i current have a 3" tbe, i bought a new catback system because it has a resonator in the mid pipe (and more stock looking design) to reduce exhaust sound. it turns out that the new system is only 2.75". how much power will i loose if i go from the 3" catback to the 2.75" catback and keep the 3" downpipe and 3" hfc. my goal is 350whp on stock turbo and no alky, i don't mind if i loose 5 whp but have a more quite exhaust. i'll just make up the hp some where else.

if you do end up loosing a lot of hp, i'll just keep the 3" catback for track days.

 

no input from anyone?

 

I believe there were a few threads posted within the last week or so about this very topic. I don't recall any consensus being reached about how many whp that quarter inch is worth, just that the 3" is obviously better.

Assuming you are doing the install yourself, why not? You want a quieter car for the street, and you say you're planning on keeping the 3" for track use... sounds good to me.

P.S. If you get the chance, it would be really cool for you do to back to back dyno runs with the 2.75" vs. the 3" unit.

 

Well it doesn't make sense to have a 3 inch dp and tp and a 2.75 inch catback. There will be lots of turbulence caused by the smaller 2.75inch flange on your catback. It would be better if you had a 2.75 inch dp, tp and a 3 inch catback or 2.75 inch or 3 inch full exhaust. It's just better if your exhausts diameter gets bigger and bigger, or stays the same rather than getting smaller towards the muffler.

 

well, my theory on that is that exhaust gas loose velocity as it cools down (taking up less volume) when traveling down the exhaust. so while the pipes get smaller towards the muffler it helps keep the exhaust velocity high, as long as there's a smooth transition from larger to smaller diameter. i'm not a fluid dynamic engineer by any means.

 

Reducing the diameter of the pipe won't help anything, period. As to if it hurts anything, that depends entirely on the anticipated power output. If you're goal is 350whp as measured by a Dynojet, that shouldn't prevent you from getting there.

 

This theory is applicable to a well designed extractor for a NA engine... after the turbo, there is no such effect... here, read this:

https://www.evolutionm.net/forums/sh...5&postcount=57


Any sudden step in exhaust diameter is bad, actually. Either way it can hurt flow. Ideally, with a stock turbo Evo, you should have a 70mm (2.75") O2 housing with a mandrel bent 2.75" turbo back exhaust, or a 76mm (3.0") O2 housing with a mandrel bent 3.0" turbo back exhaust. Of course, the 3" O2 housings available on the market are rather expensive...

Now, taking the above into account as the theoretical "ideal", the following questions arise:

- Will a full 2.75" system outperform a 3" TBE with the stock O2 housing? (Maybe?)

- Will it outperform a 3" TBE with a 2.75" O2 housing? (I doubt it...but I could be wrong)

I have not seen any comparative tests of these setups, so I can only guess.

 

i read that post a couple times to make sure i understood everything, it was very informative.

i know going to a 2.75" catback from a 3" would definitely hurt the performance, but to what extent is the question. i guess i'll just have to install the 2.75" catback and see if the butt dyno or the gtech meter can tell any difference. if the butt dyno can't tell any difference, that means the hp loose was minimal.