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!

[Smacker]

mmm just a thought. Air intake is cold Dense and a greater mass through a 50mm air inlet. the gasses that are expeld are forced throught a 45 mm outlet. There´s got to be a airflow jam or are the burnt exhuast gasses a less amount after burning?

[rockhopper]

The intake charge will be compressed by the piston.

[Smacker]

But i thought compression of gasses creates heat and then theres the combustion, so whats left after that? the same amount or less

[rockhopper]

I'm thinking out loud now!

The cylinder volume is fixed. The intake charge consists of air and petrol. To get more power need more petrol but you also need to more air to maintain your 14.whatever to 1 ratio. I'm supposing that the cylinder isn't 100% full due to restrictions in the intake, maybe around the valves which get in the way hence some have waisted stems.

Also as revs increase the intake charge won't be able to move fast enough down the intake to get in before the intake valve shuts. If we make the intake larger then the velocity drops and there is a danger of the fuel dropping out of suspension, so i guess a large intake only works well at high revs. We could have the valve open wider and for longer but then we run into problems with the valve hitting the piston as it comes up. We could also run into problems with valve overlap.ie both valves being open at the same time which means that the intake charge can escape down the exhaust port. Some race bikes do this becasue they are not worried about emissions or fuel consumption (you can see the fuel burning in the end can). Another problem with valve overlap is that the valves can actually touch each other hence you get engine designs like the testasretta which has a very narrow angle between the valves. This means however that the spark plug cannot be in the optimum place.

So we need to get more fuel/air mixture into the cylinders. We need to therefore improve the airflow through the intake without making it too big. We need to get waisted valves and an inlet tract with no sharp edges or sharp turns. A highly polished surface is no good as a rough surface actually helps the fuel/air mixture to stay mixed. Another way to get more mixture in is to force it in with a compressor ( a turbo or supercharger).

So, once the cylinder is as full as we can get it then we need to burn the mixture and then get the burnt gasses out. The restrictions in the exhaust port will be the same as those in the inlet but are less critical becasue the the engine physically forces the gas out rather than relying on atmoshperic pressure to suck the new charge in.

I think that the difference in exhaust volume between a "normal" engine and a highly tuned road engine will be quite small becasue unless its turbo charged we are not getting significantly more charge into the cylinder as modern engines must be getting close to filling the cylinder anyhow (becasue its free power).

The stock exhaust will be designed to make the engine acceptably quiet which may well mean that the tube isn't wide enough to get all the gas out in time which means that some will stay in the cylinder which will contaminate the intake charge so it will burn less efficiently. So we need a bigger pipe which will mean more noise.

The engine will have been tuned to account for the fact that the cylinder isn't full of clean fuel/air (it will be mixed with exhaust gasses which didn't have time to get out) so if we now put a bigger exhaust on we get a cleaner charge in the cylinder but there may well now be not enough fuel in it because the injection system will only inject the amount it was programmed to so we need to remap the ECU by changing the chip ( Unless the injection system is runnng a lambda sensor which most Ducatis don't).

Discuss...