Valve bounce and valve float, explain

[TheNeedForZ]

What is valve bounce and valve float? Are they different subjects?

 

Say an engine has light valve springs and is revving too much, do the rocker leave the lobe surface of the cam thus leading to the valve not closing at the proper time? Is this called valve float?

 

Say if a valve closes and bounces off the seat and sorta palpitates near the valve seat, is that called valve bounce?

 

What causes valve float/bounce and what are the general remedies?

 

In a desmodromic valve system where a valve is slammed shut by a rocker arm at high rpm, are the valves prone to bounce since it is slammed shut?

[tfreer85]

Valve Float according to Automotive Engines by Tom Gilles states that "valve float is when the valve stays open at high speed instead of following the cam lobe. The resulting clearance between the valve and seat most commonly results in a burned valve. Valve float can also result in bent or broken valves, or cause pistons to crack they strike the open valve." Also on a pushrod design "lifter pump-up occurs when the engine is revved to the point where the valvetrain components seperate do the inertia, not allowing the valve to close."

 

Valve Bounce is "when weak valve springs can keep heat transfer down, which can result in valve face damage. Too little tension also results in accelerated seat wear becuase the valve bounces on the seat." So when the valve hits the seat the spring does not have enough pressure to stop it and seal it against the valve seat. This can also happen during installation in which you bottom out the spring to install valve keepers, no longer allowing the spring to expand to its proper height and causing improper clearance.

 

To check for both of these "check spring tension in both the valve-open and the valve closed positions with a tension tester. Any spring that feels loose when installed on a head should be discarded." Also any spring that appears bent or dis-colored should be replaced. If you are inspecting a head and you notice improper valve-to-seat clearance this is most likely a result of either problem or even a bent valve or valve stem. You can also check by using a spring micrometer to ensure that the spring height is to spec with the manufacturers.

 

As for a Desmondronic valve system, I've never seen one used on anything outside of a street bike recently. Engine Masters Magazine had a write up about it, but just remember its a better system for higher RPM's than the pushrod design. It also did well in the early 1930's in high speed racing, I'll try and find it. Hope this helps,

 

Tyson