Is a 3 inch Downpipe big enough?

[antnicuk]

I was just wondering weather to change my down pipe from a 3 to a 3 1/2  with my new set up as i have more porting and a bigger exhaust housing.

I have seen people get very good power with a 3 inch but also a lot of people seem to be using 4 inch now

What do you think?

[Jono FD3]

Bigger will give you more power potential..... My theory is, go overkill then you wont have to upgrade the componant again!!!

Jono

[Prof]

DirectFreak\'s FB with a Cosmo 13B RE lump in it was running 460 RWHP with a 3" exhaust - see here  

[antnicuk]

the thing thats worrying me is with the large exhaust housing and BIG ports i dont want it to be too laggy, a more moderate sized exhuast can help it spool a little quicker at the expence of top end power.  Although Bramble\'s B/port had a 3 inch exhaust when it made 527 rwhp so i know i should still make good power but i will probably make more with a bigger set up.

The second half of my system is plenty big enough as its two 3 inch pipes from the middle back to two 3 inch straight through rear boxes.

[Jono FD3]

Ooooo that sounds interesting mate..... any pics of your system??

Jono

[antnicuk]

I\'m looking for 400 ATW\'s so i should be able to do it .........hopefully


its a turbo II,  i like the twin exit.   3 inch down pipe into a 3 inch straight through mid box into a Y splitting to 2 more 3 inch pipes to the back

[Fish]

Depends on the tubby your gonna run.

Fish

[antnicuk]

gt35/40, T4 flange with a .72 comp side and a 1.15 hot side

[kev-yorks]

The t4 flange will help spool up anyway Tony. The 3inch will cope with those sort of power levels but don\'t forget a smaller pipe also keeps the heat in and around the engine where a larger one helps get it away.
Kevin

[MikeLMR]

if it is enough for at least 400hp (as has been shown above) why worry, going bigger will just make it harder to fit in and cost more (slightly) I\'m not entirely sure on turbo\'s but the turbine works as a complete system with the exhaust and is not isolated from it totally ... so it is possible that there is an optimum size and going too big will loose out.  You probably know more about it than me (no turbo expert, just thinking aloud) but my £0.02 worth

[Ricco_04]

I would say 3" is fine.  As the exhaust gas expands on it\'s journey down the exhaust so you have already catered for this with your 2 3" pipes :bandit

I used to have a nice looking but restrictive cat back exhaust on the FD.  It made 300FWBHP (would have been around 320 but we never fine tuned it) and its internal bore was just 1.7 inches!!!!!!! :eek :eek :eek

[Carl]

You cannot lose out by going bigger.The only way you can cause a problem is by going too small,or having a poor flowing exhaust,flow is crucial,so nice smooth Mandrel bends are key.
A bigger downpipe does not affect spool up,in fact it may just improve it slightly.
Since the exhaust is after the turbine,even running NO exhaust can only offer improvement.
Put the biggest you can in as long as it is not too much like hard work.
Regards
Carl

[Carl]

Think of drag cars with virtually no exhaust at all on the turbo\'s!
Regards
Carl

[MikeLMR]

Just because an exhaust is after the turbine does not mean it does not affect it, the turbine and exhaust work as a coupled system

[kev-yorks]

In a N/A Mike then yes you\'re right as it relys on the exhaust pulses to scavenge the next batch of exhaust gasses to improve power levels but with a turbo motor the golden rule is back pressure is bad so anything at all that you can do to reduce it is the way to go. Hence why my new set up is 5inch downpipe and twin 3inch main pipes with not a baffle in sight.
Kevin