Exhaust Size again!, Dowpipe to Straight pipe

[JamesPT]

Just Thinking out loud on this one:

Seeking low end Torque and not High RPM Horses, what Diameter pipe to use?

Info:
The ID of my downpipe is 46-48mm ish., Small Hybrid KKK04 Turbo.
The system will be De-Cat, De-Boxed , straight pipe to rear exit. (Most likely)
The stock pipe is 55mm OD, to the Cat, than drops down after the Backbox to 50mm OD

A 2.0"  (51mm) pipe is a exact match for the ID of the downpipe, and therefore would not interrupt flow velocity (Torque).
A 2.25" (57mm) pipe seems the a good balanced size (matching stock, but all the way through to exit of course).
A 2.5"  (63mm) Best for power, and probably a nice throaty sound. But ID is about 10-12mm greater than Downpipe exit...No issues? 
A 3"     (76mm) OverKill for my setup really.
Also worried about Droning sound, If that happens I'll drop in a Flowmaster DBx muffler.

Now that I've written it out, I'll probably just cut before of the Cat, and take it from there with a 57mm to the rear exit.

Your Thoughts?

[Nath140889]

3 inch down pipe to 2.5 inch straight through sounds like the best exhaust to me , would flow well u want as little down pipe restriction as possible , will help your turbo out

[Ruan]

Frankly makes no difference after the turbo.

2.5 all the way will be more than adequate for your purposes

[simonh]

The elbow/ downpipe section is really the only important size in the exhaust. After that it's really just the standard baffle type silencers that cause restriction rather than the actual size of the pipework. (at stage 1 levels anyway) Anything bigger than the downpipe is a bit pointless unless you're planning on upgrading the front section at a later date.

Assuming you're not upgrading your downpipe then 2" would be ideal but 2.25" shouldn't lose any power. 2" will probably sound slightly better as the gasses will be exiting faster but there's unlikely to be a noticeable performance difference between the two. 2" is also easier to work with. :-)

[Frosty]

Can't go wrong with 2.5" IMO

[Chairchild]

2 1/4" is good for 200Bhp*

Edited, as I'm a tit

[Ruan]

Where on earth you get that figure from!!!

[simonh]

I've had a Fiat Coupe run 250bhp on a 2" system with a standard cat and have seen a V6 Astra run 200bhp at the flywheel with a 2" system. It doesn't seem too critical. A civic type r (ep3) runs almost 200bhp standard and only has a 1 3.4" system from the factory.

[Ruan]

It's absolutely critical on these motors, I can tell you that from experience!

Remember these engines consume a significant quantity more air, you're running petrol motors at an AFR of about 11:1, these engines you're having to run 18:1 or leaner to keep smoke down, then consider that they're heavily turbocharged... It makes a massive difference to how they respond to exhaust sizing! An XUD9 runs a 2.25" downpipe from the factory and the 16v HDi runs a 2.5" downpipe... These engines need a large amount of airflow and are affected negatively by small exhausts.

2.25 all the way back will support 200hp OK, but I would put a fiver on some horses being released from an exhaust upgrade! Remember with these engines that when you start restricting airflow, smoke levels go up and instantly exhaust gas temperatures rise... It's detrimental to the engines longevity to be running with restrictive exhausts, all sorts of problems start happening.

Put it this way, on my XUD9, there was a full 2.25" system from the downpipe back, if you took it off after the 2.5" downpipe, you'd gain an extra 5-10hp or so, simply from the restriction of the 2.25" pipe back... That's only making somewhere in the region of 175hp!

[toseland]

Where on earth you get that figure from!!!

Piggy haha

[mr_fish]

[Ruan]

[Chairchild]

Where on earth you get that figure from!!!

oop, obviously wasn't paying attention to what I was typing

Yes, was meant to say 2 1/4" (or as you've also put - 2.25")



my bad :p