Valve Unseat Pressure Calculator

(The Calculator is at the bottom of the page if you want to skip ahead)

Running a lot of boost in your Diesel and want to see if your seat pressure is high enough to control your valve? Want to find out what pressure a valve of given size will start to open at? This tool will help get you started. The math is pretty simple -

Pressure = Force / Area

Pressure = Pressure applied  (in our case required to overcome the valve spring)
Force = The force applied to the valve
Area = Exposed surface of the valve

So for a sample if you have these parameters  -

Valve Diameter = 1.680"
Stem Diameter = 0.3122"
Spring Pressure = 100 Lbs

The Pressure required to unseat the valve is calculate as follows -

Total Surface Area =  Surface Area of the Valve  - Surface Area of the Stem
Total Surface Area = (3.1415 * (1.680 / 2.0)) - (3.1415 * (0.322 / 2.0))
Total Surface Area =  2.133

Pressure to Unseat = 100 / 2.122
Pressure to Unseat = 47.125 PSI (the calculator truncates the fraction)

This is often useful in high boost application. Typically in Diesel vehicles where you can create a large amount of boost. It's also useful if you want to make your own blow off valve or waste gate. The value to lift the valve is likely a bit more due to friction in the guide and like a bunch of other forces but this is a general formula to get you started.

Note : units don't really matter can be inches, mm or kPsi as long as its understood if you use Pounds and Inches you will get PSI, Metric is the same idea.
Enter any Valve Diameter, Stem Diameter,  and the Spring Pressure in the fields above then press CALCULATE to find other values. Hit RESET to clear the form and SAMPLE if you want to see a sample calculation.


Pressure to open valve
Valve Diameter
Stem Diameter
Spring Pressure
Enter the following values to calculate the unseat pressure

Valve Diameter - The exposed size of the valve. Typically this will be just above the valve seat area which is smaller the the valve face dimension.

Stem Diameter - This is the diameter of the valve stem. This is used to reduce the size of the valves area since it should not be counted as part of the valve area.

Spring Pressure - This is the force that the spring makes at the installed spring height with valve installed.
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