exhaust differences

[bood]

May be a ' how long is a piece of string question' but are there noticeable differences between different diameter exhausts?

I realise that a 2.5 inch sidey will flow more than a 2 inch scorpion but is it noticeable on the road or power wise?

I ask this because mine has a 2.5 sidey ( with big Jim down pipe a t25 and tubular manifold) i would like a rear exit exhaust which there are lots of 2 inch about.

The car runs 21psi but i am by no means a racer or track day goer so 5 or 10bhp loss would not bother me.

Just after your advice/opinion/experience

many thanks

[Brigsy]

Suck it and see? Before the days of sidey's everybody used to run scorpions and the like.

All i noticed is better response with a bigger bore exhaust. If you have a 2.5" downpipe it wont fit the smaller exhausts though, usually blows like feck

[Goobie]

Worse case go for a 2-1/4 dia or get the end chopped off and have custom pipe work and back box, job done! :agree:

[bood]

Cheers guys.

Don't you use a reducer if going to a smaller bore exhaust?

If it was just a bit of response loss i could live with that.

[Mart]

Measure the back-pressure of a 2" bore system, before writing off fitting one ;)

[bood]

Your hinting at what i was thinking. is there really much loss or gain or difference on a road car

[TNT ANDY]

Rule of thumb:-

1 inch per 100bhp, so if you think you're over 200bhp, stick to bigger, but as Mart says, measure back pressure first.

Have you ever done this Mart? if so, what did you find, and what is acceptable and what is not. I have an adaptor to insert into my lambda boss to measure that pressure, but never done it yet.

[Mart]

Your hinting at what i was thinking. is there really much loss or gain or difference on a road car

For the likely power you're running, providing the bore is 2" through the backbox as well, then you'll be fine.

Andy, yes I did, a good while back - Standard elbow, downpipe, old type 2" bore Scorpion system, but with the latter Scorpion backbox, yielded a ~5psi peak back pressure, iirc. It was low enough not to cause a concern anyway.

I've never measured a complete o.e system, but would imagine it to be a good few psi more. Maybe ~10 perhaps.

A 2.25"/2.5"/side-exit with a shorter gas path would probably have a better (less) PD, but whether it's noticeable or you'd see a gain, everyone has their own opinion on that.

Put it this way though, you don't see a Veyron (with cats in situ too) running a 10" bore system ;)

[jesus in the seat of a 5]

anyone know where you can buy the reducers though...:scratch:

[Mart]

Tweeks. Rally Design. Ebay. Local exhaust fabricator.

[bood]

Good information.

thank you

[TNT ANDY]

Here here:agree:

[ScottKinnear]

I've got a 2 1/4 rear exit exhaust and it fits my 2 1/2 inch down pipe with no problems.

Got mine through bb tuning along time ago and the down pipe from ktec. I'm sure someone on here knows who really made them.