Valve spring pressure and boost.

[434nova]

I have been thinking about this for a while. Why is the seat pressures so low on a hydraulic roller cam that will see 30 #s of boost ? I am talking about 130 to 140 #s on the seat. Wouldn't the springs get worn fast with this low seat pressure ? How important is open pressure on a spring ? So what I am really asking is do you need alot of seat pressure when boost is applied to keep the valve closed ? Thanks

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[earlbrown]

Boost only hits the intake valve (and those springs theoretically run cooler). When it comes to springs, you need enough to keep it sealed and not bounce around on the seat and act goofy. Your open poundage is to keep the tappet from launching off the cam nose General Lee style.


You bring up a good point on the boost. The backside of the intake valve is exposed to manifold pressure, for every sq inch of valve area, you're spring will "lose" seat pressure directly proportional to your boost level.

 

[turbobitt]

The intake valve is pressure balanced in the fact that cylinder pressure rarely drops below exhaust manifold pressure and will behave much like a N/A application. Exhaust valve has its own challenges.
AG

 

[bison]

The intake valve is pressure balanced in the fact that cylinder pressure rarely drops below exhaust manifold pressure and will behave much like a N/A application. Exhaust valve has its own challenges.
AG

^^^this. You can't argue with the physics. There's actually back flow near the intake closing as the piston passes tdc till the piston is down far enough to equalize the pressure. Typically ex closing at .050" is 10btdc to tdc.


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