Downpipe done !

[ryan_GTI]

i thought a 3" dump-pipe has already seen decent gains from the dyno.

Just wondering whats the advantage of goin 2.5" ?

[philthy]

Hey guys. What size is the stock downpipe? I thought the exhaust was 2.5" all the way through? (admittedly haven't looked that closely myself)

[mitch_gti]

I just ran outside and looked at the standard down pipe and flex pipe. Looks to be just under 2inch. Very odd shape aswell. Instead of being circular, alot of the pipe is square

If you were upgrading the whole exhaust 2.5 would probably be the best way, smallish motor, small turbo means you should probably keep a certain amount of back pressure there.

But if you just upgrading part of the exhaust (downpipe/flexpipe) go as large as you can ie. 3inch.


Remainder of standard exhaust is about 2 or 2 1/4

[Bubu]

"Back pressure" On a turbo car??? WTF

Turbocharged cars do not need exhaust back pressure. The air is compressed in to the engine at (insert boost amount here). This pressured air is then heated through the combustion process and comes out the exhaust manifold at even more psi then it went in. Air expands when heated and contracts (if those are the right words) when cooled.

Arguably the dump pipe on a turbocharged car is the most important because the exhaust air/fumes drop in temperature by the time it travels through the pipe to the atmosphere. Therefore the exhaust system experiences higher pressure at the dump pipe as it is the hottest part of the exhaust.

I would rather have a 3' dump pipe and 2.5' exhaust, than having a 2.5' dump and 3' exhaust.

[99gti]

"Back pressure" On a turbo car??? WTF

Turbocharged cars do not need exhaust back pressure. The air is compressed in to the engine at (insert boost amount here). This pressured air is then heated through the combustion process and comes out the exhaust manifold at even more psi then it went in. Air expands when heated and contracts (if those are the right words) when cooled.

Arguably the dump pipe on a turbocharged car is the most important because the exhaust air/fumes drop in temperature by the time it travels through the pipe to the atmosphere. Therefore the exhaust system experiences higher pressure at the dump pipe as it is the hottest part of the exhaust.

I would rather have a 3' dump pipe and 2.5' exhaust, than having a 2.5' dump and 3' exhaust.

Couldn't agree with you more. The turbo provides more than sufficient back pressure to stop the car from stalling. You could (illegaly) run a dump pipe straight to atmosphere and your car would love it (neighbourse or EPA wouldn't).

[shaneth]

this is all well and good but from what i heard guy telling me, although the car will run fine it can change where you get your gains i could be wrong but i think the 3" gave more noticable gains on the upper end of the rev range where the 2.5" was more mid rev range. I also pretty sure he tried both 3" and 2.5" inch and the total power gain was minimal if at all. Hopefully he will come on and add his 2 cents.

[mitch_gti]

I said A CERTAIN amount of back pressure.



If this is not correct then why didnt APR make the GTI polo Exhaust 3'inch or 3.5'

Most likely because 3-3.5 would make no significant gains over using 2.5 (when doing the whole exhaust)

As shanneth said, '' '3" gave more noticable gains on the upper end of the rev range where the 2.5" was more mid rev range. ''

Our cars have a tiny inzy wintzy turbo, if you upgrade the whole exhaust surely 2.5 would be better, to keep exhaust gas velocity up, creating faster spool up and more low/mid range.