Lets make it simple. Assume a mildly modded engine capable of revving up to 7000rpm. Assume that a 4-1 extractor is used and a single muffler is used at the tail pipe. What would the reccommendations of piping sizes be for these engines?..( in inches ) ....thanks mates....

1.) A12 -
2.) A14 -
3.) A15 -
4.) L16 -
5.) L18 -

Yes that'll be ok, as the collector has already brought the size down to this, and you wouldnt gain much by going bigger.

We made our own collectors from 80mm pipe, and thus used a 80mm "tailpiece"

If i stick to the size that comes from the extracters its about 63mm will that be ok,am i correct in saying that a bigger pype behind the extracter will not have an effect?

A well designed set of extractors, have already done their job when all it's runners have joined into one, that's what they are there for. Anything after that would make absolutely no difference in performance other than to restrict flow. The main reason people have any exhausts/ silencers on their racing cars these days is mostly because of noise.

For our hotrod, we used to take a piece of pipe about 80mm ID and about 2m long, enough to stick out the side of the car just behind the front wheel. It was polished up to a shine in the lathe, with some very fine sand paper and some brasso. We then either spot welded it to the collector or tied it with some wire, but tried to make it air tight.

The car was started, and the throttle was gunned up to high revs in short bursts, until a blue ring formed around the polished pipe somewhere in it's length. The pipe was then cut diagonally across this ring at a shallow angle, as this was where a high pressure zone was formed. When cut across this zone, you ruled out most if not all back pressure, but pressure was also high enough that cold air wouldn't find it's way up the exhaust and onto your exhaust valves.

Many guys still runs their rods this way, if it's not to loud, and I would suggest you try this if the car is only going to be used on the track.

EDIT: This procedure was repeated when any internal changes was made to the engine or extractors, as the pressure zone would shift when changes was made.

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?

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).

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.

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:
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....

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.