Jack Worstell Posted February 17, 2017 Share Posted February 17, 2017 If I run a compression test on a 248ci straight eight ( with, say, a 6.3 compression ratio....a factory number ) then what pressure should I expect to see from the test ? Jack Worstell Link to comment Share on other sites More sharing options...
JoelsBuicks Posted February 17, 2017 Share Posted February 17, 2017 Jack, theoretically, 6.3 ratio would be 6.3 x 14.7 = 93 psi. But consider heat of compression which causes a temperature and therefore a pressure increase, multiply by about 1.3 and you get 120 Psig. Now, because valves and rings are not perfect, you'll lose pressure. So, if you run a compression test and are between 95 and 110, I'd say that was pretty good. Lots of experts here, hopefully you'll get insight from others. Joel Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted February 17, 2017 Share Posted February 17, 2017 Jack, If I remember right I think mine was about 90 lbs across the board with one cylinder about 3 pounds less. My test was done cold with the throttle open. What we are hoping for in a compression test is that the cylinders are even or within 10% of each other. Link to comment Share on other sites More sharing options...
Jack Worstell Posted February 17, 2017 Author Share Posted February 17, 2017 Joel and Dave...thanks for the help. Joel...thanks for your nice easy explanation of why it's hard to predict compression test results in a simple fashion using the compression ratio.. ..... And I've read that camshaft overlap/timing also enters into this and further complicates predicting compression test results from the compression ratio even further..... am I right in assuming the valve/camshaft timing/overlap always tends to lower compression test pressure results vs the "theory" calculated from using the compression ratio ?? (And I understand that the temperature increase happens because of "adiabatic" compression.)....... Dave.... we ran a compression test on our 1937 Special today. We where pleased to get about 110 psi on all cylinders ( except one or two were down to about 105psi So why ?............did a previous owner "shave' the head ? Or maybe installed dome pistons to replace the original flat pistons ?? ( If you get 90lbs on your 1938...which has I think 6.1 compression ratio.....then that suggests that in the past someone has done something to get our engine up to maybe 6.5 compression ratio ?????? ). I guess to get a specific answer we would have to remove the head and see what we have.........but for now we'll just accept the good results and go onto something else..........but it would sure be nice to know the compression ratio and horsepower improvements as a consequence of whatever good work some previous owner has done..... Jack Worstell jlwmaster@aol.com 2 Link to comment Share on other sites More sharing options...
LAS VEGAS DAVE Posted February 17, 2017 Share Posted February 17, 2017 Jack PLEASE don't take my 90 pound number as a fact, that seems to be what I recall but I am not sure, maybe it was higher. The only thing I am sure I remember is that they were very even except one which was down a few pounds. Link to comment Share on other sites More sharing options...
Jack Worstell Posted February 17, 2017 Author Share Posted February 17, 2017 OK Dave Jack Link to comment Share on other sites More sharing options...
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