I read on another forum that a "rule of thumb" is to multiple the compression ratio by 20 to get the expected compression test result. So for a 9:1 compespion ratio engine the expected result would be: 9 x 20 = 180 psi. I haven'the heard of this relationship before and wanted to see if it's at all valid? Thanks.

No.

A bunch of BBC near 9:1 motors will crank 140-150psi.

Engine cranking speed and camshaft has a bunch to do with cranking compression also. Not just static/physical compression ratio.

Way back when I discovered the cranking speed thing, already knowing the cam and static compression ratio effects. Our class was introduced with a compression ratio rule. I quickly discovered by luck (low battery) that cranking speed , when slowed down, compression test would read lower. Worked great until others discovered same thing.

Edited in: Best to do compression test with fully charged battery to keep that part of test consistent.

No its not

Camshaft size plays a large role in cranking compression. I look for equal numbers across all 8 cylinders more so than I look for a target number.

The more conservative the cam, the higher the compression test will read. Hotter cam has more overlap. You can have 30 to 40 degrees total overlap wipes out that much crank speed compression.

Mercruiser V 6 at 9:4 to 1 are 180 when solid

Mercruiser 350 can be 150 to 180 and I even see 180 to 200 on certain year models with the same comp ratio

I even see some Volvo's 190 to 210 in the past with 9.4 to 1 comp ratio

big block what was mentioned a lot are 140 to 150 Merc

My 9.0:1 454's, 500+HP with mild cams are 125 PSI cranking which surprised me. I'm going to do another check after I change cams.

Just for the knowledge base because I know you like this stuff:

It's actually IVC (intake valve closing) that effects cranking compression.

Overlap, during cranking (engine not running) causes the exhaust valve to act as an intake valve also. The exhaust valve closes (when piston is starting to go down) way before bottom dead center , but the intake valve closes as the piston is going up towards TDC. Cranking speed is slow, therefore piston is pushing air out of intake valve until the intake valve closes. The longer the intake valve stays open, the more air and thus psi is lost.

So, if you take two cams identical other than LSA, the tighter LSA cam will have more overlap, but will have an earlier IVC. Thus the tighter LSA cam will have more cranking compression, This effects an actual running engine too, but that is another topic.

I think this kinda explains my low numbers (125ish) I used to get thru out the years . I have high tq starters which crank slower .

the more I think about it, it isn't the overlap that lowers compression test numbers. it is the intake closing event. that is the only open valve on the compression stroke. but there is about 60 degrees ABDC the intake is open while the piston is trying to compress the air. and the hotter the cam, the longer the valve is open, cutting into compression time.