Motor Physics Question
Road Warrior
Joined: Jun 2008
Posts: 1,319
Likes: 1
From: Deep South Texas
The dome shape helps the flame propogation at high RPM, I doubt you will see much difference on a H-D without increasing the compression ratio.
Road Captain
Joined: Jul 2008
Posts: 656
Likes: 0
From: gone
Interesting that some say that the domed piston helps flame propagation. I was always under the impression that the dome "cut" the flame front because of the angles involved. This was the reason people would put dual plugs in the shovel heads. I understood that the only reason to use domed pistons was to raise compression ratio when the chamber size was too large to achieve the desired compression any other way. It’s cheaper to buy domed pistons then new heads. I have always understood that a smaller combustion chamber with a flat top or dished piston (dish matching the chamber) with a close to zero quench was the best for flame propagation and would generate the most power at the highest efficiency. That may be different in a Harley motor but I can't see how flame propagation would differ from an automotive engines. As far as heads go it’s all about the valve placement, size, and port flow. Just because a head flows stupid numbers doesn’t make it good for your motor. You have to average the cam lift and lobe open time to get a realistic number this combined with the lobe separation angles will help you to figure volumetric efficiency and velocity.
Last edited by troy.j; Sep 19, 2009 at 11:37 PM.
Road Warrior
Joined: Nov 2008
Posts: 1,296
Likes: 3
From: Cheltenham, UK
Perhaps my earlier post wasn't entirely clear.
All I have understood in the past is that geometry had an impact on flame propogation and expulsion of exhaust gases, but I have no idea which shapes have what effect. Would be interested to see a definitive response as I have seen flat, dish and domed and I am certain this has been done for good reason on each case.
Worth adding at this stage is that good flame propogation is not necessarily fast propogation or slow propogation but rather optimised for each engine
All I have understood in the past is that geometry had an impact on flame propogation and expulsion of exhaust gases, but I have no idea which shapes have what effect. Would be interested to see a definitive response as I have seen flat, dish and domed and I am certain this has been done for good reason on each case.
Worth adding at this stage is that good flame propogation is not necessarily fast propogation or slow propogation but rather optimised for each engine
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