There is nothing magic about 28″ water. Famed engine builder Smokey Yunick picked this pressure (about 1 psi) many years ago (probably in the 1960s) as what he thought was reasonable, and it stuck. It is now the industry standard.

It is not like a running engine produces 1 psi of pressure across the valves. When the exhaust valve opens, there can be over 1000 psi pressure, and both the intake and exhaust valves can have reverse pressure so the port actually flows in the reverse direction.

The graphs below show the pressure across the valves in a running engine, simulated with our Engine Analyzer Pro. The engine is a very healthy race motor running at 9000 RPM, approximately where it’s HP peaks. The graph below shows that on the intake side, the pressure is typically about double 28″, but varies significantly.

Flow Pressure Across Intake Valve at HP Peak RPM

This other graph shows that the exhaust pressure is typically about 10 times higher than 28″ or more. And in both cases of intake and exhaust, the pressure actually reverses at certain points in the cycle.

Flow Pressure Across Exhaust Valve at HP Peak RPM

So we can see that the running engine does not run with 28″ water across the valves.  The pressures in the graphs above will change significantly as conditions change:

• Lower RPM will typically reduce these pressures.
• The more restrictive the valve, the higher these pressures.
• Turbocharging and supercharging will typically increase these pressures.

Flowing at 28″ has probably more to do with what can be produced economically with a flow bench with 2 and 3 stage vacuum motors.  Smaller benches with single stage vacuum motors typically flow at 10″, instead of 28″.  If you have unlimited funds and time, you can flow at several test pressures up to 100″ water or more.

The advantage of having an industry standard is head porters are more likely able to duplicate or follow the work of other head porters.  If Bob finds a 3% improvement by doing X, then Dave will most likely see the same improvement by doing X on his bench.

Click on the text to link to more info on our FAQ page about Flow Bench Test Pressure.