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

[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...

[loony888]

even though the swept volume will stay the same, tuning the engine can, and does need more fuel/air to make more power. after all an engine is really only a pump and the more air you can get in the more power you can make, note here i said air, not fuel, getting fuel in is the easy part, and only there to ignite the air, getting more air in is the challenge and without air, burning fuel is a waste of time. changing lift and duration of cams, going to larger valves and different port shapes and sizes is purely to help maximise flow so more air can make it's way inside. turbo'ed or supercharged engines are more efficient at this than naturally aspirated engines but none will achieve 100% capacity, most naturally aspirated engines can achieve at best 70-75% capacity, and mass production and machine line tolerances mean even less for std engines. obviously, once you have done all this work the thing will run lean so more fuel is required to achieve an optimum air/fuel ratio and thus optimum horsepower (this is after all the point of the exercise) so there must be more fuel and air being consumed by each cylinder to make more power. the waste from this has to go somewhere so a matching exhaust with the ability to allow the waste gases to travel freely away (let's not get into scavenging here)is needed. add to this high revs allowing less and less time to fill/compress/burn/empty each cylinder and it all gets too much for the std exhaust to deal with. larger bores are one way to do this, with the added benefit of being quieter, but clever design, like the "spaghetti" systems work very well too.

more discussion........

[phil_h]

Exhaust tuning is very simple.
Hot gas at combustion pressure will pass through any exhaust port into any exhaust pipe really easily, but the faster the better to reduce heat transfer into the engine.
When it is in the pipe, getting it out the other end is not a real problem by itself, cos its hot and expanding, so any size pipe will do.
Its the sound pressure waves that have to be managed in the pipe - and you simply have to choose what frequency you are going to tune for ie what engine revs you want to optimise.
You then work out the exhaust volume you need in order to get the returning pressure wave keeping the fresh change in the cylinder at that rpm.
You then simply lay out the plumbing.

So, if you want high rpm power, you need a high volume exhaust, and if you want low rpm power, you need a low volume exhaust.
Engine tuners have been doing this for almost 100 years.
Go get a copy of 'Tuning for speed' by Phil Irving.