24 replies to this topic

[Smutt]

When would the use of a 4" exhaust system be appropriate for a HP increase..
Sourcing a 4" dump would be the first step..
Such as: above 450 engine HP? or higher?

[GSRWRC]

When you're running a T51R sized turbo and are after more than 700hp. A 4" exhaust would be very hard to keep quiet too.

[Liberoz]

When you win the lottery and decide a 2.3 Cosworth stroker motor with Methanol just isn't enough HP and its choking the 3" exhaust

[milkandoj]

Waiting for baz to see this....

[Smutt]

It still be an interesting experiment to do and document (costs aside) as "the only" change to a engine set up currently running around 500 engine HP for example..
Is anyone using a 4" system at the moment?

[Talonfiend]

Hmm, why not use a shorter 3" exhaust or a 3.5" exhaust? 4" is extreme you planing on having a 800-900hp daily driver?

[mannynz]

I believe that any gain you will get if any at all from a 3" system to a 4" system will be cancelled out by the weight gain of going bigger.

[evopwr]

no point at all IMO

[DOUGMO]

i have seen a few cars now that make over 300kw @ all 4 with a 3" benefit from a 4" with a 10% increase in power from just the exhaust alone (retune to suit obviously)

it is worth it

[Trippy]

i went from 3"with no cat to 3.5"with a cat with a 3" dump and made an extra 30hp on the same boost to shut it up i run a 3.5"varex muff.just finished my new lobster back 3.5" dump.its a full s/steel system with all joins v clamped and tigged.not cheap but worth it.

[Smutt]

"trippy" - did u manufacture the 3.5" dump yourself? what turbo does it suit?

[Trippy]

yeah mate.its a custom billet turbo from mr turbo similar to gt35/40.my old dump u can see on onpsi members rides.its a 3" lobster back. i make just over 440hpatw and run 11.04 at 128.3 mph over the 1/4 10's next time.

[Smutt]

Cool, i checked that out already

Very tempting to upgrade mine to a 3.5" from dump back.......

Edited by Smutt, 07 June 2010 - 12:14 PM.
removed comment

[bazeng]

4" is beneficial for anything over 300kw in my opinion.

I've seen a bunch of Ford Falcons XR6 turbo's see a huge gain with an increase from 3" / 3.5" to 4"....

I've done some testing and have gone from a 3" straight dump to a 3" to the muffler and I lost 50kw's at the wheels. I quickly bypassed the CAT and it gained 20kw's back. Still down 30kw's.....

It is only worth it once you start to make over 300kw's at the wheels imo. Simple rule of thumb.

If you do want a good 3.5 or 4 inch exhaust made, make sure you use a GOOD fabricator!!

The dude who did mine (BJP) made if perfectly and without any clearance issues!!

[Liberoz]

Are we talking removing cat's on a street registered car (highly illegal)

If you see the fine's for running a decat pipe or get done with a punched out cat you'll wish you you just went for a larger i'd sized cat and copped the minor % kw loss.

[milkandoj]

Whats the good cats to get these days, baz you found some big ones didn't you?

How can they catch you with a cat that "blew out" anyway?
Isn't it true that they can not remove parts of the car in an inspection at the pits? or can they do it?
An insurance claim might be another matter, if they decide to check to stick you up on a claim.

[Liberoz]

decat pipe is a impoundable offence

[evopwr]

decat pipe is a impoundable offence

meh so is owning a vehicle with a BOV

[bazeng]

Whats the good cats to get these days, baz you found some big ones didn't you?

How can they catch you with a cat that "blew out" anyway?
Isn't it true that they can not remove parts of the car in an inspection at the pits? or can they do it?
An insurance claim might be another matter, if they decide to check to stick you up on a claim.

I'm running a 4" cat on my car.

IMO, just use the largest CAT you can get on any exhaust size.
They really do restrict flow.

The CAT I tested was a METAL CAT also (the genuine hi flowing one from QLD).

[D3bb4]

they can pick a punched cat by doing an emissions test.

there are plenty of race cats on the market, even in 4" - not cheap, but not as much as a fine.