If you have ever built a performance engine or been around them long enough you have probably heard of the phenomenon known as valve float. If you are hiring a racing engine builder to complete a powerplant package for you they undoubtedly discussed the camshaft, lifters, and springs with you to understand your goals for the engine, and the parameters under which it will normally operate.

Lets explore a crash course in valve float, picture this — a high-RPM engine paired with an aggressive camshaft, in a blueprinted interference environment, operating at high temperatures. As the valves open to allow the introduction or evacuation of gases they rely on the camshaft for timing and the valve springs for return action. A typical valve spring is only intended to provide enough K-ratio (or spring rate) to shut the valve with enough speed and force to keep up with normal OEM-intended operation.

As we demand more and more RPM from our engines in the search for power, the return force of the valve springs must increase accordingly to prevent valve float. The actual float refers to a condition where the valve is not experiencing pressure from the spring and is allowed to hang, un-shrouded in the chamber or bounce off the seat when it should be firmly planted closed.

While this sounds sloppy in the best terms, it can be downright destructive in the worst. Not only is horsepower escaping in the form of cylinder pressure past the floating valve — but piston to valve interference becomes a real problem. The possibility of a destroyed top end, pistons, and cylinder walls is the real danger when tangling with floating valves.

Usually this problem is remedied before it ever occurs, by pairing an aggressive cam with an appropriate set of double valve springs. When venturing into the unknown with no benchmark for a “redline,” the cam and valves can easily outpace the springs — usually manifesting in a dramatic loss of power and an accompanying noise reminiscent of a Jake-brake.

Rarely do we actually get to see this problem occur, but thanks to high speed video we see the hula-dance the valve spring performs as it spins in it’s pocket and ultimately leaves the valve with immense lash. The movement of the valve spring looks like a Slinky as the coils undulate in a harmonic fashion — nearly out of control.

http://www.enginelabs.com/?p=168979

 

cool video

 

 

Good stuff Nemesis!

 

Wow!

 

I've been researching valvetrain stability in anticipation of building a higher revving motor (7500-7700) and came across this video. Definitely shows more going on than I anticipated. We all know that higher revs brings more maintenance, cost and risk...but something about a screaming LS turns me on big time.

 

Cool video. Good luck with your future build.

 

Pushrod flex also contributes, an in our motors, likely before we run into spring issues. I found in my relatively mild build that I got more valve train stability above 6K RPM when I went to the 3/8" double taper pushrods.