An exhaust question....
Discussion
To do with pulse wave forms and such. The faster the gass comes out of the engine and down the 'pipe' the more (if it is designed right) the exhaust can help 'pull' the burned gasses out of the cylindar and even help pull new gass in. To do with soooo many variables though.
Bit like you can fit a whopping great exhaust to a car and quite happlily lose power across the rev range, lose torque, but gain at the VERY top end (which you very rarely use) etc etc.
Bit like you can fit a whopping great exhaust to a car and quite happlily lose power across the rev range, lose torque, but gain at the VERY top end (which you very rarely use) etc etc.
chilli said:
So, why do they make so much difference? I thought that the bigger the exhaust (the more pipes) the easier and quicker it would be to get rid of the exhaust gasses, therefore allowing "things" to happen quicker "under the bonnet". I guess theres' more to it than that??!!
Cheers.
Cheers.
It is a bit more subtle. Exhaust gasses do not flow continuously, rather they're emitted in a series pulses on the opening of the exhaust valve. Whenever the pulse encounters a dramatic change in the diameter of the exhaust pipe, for instance the section where the pipes merge, or the exit to the exhaust, a further reflected pulse is generated going back up the exhaust system in the opposite direction. This pulse will in time bounce off the back of the exhaust valve.
Now, if you can time things such that the exhaust valve opens just after a previously created reflected pulse hits the valve (and is reflected once again), you a bit of get extra assistance. The reflected pulse and the pulse from the now opened exhaust valve combine, giving an extra draw on cylinder, drawing additional mixture into the chamber to be burnt on the next power stroke. With an exhaust system where the pipes from each cylinder merge, the reflected pulse from one cylinder will travel back to the exhaust valves of every cylinder, meaning that the exhaust pulse from one cylinder can assist the others in the engine.
The length, diameter and shape of each of the sections of the exhaust have bearing on how at what engine rpms the exhaust tunes in, how well it does so and so on, so yes it is complicated. This analysis is done using a computer based model and then engine dyno testing. The bigger pipe is better approach of most chav chariots is certainly well wide of the mark. Other factors are that changing the cam changes the valve opening and closing times in the 4 stroke cycle and hence the affects at what rpms the exhaust chimes in. There is also a similar pulse tuning effect to this occurring on the induction side of the engine with the inlet valve, which engine manufacturers also use.
Hope some of that made sense,
Steve.
Edited by Steve_T on Wednesday 13th December 14:32
chilli said:
Cheers Steve. If I'm honest not all of it has sunk in, but I get the jist!!
ps - Are you really a fisherman? I'm guessing by the cars & bikes owned, your pretty good at it??!!!!!!!!!!!
ps - Are you really a fisherman? I'm guessing by the cars & bikes owned, your pretty good at it??!!!!!!!!!!!
Really good stuff here, pages 231->263, but it might be heavy going.
www.proracingsim.com/downloads/ProgramDemos/Dynomation-UsersManual.pdf
Steve.
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