Optimum exhaust bore sizes

[obi_waynne]

Is there a formula or way to calculate the optimum bore size for an exhaust?

All I seem to turn up are best guesses and theories?

1.4 = 1 inch
2.0 = 1.5 inches
3.0 = 2 inches
4.0 = 3 inches or twin 1.5 inches

Any comments or suggestions would be welcome. Isn't power output a better guage than the engine size?

 

[stamford]

For us everyday folk we leave it to the experts. We presume they know what they are doing and do it right. How can we judge if similar ones are any different? I guess the likes of professional race teams have them developed using calcs and dyno them for results. I guess a combination of horsepower, emissions requirements, decibel levels dictate this in the end result. For road cars I guess there has to be compromise still.

 

[old-git]

Is there a formula or way to calculate the optimum bore size for an exhaust?

All I seem to turn up are best guesses and theories?

1.4 = 1 inch
2.0 = 1.5 inches
3.0 = 2 inches
4.0 = 3 inches or twin 1.5 inches

Any comments or suggestions would be welcome. Isn't power output a better guage than the engine size?

One problem with this list is the suggestion that 2 x1.5 inches is equal to 3 inches. A 3 inch pipe has a cross sectional area of just over 7 inches. 2 x 1.5 inch pipes are just over 3.5 inches in total, half that of a 3 inch pipe! Even a pair of 2 inch pipes only add up to under 6.3 square inches (if my maths is correct).

What I can say with confidence is that most modified systems are too big. The exhaust on my Elan is 3" and will be dealing with over 600bhp. This is only a little smaller than the one fitted to the Pikes Peak RS200 powered by over 950bhp (and 1150 with nitrous). My engine builder, Geoff Page, built both engines. See:
http://www.mach2racing.com/the_rs200e.htm

Big 'poser' pipes reduce gas speeds so reducing the scavaging effect