Deano1565

iTrader: (2)

Right fellas,
I am no Subaru expert or mechanical expert for that matter but was just trying to get my head around a few things after a heated discusion at work. I was wondering how a larger than standard exhaust from the Turbo back could have a negative effect on the cars performance, mainly Torque? I was under the impression that the larger the exhaust from the Turbo the less restriction to flow. I was ulso under the impression that Turbo spool time would be affected the larger Dia the headers?

So am I right in thinking that you want the smallest Dia headers as possible to increase flow speed to make spool up quicker but not small as to cause a back presure to the cylinders. Then after the Turbo as little restriction as possible to prevent back presure back to the turbo.

Any sort of not to geeky or complicated answer would be appreciated .

alcazar

iTrader: (2)

I'm no expert, but would imagine it has to do with gas speeds?

If the exhaust is TOO large, the gases just expand into it, slowing, causing the flow to be less?

Deano1565

iTrader: (2)

Cheers mate, I suppose that makes sense!! but would it have any real effect on the torque?

alcazar

iTrader: (2)

Pass. You need to wait for some of the gen men to come on, David at APi, splitpin, Harvey etc.

CrispyK

iTrader: (3)

It was described to me like water if you have a volume of water in a hose coming out a large pipe it would dribble out,but if it was through a smaller pipe the water will fill the pipe and become pressurised abd will force its way out spraying out with force,

Same with exhausts if it to big all pressure would be lost unless the air volume is increased to increase back pressure so it forces its way out... Right hope that makes sense lol

Deano1565

iTrader: (2)

I take it your talking about the headers mate? I understand that part... (I think) Its the bit after the Turbo that is slightly confusing!!

Why would you want back pressure after the turbo? surely that would just slow the spool up?

CrispyK

iTrader: (3)

Sorry it not really back pressure but pressure in the exhaust maintains gas velocity By creating positive pressure

CrispyK

iTrader: (3)

Increased gas velocity will create a vacume maintaining the turbo spool hope this helps

Deano1565

iTrader: (2)

Cheers bud.

APIDavid

iTrader: (4)

This is a complicated issue. Exhausts need to be tuned to cars and their engines, just as much as you would tune induction. On a road car there is too much that cannot be done due to space, emissions, noise etc., etc. to get it perfect as a race car would be.

You need an amount of back-pressure in the exhaust [ IE not too big ] to help the scavenge of the exhaust gases from the cylinder heads. As a very general rule of thumb for a Subaru; 2.5 inch systems are OK and advisable up to later 300's of horsepower. 3 inch system will work but, you'd almost certainly get better power and torque figures on a smaller system.

Headers need to get the gas out of the heads and away as fast as possible, but then needs the gas to be forced into a restricted space to increase gas speed to help the turbo spool. If [ in the case of an Impreza ] the up pipe is too large, turbo spool will be compromised and a later starting spool will be evident.

But no two cars have the same characteristics and what works on your, apparently similar mates car, doesn't mean it will work on yours.

David APi