Here's some confusing blather from the service bulletins: To test cylinder compression, engine warm, plugs out, full throttle--Formula: Normal compression=atmospheric pressure X compression ratio + atmospheric pressure + 5. Go get the barometer... The dyno chart for a 99A (prewar Merc) has a compression pressure curve, but it starts at about 700 RPM, well above cranking speed, and is rising steeply there...at 700, it's about 130, and goes up to 145 above 2000 RPM... And RPM is a real issue here--a 12 V setup will give you a higher pressure, because of higher cranking speed--a tired starter and lots of resistance in the circuit will give a low reading--so actual numbers can be very induvidual and hard to compare to anything else. I assume the "compression ratio" in the formula must mean specified ratio, which is often considerably different from reality...
it depended on the year and the compression ratio of that motor, from memory it was about 85-110 psi MOST 8ba style motors were 100-110 ( mercs) so I have always used about 100psi as a good indicator, ask psycho billi boi what the comp on no 6 cylinder was on his flatbanger scarey stuff...
Finally found a Ford 59A number--115PSI 10 lb tolerance/variance. Haven't gound a late model number yet, but that shouldn't be much different, maybe a bit higher in '52-3. More compression/more displacement/12V starter would up that, radical cam would likely lower it and move compression curve upward... I never looked at that Ford formula posted above before--it gave me ridiculous numbers. I believe there are at least two scales for atmospheric pressure measurement, and I don't know squat about any of that, as long as there's enough atmospheric pressure to keep my pith helmet from floating away...did anyone else try some numbers in that?
Josh: There were some numbers posted last week on the msn site http://groups.msn.com/FordFlathead1932to1953/general.msnw?action=get_message&mview=0&ID_Message=37686&LastModified=4675509634373080408 Lots of good info there about flatheads (and lots of opinionated b.s., just like other forums ) Mike
The Jalopy Journal
