- Joined
- Nov 26, 2009
- Messages
- 3,081
We've heard all about valve loft and bounce. What's interesting is that at extreme RPM the lifter doesn't even touch the cam when the valve spring loses control and allows the lifter to loft over the cam nose and should not wear the nose of the cam or mark the mid point of the lifter (this needs verification because it disagrees with other statements that have been made on this forum).
See image below.
This actually increases HP by increasing valve lift as observed in early AJS race bikes and recently in NASCAR racing. Read interesting details in link below:
https://www.cycleworld.com/valve-float-or-valve-loft-kevin-cameron-top-dead-center
Then there is valve bounce - another result when the valve spring loses control. See image:
Valve bounce happens when the valve slams back down on the seat at extreme RPM. This hurts your performance by causing loss of compression when the intake valve bounces and allows exhaust gasses to contaminate and dilute the intake charge when the exhaust valve closes late. Its especially dangerous when it gets so bad that the valves tangle to the point that they hang up - the piston hits them and you're looking at a ruined top end.
A brief link to these images and explanation:
https://help.summitracing.com/app/answers/detail/a_id/4765/~/valve-float
When I raced I didn't use a tac - I just revved it as high as it would go because there was a faster Ducati or something in front of me. When the valves floated you could hear the motor start to miss and sound fuzzy. When missing a gear and the revvs hit the moon, you could hear the valve noise over the 100mph wind. On tear down you could see where the valves rubbed each other and bounced off the pistons. Those times I was lucky it wasn't worse. Finally - I bought a rev limiter.
See image below.
This actually increases HP by increasing valve lift as observed in early AJS race bikes and recently in NASCAR racing. Read interesting details in link below:
https://www.cycleworld.com/valve-float-or-valve-loft-kevin-cameron-top-dead-center
Then there is valve bounce - another result when the valve spring loses control. See image:
Valve bounce happens when the valve slams back down on the seat at extreme RPM. This hurts your performance by causing loss of compression when the intake valve bounces and allows exhaust gasses to contaminate and dilute the intake charge when the exhaust valve closes late. Its especially dangerous when it gets so bad that the valves tangle to the point that they hang up - the piston hits them and you're looking at a ruined top end.
A brief link to these images and explanation:
https://help.summitracing.com/app/answers/detail/a_id/4765/~/valve-float
When I raced I didn't use a tac - I just revved it as high as it would go because there was a faster Ducati or something in front of me. When the valves floated you could hear the motor start to miss and sound fuzzy. When missing a gear and the revvs hit the moon, you could hear the valve noise over the 100mph wind. On tear down you could see where the valves rubbed each other and bounced off the pistons. Those times I was lucky it wasn't worse. Finally - I bought a rev limiter.
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