Pressure/Leak down testing

[Larry's Shee]

Reading the thread about turbocharging got me thinking. Now I just looked it up. The atmosphere exerts about 14.696 psi @ mean sea level. How is it that an engine could leak if only tested to 6-7psi? Also, I see never excede this pressure as you will blow the seals out. How could this be? It's still only 1/2 atmosphere. Also you would be saying that it only creates 7psi max just before the transfers open.

ALL answers, opinions, and theories welcomed I:I

 

[Wheels]

You only need to test a Blaster engine to about 7 psi above whatever the atmospheric pressure is around your workbench. The critical areas of your engine actually operate in a vacuum but this is harder to replicate and test than positive pressure with some soap bubbles. If your engine leaks in the wrong area it will suck in more air which will cause a lean condition and eventually smoke your top end.

 

[enalkarion]

The 8 psi on the gauge is actually PSIG pounds square inch gauge. its actually 22.696 total pressure.

 

[Larry's Shee]

Actually thought about it more and 7psi would be above atmosphiric, or about 21 total if you were in a vacume. As to the engine it is actually working at both vacume and pressure alternately. Vacume as piston goes up and pressure when piston goes down. That is how it forces the mixture up into combustion chamber. Would still like to know what pressure it achieves at this point.

 

[Wheels]

I guess I have a better understanding of your question now. Maybe we can get KOR or another experienced engine builder to chime in here. I'm not sure what the real forces inside the bottom end of a Blaster motor are, I just leak test to 7 psi because experience says it works.