helicopterjim
Hero member
- Location
- near Lone Butte, BC, Canada
Engine poppet valves are a simple device, but for some reason, they are widely misunderstood.
There are several dynamic things that can go wrong with valve trains. Valve bounce and valve float are the most commonly mentioned.
Valve bounce is exactly what it sounds like. The valve bounces off its seat momentarily after it closes. The cause can be a closing ramp that's too aggressive, or too little seat pressure from the spring.
Valve float is when the cam follower fails to stay in contact with the cam lobe. This is where the misunderstanding comes in. Most people think it happens as the valve is closing, and the spring can't close the valve quickly enough to keep up with the closing cam lobe.
To illustrate that this is a widely held view, when I googled valve float, the first article that came up stated "Valve float occurs when the valve springs are incapable of holding the valve train against the camshaft lobe after peak lift"
Wikipedia also gets it wrong with "Valve float is an adverse condition which can occur at high engine speeds when the poppet valves in an internal combustion engine valvetrain do not properly follow the closure phase of the cam lobe profile".
The notion that valve float only happens during the closure phase of the valve after peak lift is a fallacy. If it happens at all, it'll begin to happen before peak lift, while the valve is still opening.
The valve train isn't quite as simple as the cam lobe pushing the valve open to its maximum lift, and then the spring taking over to push it shut again.
It's difficult to find a technical article that explains valve float correctly. I know they're out there, so if I find one, I'll post a link.
Failing that, I could explain it myself if anyone is interested. I just didn't want to launch into a technical diatribe that nobody would read.
There's also such a thing as "valve loft", where the valve train components don't lose contact with the cam lobe, but flexing of the components allows the valve lift to profile to differ from the cam profile, sometimes opening the valve further than the max lift of the lobe. This applies to valve trains where flexible parts like rockers are employed.
I actually found an article that explains valve loft and bounce, although it glosses over float:
SpinTron Secrets: Valve Float, Bounce, and Loft Explained!
In this segment of Spintron Secrets, we dive into the concept of Valve float, the phenomena that cause it and a few valvetrain topics that can make or break a race engine. Rev on!blog.trendperform.com
BTW. Spring bind is more of a dimensional cock-up than a dynamic effect. If a spring is going to bind, it'll happen as soon as you try to turn the engine over and find that it's hard to turn over maximum valve lift (that's if it doesn't stop with a clunk just before max lift). It's not related to engine revs.
If you're getting spring bind happening with a correctly sized spring, then you're already well into valve float territory. In that case, the binding is not the real problem.
You do realize this is a thread about selling an SF project.
Just sayin' ......
Jim
(I miss those old emoticons where the little dude disappeared in a cloud of smoke)
