Quote:
Originally Posted by ike84
How is that possible? Not questioning your experience, but the math there doesnt add (or multiply) up lol.
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Well you can start with things like valve overlap.
Valve timing. air temp.
The simple math formula of multiplying atmospheric air pressure by CR assumes things like.
Intake valve wide open exactly at BDC and closed at exactly TDC with no exhaust valve overlap.
Piston cranking speed is another example. Watch the needle on the vacuum gauge in the video's in this thread.
As RPM increases vacuum increases. = increased cylinder fill.
So the simple math formula of multiplying atmospheric air pressure by CR will not work to give you cranking PSI.
Look up any engine specs. you can find that show CR and cranking compression PSI
That simple math formula will not give you stated cranking PSI
As for leak down tests being more helpful than compression tests:
A leak down test is a "static test" of the cylinders ability to seal. (hold pressure)
A compression test is more of an operational test of volumetric efficiency (cylinder air filling ability) which is directly related to vacuum
and the ability to "build pressure"
Very closely related but you can get a little different info from each test.
So it depends on what info your after on how helpful a given test method is.