2.5" downpipe to 3" exhaust=WHY?

[sweet97]

Why do some guys run 2.5" downpipes and then use 3" catbacks. I have a turbo back 2.5" from buschur and am wondering what advantage I might gain going to a 3" catback. It does not seem worth the expense or effort. Please explain! Mark

 

[pboglio]

Why do some guys run 2.5" downpipes and then use 3" catbacks. I have a turbo back 2.5" from buschur and am wondering what advantage I might gain going to a 3" catback. It does not seem worth the expense or effort. Please explain! Mark

Mark,

I did a before and after datalog. Please note I'm using DSMLINK for h.p. calculation so the values may be high but its the differential that should be noted. With my Greddy 2.36" catback I got a repeatable 345-348 w.h.p. @ 5600 rpm (DSMLINK). Swapped to the 3" Thermal and I did 2 datalogs on different days and I always got a peak of 363 w.h.p. @ 5850 rpm. No fuel changes, no ECU resets, no boost changes. Just swapped exhausts and datalogged. So I picked up roughly 15-18 w.h.p. and shifted my power curve up 200 rpm. This is still on the 2.5" downpipe/testpipe. Its worthwhile but its not a huge difference. Every little bit helps.

 

[sweet97]

Gene that is good, 15HP is significant to me. It seems that I should try the RNR 3" turbo back. Again I see guys running 2.5" downpipes and then go to 3", is this supposed to stop creep?
The RNR shipped in a 3" SS turbo back with mffler is $658. shipped. I cannot get ahold of extreme motorsports. Their email link does not work with my webtv system. I am concerned their cat back might not line up with my Buschur 2.5" test pipe. Gene your system looks great, all pipes polished and te resonator is what I want. What exactly dd you use for the resonator, can you give me a Borla part# and supplier? Mark

 

[Steve93Talon]

There are two schools of logic on this:

First, you should theoretically swap the DP to 3" first because as the gasses get further from the engine they cool and therefore contract, meaning that bigger pipe closer to the engine would be more important.

Second, the cat-back is longer than the DP, so you can calculate the amount of pressure drop based on length, number of bends.... and say that replacing the cat-back would be more important.

I say stick with 2.5" until you really need 3", then do a full 3". I know someone who's gone 11.7 @ 121.5 on a press-bent 2.5", and the his best to date on 3" is 123.6mph. If you're under 400whp, your money is probably better spent elsewhere for now.

 

[pboglio]

IMHO, the point of a 3" exhaust is at somepoint, the 2.5" exhaust creates enough backpressure in the motor to make tuning difficult. The 3" exhaust pushes that envelope a little further. You can push a little more boost, a little more timing, and lean the car a little more. The extra power from reduced pumping losses is a bonus.

 

[daren_p]

Correct the exhaust gasses get cooler as they get further away from the engine but this doesn't mean a larger pipe is required closer to the engine, its the opposite. When the exhaust gases are the hottest they have the greatest velocity that is why some people have a dp that starts at 2.5" & then expands to 3" after the flex and continues 3" all the way back. Having the dp start at 2.5" also allows a little extra room for oil filter clearance.

 

[1fastalon]

With a 2.5 inch downpipe and a 3 inch cat-back i still have boost creep, and thats with my MBC turned all the way down to stock boost. It runs at about 14 PSI then starts to creep up till I hit fuel cut. I need iether an FDC or a ported or larger wategate. Then again with 80,000 miles on the clock it may be time to start looking into a new turbo with an external WG anyway.

 

[goofynick6]

Basically you have a pressure system, just like the weather, just like an inflated baloon. What it wants is for the high pressure to equalize, or therefore become lower. Exhaust manifold pressures can reach 30+psi before the turbine, so it just wants to rush out to normal atmospheric (i.e. popping a baloon and the air rushes out).

So, that being said, when the hot exhaust gases exit the turbing housing's 2.5" hole, it is trying to equalize and become less pressure and expand from its compressed state. So, when you have a 2.5" pipe for it to expand into, then it has pi X 1.25" squared area to expand in (about 4.9 sq. inches of space) So, when you have a 3" pipe to expand into that gives the exhaust gases about 7.065 sq inches of area in which it can equalize itself.

So, the 2.5" o2 housing isn't a big deal because it is basically an extension of the turbine housing, but the 3" dp and exhaust are awesome for allowing the gases to cool and expand and escape from the turbo. Not to say a 3" isn't a good thing, I'll certainly always run a 3" dp on any turbo setup because of the gains (I know people run 10's on a 2.5", but they could be faster with a 3"). 2.5" exhaust is usually adequate for 250hp, 3" is good for 400 roughly...etc. You can always make more power on smaller pipes, etc, but that doesn't mean it isn't strained.

Anyhoo...enough of my babbling, just giving a little info..

Nick

 

[turbo90gsx]

My input... With my 3" turboback, you can hear the turbo despool all the way out the back. IMO thats worth the extra. J/K but it is pretty cool bounus. I went from stock to 3" right off the bat cause I don't want to end up buying a whole nother exhaust when I want to upgrade to a ginormous turbo. If you have the funds go 3"

 

[sweet97]

Yeah I just bought a Buschur 2.5" SS turbo back last July because at the time I was running an EVO III 16G but have a 50 trim now. If I can sell the Buschur for say $400. I will upgrade to the 3", otherwise it will have to wait. The Buschr` is a perfect fit with all pipes flanged, very nice. I hate to take a chance on something that did not fit that well and I am speaking of the RNR system. Mark