I run a very modified A14 engin in Gx body that i only use for 1/4miles.I have got a set of 40webers and a flowed head ext.
Now my question is what size exaust must i fit cause the one thats is on at the moment is rotten.I was thinking about 57mm but will that be ok or do i need to go bigger?

Its hard to pick what size exhaust you will need. I saw a formula somewhere once, but it was only any good for calculating for standard engines. 2.5 inch sounds reasonable, in fact probaly the biggest you would want to go for a non turbo engine.

I have a 2 inch on my modified a12 and its great for it.

Oh yeah to clear it up a bit further, a standard exhaust at 2.5 inches but a mandrel bent one should be fine a 2 inches, but its all guess work really.

For a A15 that has done slight port & polish and milled head and a non-stock carb, is 1.6 inches okay?

57mm would be better known in many places as 2-1/4" & if this is a racer, then I expect that you will be wringing its neck for all its worth.
Under the circumstances, I suspect that 2-1/2" [63.5mm] would be an improvement.

The 57mm pipe has 2552 sq mm of cross sectional area, while the 63.5mm pipe has 3167 sg mm of cross sectional area. This represents an increase in diameter of only 1/4"/ 6.5mm or 11.4%, but an increase in cross sectional area of 615 sq mm or 24%.

A small increase in diameter goes a long way when dealing with pipe sizes.

This link should help with checking my maths.

Quote:
now i feel like a complete mathmatical twit!!

I blame it on the night shift..... its almost time for me to knock off... about 45 minutes left....

if it really is a race car for 1/4 mile
you will want 75mm and have the pipe exit under the passenger side front door
or you could have a two inch system with a couple of hot dogs and a chrome tip mmmmmmmmmmmm

Quote: For a basically stock A15 like yours I would suggest that 1-3/4" [45mm] would be a minimum with 2" [50mm] probably being a good size as it leaves scope for further improvements later on.

Keep in mind that in stock applications, the exhaust gas cools rapidly on its way down the pipe & contracts as it cools. This is why the outlet on many stock mufflers is smaller than the inlet. This is done in an attempt to maintain gas velocity with the lesser volume of gas down the back end. Since even exhaust gas has mass, the theory is that the swiftly moving mass has inertia that sort of 'pulls' the gas charge that is behind it along.

Does it work? Beats me, but many factory engineers thought it did.

The idea is to have it come out just before the left rear wheel,but I can decide what size to go for.
The car will just be used for fun not on the road at all,and will be reved around 8000rpm(shift light comes on at7500rpm).

Because of the high revs that are used on it, I would also say a 57mm will be good with some freeflow silencers.Maybe even a straight through pipe without silencers.
It is not going to be used on the street, right?