 3" DP and a true 3" DP. As a matter of fact, you'll have hell of time trying to bolt up your O2 housing to the 3" flange.

 

[GRNDSM]

I bought an ATR o2 housing eliminating full 3" downpipe. Still need other exhaust parts, so it's not bolted up yet...

Why don't you want a true 3" downpipe?

What I said was referring to bolting the DP up to the O2 housing. Since stock O2 housing has a flange of around 2.25", there are fitment issue between it and a TRUE 3" downpipe (which would have a 3” flange).

And think about it, the O2 housing is ~2.25 at its most narrow point. What do you think you will loose if you go with a 2.5” section for 2 more inches before you transition to a 3” pipe?

What you bought (O2 housing eliminator downpipe) is a totatally different animal and doesn't even belong in the same context. I have run 3" HRC eliminator DP years ago and they had their own "issues".

BTW, I just had an opportunity to dissect APEXI N1 1G exhaust (which is a fairly popular DSM exhaust system) and have some interesting observations:

3" is actually 2.75"
Resonators neck down to 2" (and there are two of them)

In my case, I was using it for my GVR4 daily driver, so didn’t really care. But I wouldn’t want to use it on a low 11/10 sec car

 

[inspector-111]

If you're using a stock or 2.5" tubular O2 housing, there is little to no difference between a 2.5" - 3" DP and a true 3" DP. As a matter of fact, you'll have hell of time trying to bolt up your O2 housing to the 3" flange.

I have been condsidering the FPM one piece DP as an option, or the more affordable choice would be some e-bay parts; 2.5" O2 housing, and a 2.5"->3"DP.

Just trying to make the right choice.