Valve spring pressure vs boost

oil-burner

oil-burner

New member
Started looking into valve spring options, wondering what effect boost has on the seat/open pressure. found this tech info from a cam company.


Is Choosing The Right Valve Springs for Supercharged Engines Critical?

Selecting the proper valve spring for any performance engine is important; but it is critical to proper operation of supercharged engines. Consider the fact that when the engine is in a "boosted" condition, the supercharger (or turbocharger) is trying to blow the intake valve open. The boost pressure actually reduces the intake valve spring seat pressure. This is extremely critical on engines with hydraulic lash adjustment. Proper seat pressure (working through the rocker arm and pushrod) is necessary to keep the hydraulic lifter plunger centered in the lifter body to prevent "pump-up." If an engine has 2.25" dia. intake valves, there is 4 sq. inches of backside valve area. Now add 12 (psi) of boost pressure, and you have reduced your effective seat pressure by 48 lbs. (12 lbs/sq.in. X 4 sq. in.). If you started out with 120 lbs. of seat pressure (static), you now have 72 lbs. of operational seat pressure. There is no way that 72 lbs. of pressure is going to control a 2.25" valve!


So if this is correct, the following should be for a 12v

Valve dia is 1.772 with a sq/in of 2.466 (1.722 devided by 2 = .886, .886 x .886 = .784996, .784996 x 3.1416 (pie) = 2.466)

Now take sq/in of valve dia x boost level = spring pressure needed to keep valve seated (ex: 2.466 x 75 (lbs of boost) = 184.95 lbs of seat pressure needed to keep valve on seat)

From what I found a stock 12-v spring is rated at 59lbs @ 2.00 (seat pressure) and 180lbs @ 1.550 (open pressure)

So with stock valve springs and 75lbs of boost the valve is never seated when under full boost ?

scot
 
Also subtract out the stem diameter. Use the portion of the valve that actually sees boost.
 
Don't forget about cylinder pressures and drive pressures, which are many orders of magnitude larger than boost pressure.
 
We made a plate to go over the heater grid. We put in an air fitting and 2 gauges, one to check against the other one. On a 24 valve with a 93 psi seat pressure the intake valves were blown open at 82 psi. We had removed the exhaust guide seals, to increase valve guide life. We didn't check the exhaust, but during the check of the intakes, we found the valve stem seals contained the boost. No air was coming up the stem, a loss of pressure was present as the valves were blown off of their seats. We had a similar formula to check what seat pressures were needed to contain the boost. We found the formula to be inconsistent with reality. The dynamics and cycles of a running engine, will only allow for a short time of the valves to be blown off their seats. Anytime cylinder pressure is greater, than dynamic seat pressure, the valve will be forced closed. The times that the valves can be unseated or not controlled by the camshaft, are at the end of the exhaust cycle during overlap and during the intake cycle. Obviously overlap and drive pressures, can cause some exhaust gas reversion into the intake tract.
 
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