Boost induced valve float....

Big P · Post by **Big P** » Thu Jun 15, 2006 5:10 am

Just curious to see if any of you guys ever ran into valve float that was associated with running high boost pressures. I was thinking about it one day. I mean, it is really a possibility seeing as how the extra pressure in the intake charge could actually try to keep the intake valve open. If this were to happen, then piston to valve contact would almost be guaranteed. Would simply running valve springs with a higer seat pressure be more benefical at this point especially as a safety measure? Just looking to see what you guys think about it...

MMamdouh · Post by **MMamdouh** » Thu Jun 15, 2006 8:20 am

i don't think high boost is enough to open a closed valve or cause it to float.

how much pressure does a standard issue valve spring apply? i don't think a turbo pumping 25 PSI can oppose such big force.

MMamdouh

ubuyau · Post by **ubuyau** » Thu Jun 15, 2006 9:08 am

MMamdouh wrote:i don't think high boost is enough to open a closed valve or cause it to float.

how much pressure does a standard issue valve spring apply? i don't think a turbo pumping 25 PSI can oppose such big force.

MMamdouh

+1.

--Tim

Post by **PrecisionBoost** » Fri Jun 16, 2006 12:21 am

Valve float is more likely with boost... but I personally don't think it's a huge deal.... RPM is the biggest factor in valve float.

I'm pretty sure Tom's turbo engine is using a higher rated spring.... which will also allow for a higher RPM redline.

Stefan · Post by **Stefan** » Sun Jun 25, 2006 11:20 am

The reason you normally get valve float in the first place is due to the springs inability to control the inertia forces in the valve train at high rpm. Those forces are strongly influenced by the mass of the valvetrain - one reason race engine builders use lightweight valve train parts.
Take a 2L OHC turbo engine like mine. It has a 43mm dia intake valve weighing no more than 3oz. If we take off a few mm for the seat width and also subtract the valve stem dia of 7mm we have approximately 1.79 square inches of area. Now at 25psi of boost that's a force on the back of the valve of nearly 45lbs... As far as the spring is concerned it's an extra 45lb force added to the "stock" inertia forces it was designed to control. Depending on valve area it can be quite a difference...
Most production engines can cope with that extra force at production level boost pressures and rpm limits, but if you plan on raising either it is good insurance to install higher rate valve springs.
You could shim up your valve springs, just make sure that at full lift you have sufficient clearance between the coils. Be aware that you will also be putting the stock spring under more stress by shimming it.

Stef

Post by **PrecisionBoost** » Mon Jun 26, 2006 3:45 am

Hmmm.... never thought about shimming the springs... I suppose that would increase the spring force... I wonder what the maximum compression of a stock spring is... I can't imagine there is a whole lot of room for shimming.

One thing to consider.... once you get to a certain amount of spring strength (or double springs) some manufacturers will suggest the use of solid cam follower.... and the increased pressure will decrease the life of your cam shaft.