Ducati Sporting Club UK > Model Specific - Discussion, Technical Problems and Help > 851 / 888 > Exhaust theory.

[jgriff]

I found this at http://www.nsxprime.com/FAQ/Miscella...austtheory.htm
I think it helps in the great 45mm/50mm quandary.

Pipe Sizing

We've seen quiet a few "experienced" racers tell people that a bigger exhaust is a better exhaust. Hahaha… NOT.

As discussed earlier, exhaust gas is hot. And we'd like to keep it hot throughout the exhaust system. Why? The answer is simple. Cold air is dense air, and dense air is heavy air. We don't want our engine to be pushing a heavy mass of exhaust gas out of the tailpipe. An extremely large exhaust pipe will cause a slow exhaust flow, which will in turn give the gas plenty of time to cool off en route. Overlarge piping will also allow our exhaust pulses to achieve a higher level of entropy, which will take all of our header tuning and throw it out the window, as pulses will not have the same tendency to line up as they would in a smaller pipe. Coating the entire exhaust system with an insulative material, such as header wrap or a ceramic thermal barrier coating reduces this effect somewhat, but unless you have lots of cash burning a hole in your pocket, is probably not worth the expense on a street driven car.

Unfortunately, we know of no accurate way to calculate optimal exhaust pipe diameter. This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game. For engines making 250 to 350 horsepower, the generally accepted pipe diameter is 3 to 3 ½ inches. Over that amount, you'd be best off going to 4 inches. If you have an engine making over 400 to 500 horsepower, you'd better be happy capping off the fun with a 4 inch exhaust. Ah, the drawbacks of horsepower. The best alternative here would probably be to just run open
exhaust!

[loony888]

and this relates to what exactly? when my 888 makes 500hp and i have to figure out how to route a 4" pipe through my swingarm i might find something useful here, till then.........nah!
besides, there's no real "quandry" if you have the cash to make big horsepower, chances are you have the cash for a 50mm system. if you're doing the open airbox, chip and k&n bit a 45mm system is enough.

[jgriff]

Originally Posted by loony888

and this relates to what exactly? when my 888 makes 500hp and i have to figure out how to route a 4" pipe through my swingarm i might find something useful here, till then.........nah!
besides, there's no real "quandry" if you have the cash to make big horsepower, chances are you have the cash for a 50mm system. if you're doing the open airbox, chip and k&n bit a 45mm system is enough.

This is mainly due to the random nature of an exhaust system -- things like bends or kinks in the piping, temperature fluctuations, differences in muffler design, and the lot, make selecting a pipe diameter little more than a guessing game.

[851neil]

bigger bore pipe generally means less noise also - think I read somewhere that bigger bore sbk racing zorsts were brought in mainly to meet more stringent noise regs

[jeff st4s]

Seems logical, running at higher rpm needs to get rid of more hot gassses so bigger exhause system used, so if your not running the engine in that power band then a standard exhaust could be better for you.

[rockhopper]

Or you could look at like this.. No matter what power the engine is making it is still producing the same volume of exhaust gas for each cycle. The only way that will change is if you change the capacity of the cylinder. Therefore the diameter of the exhaust pipe must have some other effect on the engines performance.
What that is though i have no idea!