Working out piston crown volume.
Posted: Sat Mar 27, 2021 8:42 am
Hi all.
My KMX is in bits again now and I've got 2heads sent away for proper squish band modifications and to get a sensible compression ratio.
However I can't measure the actual total combustion chamber volume with the head on and the engine in the bike, due to a lack of room above the engine and the fact that it's inclined forwards.
I've got a bit more hopefully accurate data now, and I'm trying to work out how to measure one thing to get a cc volume.
The piston is 55.94mm diameter.
The crown height is 3.2mm
The piston edge sits 0.3mm below the cylinder deck height.
What I'd like to work out is the amount of cc that that is leaving in the cylinder to add to my combustion chamber volume.
A simple calculation for a 56mm piston in a 0.3mm stroke cylinder would give about 0.73cc.
The piston crown is way above the cylinder deck height at TDC but the edge of the piston is 0.3mm below deck height.
This is annoying as I really want to know what the space left in the cylinder is as I'm calculating my combustion chamber volume instead of measuring it assembled.
My thoughts are that it's likely to be no more than 0.3-0.4cc left in the cylinder, but it could be alot less due to most of the piston being above the cylinder at TDC.
Does anyone know a formula I could use to try and nail down this volume a bit more accurately.
Finally when I was doing my earlier compression ratio calculations they were way out as I was using the wrong effective stroke length due to getting my exhaust port height wrong.
I'd actually been starting and running the bike with a compression ratio of 9.3:1 from the exhaust port closing, so no wonder it was so hard to start for a 125 (134cc).
I still don't know what compression I need or can get away with for super unleaded fuel, but I've left this up to the experts who are going to drastically reduce it compared to where it is now.
If It runs great after all this I'll be very happy indeed. However if I lose power at low-mid revs compared with standard then I'll be looking to increase the compression again and find a higher octane fuel than pump unleaded. I don't want a bike that screams to the moon but between 4-7000rpm would get left behind by a standard KMX.
My KMX is in bits again now and I've got 2heads sent away for proper squish band modifications and to get a sensible compression ratio.
However I can't measure the actual total combustion chamber volume with the head on and the engine in the bike, due to a lack of room above the engine and the fact that it's inclined forwards.
I've got a bit more hopefully accurate data now, and I'm trying to work out how to measure one thing to get a cc volume.
The piston is 55.94mm diameter.
The crown height is 3.2mm
The piston edge sits 0.3mm below the cylinder deck height.
What I'd like to work out is the amount of cc that that is leaving in the cylinder to add to my combustion chamber volume.
A simple calculation for a 56mm piston in a 0.3mm stroke cylinder would give about 0.73cc.
The piston crown is way above the cylinder deck height at TDC but the edge of the piston is 0.3mm below deck height.
This is annoying as I really want to know what the space left in the cylinder is as I'm calculating my combustion chamber volume instead of measuring it assembled.
My thoughts are that it's likely to be no more than 0.3-0.4cc left in the cylinder, but it could be alot less due to most of the piston being above the cylinder at TDC.
Does anyone know a formula I could use to try and nail down this volume a bit more accurately.
Finally when I was doing my earlier compression ratio calculations they were way out as I was using the wrong effective stroke length due to getting my exhaust port height wrong.
I'd actually been starting and running the bike with a compression ratio of 9.3:1 from the exhaust port closing, so no wonder it was so hard to start for a 125 (134cc).
I still don't know what compression I need or can get away with for super unleaded fuel, but I've left this up to the experts who are going to drastically reduce it compared to where it is now.
If It runs great after all this I'll be very happy indeed. However if I lose power at low-mid revs compared with standard then I'll be looking to increase the compression again and find a higher octane fuel than pump unleaded. I don't want a bike that screams to the moon but between 4-7000rpm would get left behind by a standard KMX.