Ok, but that would only be signifigant if there was an extremely large pressure drop over the valve. Lets say at peak valve lift their is 20psi in the intake port, pressure probably isn't much lower in the cylinder at that point and a few psi differential isn't going to be enough to start lofting the valve unless the springs were marginal to begin with.

 

There are 2 types of valve float: loft and bounce. Loft occurs at peak lift or after peak lift and is generally not detrimental to power, and can actually increase power in some cases (ever hear of the "launcher" series of cams?). Bounce is when the valve closes and bounces off the seat a few times. This is detrimental to power and can be fixed by using more seat pressure.

I agree with INTMD8, that the only time FI needs more spring pressure is in a turbo motor on the exhaust side. As he said, that's the only time when you're trying to close the exhaust valve and you actually have alot of dP trying to hold the valve open, which can contribute to bounce. When you are closing the intake valve, cylinder pressure should be at or above boost, so the valvespring will not be affected.

Mike

 

So we don't need a valvespring to close the intake?

Rich

 

No! The Brad Anderson blown alcohol hemi I was looking at yesterday runs over 1000 pounds open simply because of the RPM and cool factor.

 

Not any more than a naturally aspirated motor.

 

I do disagree, that was a tongue in cheek question

 

I'm with Rich on this one... you can easily calculate the additional required seated pressure on the intake valve if you know the valve size and boost amount.

Bret