in preparing for the build for the 83 D-150 motor I used the online RB calculator to set a target cranking pressure that was dependent upon cam timing, combustion chamber volume...

remember for this build I am trying to stay with regular gas, so I was wanting to push the compression a bit, but not get way out of bounds.

back in 2008, on the 100K stock motor, with the engine warm the compression was
120,150,150,129,150,148,, avg 141.1 range of 30

with the rebuilt motor, engine cold, about 30 minutes total run time on the engine the compression is
150,160,150,165,162,162,, avg 158.1 range of 15

I will check the cranking pressure again after I get a couple of hundred miles on the motor and the rings get well set,, but so far the RB calculator cranking pressure calculator looks to be fairly accurate, with my cam set at 102 intake center and 53 cc chambers and 8.49 static and 8.17 dynamic it predicted the cranking pressure to be 163.3,, so the average cranking pressure on the new rebuilt motor is within 3% of the calculated value.

Cool!

Lou

_________________
Home of Slant6-powered fun machines since 1988

Great report... and "cross-check" on your calculations.
The old saying: "the numbers don't lie" rings true... that is why I constantly answer engine build questions with "what is your DCR" or "what is your cranking compression"??

I'm just thankful that a long, long time ago, a good engine builder took the time to teach me ( a "know-it-all" punk kid, at the time) how to calculate DCR and showed me the relationship between those numbers and cam selection. ( Intake valve closing point, to be exact) This was no small task, back then... seeing that doing compression and DCR calcs was all "pencil & paper" math work. (and what kid likes doing math?)

Today... we all have it easy with the on-line calculators. I get DCR results in a minute or less, for something that use to take 10 or 15 minutes to work-out.

So the bottomline... take the time, "run the numbers" and set the engine up to what the numbers say... and be rewarded.
DD