2.5 inch DP to 3 inch catback... pros vs cons

[Sephroth57]

In my local DSM group, its pretty much agreed that a 2.5 inch DP is all you really need unless youre runnin a huge turbo plus youre saving yourself some boost creep by not getting a 3 inch. I have living proof that a 2.5 inch DP to a 3 inch catback works fine cause my friend runs 11.4 with that setup...

so anyway, i have the same thing and im having some boost problems, unrelated to the DP install, and i called buschur to see if they would have some advice. i was telling him (Jarred) my setup and hes like oh its probably your different size piping for the exhaust.... im like ummm whaat?? first off i told him i had the same problem before i put my downpipe on, wouldnt upgrading from the restrictive stock to a 2.5inch make it better if that was the problem.. which it isnt. he said he cant really tell without seeing my car which i understand, but i have no idea why he would say going from 2.5 to 3 is bad, its like a cone opening up as it goes longer, makes no sense to me..

anyway heres my other thread regarding the boost problem - http://www.dsmtuners.com/forums/showthread.php?t=126481


but i was just wondering what you guys thought about him saying 2.5 to 3 is bad, i see no problem with it and my friend runs 11s with that setup so i dunno...

 

[VRMAN]

I think it is that the smoother the air travel both in and out of the engine the more HP is generated and smoother the car will operate under load.

So 2.5" to 3" is sort of like pissing into a funnel backwards. Bigger is always better in regards to turbo'd cars exhaust. The old addage of the best exhaust is no exhaust rings true.

 

[paranoidTSi]

I had this setup, the stock downpipe, 3" Hiflow Cat, 3" Cat-back...then I added the 3" downpipe, it made quite a noticeable difference...the RPMs climb much quicker..same thing your situation. You are essentially only getting 2.5" of flow.

Now imagine, take a 2.5" pipe and a 3" pipe together..you put a hose that is capable of more than. 2.5....whats going to happen? its simply going to fill that pipe up and drip down to the 3" pipe with PLENTY of space to hold more water.

Put in a 3" DP, trust me..you'll be more happy. Either that, or sell your 3" and get a 2.5" since it would be a waste to have on there.

 

[Sephroth57]

well although everyone usually buys 3 inch exhausts, 2.5 is good to 400HP and i dont think im gonna go over that really. so lets just say my 2.5 to 3 inch is really only giving me the effects of a 2.5 all the way, how could that be somehow bad? i just found his statement retarted and i soon after hung up because i didnt want advice from someone who couldnt comprehend how an exhaust works

 

[mitsuclipsegsx]

what place sells a 2.5 inch exhaust for a 2g? casue i am getting an exhaust and dont know what the hell to get 2.5 or 3 ichDP really confused

 

[Eclipsed]

what place sells a 2.5 inch exhaust for a 2g? casue i am getting an exhaust and dont know what the hell to get 2.5 or 3 ichDP really confused

Click here:

http://www.dsmtuners.com/parts/default.php?cPath=3_55

Mistsu, it all depends on what your plans are for the car. Like Sep mentioned, 2.5 is good to 400. Personally I went with a 3 inch dp just because I was going 50 trim and I didnt want creep, also helping the creep is the 02 housing dump.

Sep, go for the 2.5 is you dont plan on going all out with your car. I'm not to sure on prices but arent 2.5 considerably cheaper? The 3 inch would probably be best, but if you only plan on going 16g, which is pushing it, or smaller get the 2.5.

I dont really see any gains from going with a 2.5 to 3 inch though. Like mentioned the max flow velocity and flow amount is limited by the smallest part on your exhaust, ie the 2.5 dp. So by adding a 3 inch cat back you are in a sense taking off the exhaust after the 2.5. If you can understand that, I really dont know how else to explain it. Just that the flow rate will just improve from the 3 inch on.

I really cant see more gains in terms of horsepower coming from a 2.5 to 3 inch instead of full 3 inch, but there are other reasons to go 2.5. To name a few would be that they are cheaper, you wont be as laggy at lower rpms, and it would be a tad quieter.

As for your problems in the other thread, the exhaust isn't causing it. Seems like a new oxygen sensor would help, but I think the main thing is that your coolant sensor isn't working properly so it is causing the ecu to go into closed loop. Fix those two things and get back to me.

 

[miteclgst]

I am running the same setup, 2.5" dp that switches to 3" under the oil pan. So if my setup is like "pissing into a funnel backwards" then anyone who has a ported stock or Evo 3 02 housing is simply wasting their time with an exhaust bigger than 2.5 right? I mean the largest outlet that you can port on either is around 2.5 so they too are pissing into a funnel backwards. The flow of heated gasses through an exhaust in no way has anything in common with water. When gasses heat and cool their dynamics change, this is unaccounted for in your analogy b/w the gas and water. And to make a comparison when you swapped from your stock dp (diameter 1 3/4") to a 3" is unfair. That would be like me saying that my car was noticably faster when I got my Evo 3 16 G then when I had my T-25, this means that the 14b is no good. Come on guys.

 

[VRMAN]

I don't think anyone was saying a 2.5" exhaust is bad persay just that the 3" is better.

Using your own analogy: If both the 14B and 16G were the same price brand new, why wouldn't you go with the 16G?

 

[miteclgst]

I can see where you are coming from but it just seems that everyone on this site automatically assumes that you need to go with the biggest, baddest setup no matter what your long term goals are. Observe any ACT 2100 vs 2600 thread an you will see what I mean. People only suggest the bigger clutch because it is bigger, in that case if you are looking to make 300hp max why overkill your drivetrain with a 2600 when a 2100 would work perfectly fine. I wouldn't want to deal with a stiffer clutch pedal and a greater risk of crankwalk if it was totally unnecessary. I personally went with a 2.5" dp because I read somewhere that the super hot exhaust gasses flow better through the smaller diameter until they start cooling, then they need the 3". Maybe this was misinformation but I followed it and have no complaints. My major gripe would be the way that people back up their posts. Give some cold hard evidance why one setup is better than another, speculation about how gas might flow and what each person thinks is best is a useless case to base a theory on. People will always defend whatever it is they do to their cars because they think it's right, and that is fine and dandy more power to you, but don't pass it off as law simply because it gave you good results. VRMAN i'm not attacking you in anyway, I'm sort of venting but it's in no way directed towards you, I'm sorry if I gave you that impression. Everyone needs to remember that with everything comes a trade-off, a larger exhaust means more top end flow but less low end back pressure as stated before. If you want to sacrifice the low end for the high go with the 3. otherwise go with the 2.5. /rant

 

[herovueboi]

i'm running 2.5 punishment downpipe and 3" Test pipe with RNR 3" exhaust. Spools faster but i think 3" turboback just feels better. My friend with a 1g tsi fwd feels like faster spool up, but maybe it's just me.

 

[mitsuclipsegsx]

i coundt find a place that sells 2.5 exhaust systems??? Is their anyway i can make a greddy evo exhaust fit my GSX

 

[tigerstalon]

i coundt find a place that sells 2.5 exhaust systems???

Extreme Motorsports does. I have one on my car.
http://www.extrememotorsports.com/g2cat/exhaust.htm