Exhausts

[av8rirl]

While reading in the Formula 1 forum about "Renault to abandon wide-angle engine" I started wondering about exhaust lengths (and numbers) on other racing cars.

For a four stroke, four cylinder engine with a V of 180 degrees (ie flat), what would the optimum exhaust length be? Should it be short (like current F1 exhausts)? Or should it be long? Now I'm sure that there are formula's for working this out, but what is the formula? does it depend on cc, compression ratio, exhaust diameter, exhaust bends, etc???

Also would it be better to have 4-2-1, 4-2, 4-1 configuration? What are the pro's and con's for each? Should the exhaust have its exit facing downwards? horizontal? or upwards (like the current F1 trend)??

I'm sure there will be much debate as to what is best and what is personal opinion but ALL answers are much appreciated.