| 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...  |