BAMF87 Posted January 19, 2014 Report Share Posted January 19, 2014 I have maybe 15 hrs on a new top end with hjr dune port and milled stock head for pump gas. I'm at 2500asl and both cylinders are at 120psi. Shouldn't I be higher on compression? Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
uman1030 Posted January 19, 2014 Report Share Posted January 19, 2014 As you your altitude increases the air thins giving you a lower cranking compression. this doesnt change compression ratio but just effective compression. at 2500 ft asl your cranking compression reading will be about 93% of what you would be at sea level. also other factors that come in to play like air temperature as well Quote Link to comment Share on other sites More sharing options...
BAMF87 Posted January 19, 2014 Author Report Share Posted January 19, 2014 Doesn't that still seem low? I'd like to be in the 150 range at sea level, guess it's time for a cool head Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
uman1030 Posted January 19, 2014 Report Share Posted January 19, 2014 depending on port timing also it can show lower numbers during cranking compression test cause ports overlap letting some compression leak out. also air temp changes density so 93% would be at 59F wich is what most altitude compensation charts use. general rule of thumb is about 4% per 1000 ft Quote Link to comment Share on other sites More sharing options...
BAMF87 Posted January 19, 2014 Author Report Share Posted January 19, 2014 Ok thanks Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
thomas h. Posted January 19, 2014 Report Share Posted January 19, 2014 I got a little more at sea level with my HJR port. I though I wanted more too but the crap 91 we get here makes me Change my mind. Quote Link to comment Share on other sites More sharing options...
uman1030 Posted January 19, 2014 Report Share Posted January 19, 2014 I got a little more at sea level with my HJR port. I though I wanted more too but the crap 91 we get here makes me Change my mind. whats your cranking compression at sea level? and is it a dune port also? Quote Link to comment Share on other sites More sharing options...
thomas h. Posted January 19, 2014 Report Share Posted January 19, 2014 D port 4 mill 127 Quote Link to comment Share on other sites More sharing options...
Surfrjag Posted January 19, 2014 Report Share Posted January 19, 2014 Could also have wore those rings out. Quote Link to comment Share on other sites More sharing options...
uman1030 Posted January 19, 2014 Report Share Posted January 19, 2014 D port 4 mill 127 then with a 93% altitude compensation compression adjustment that puts him not far off then with in a couple psi. according to the chart he would be at about 128 at sea level. close to the same assuming you both have similar combustion chamber volume. Compression Test Altitude Compensation Factors Altitude Factor 500 0.987 1500 0.960 2500 0.933 3500 0.907 4500 0.880 5500 0.853 6500 0.826 7500 0.800 8500 0.773 Quote Link to comment Share on other sites More sharing options...
thomas h. Posted January 19, 2014 Report Share Posted January 19, 2014 I told HJR I'm looking for longevity not race Quote Link to comment Share on other sites More sharing options...
thomas h. Posted January 19, 2014 Report Share Posted January 19, 2014 Did you get a fresh bore? Quote Link to comment Share on other sites More sharing options...
thomas h. Posted January 19, 2014 Report Share Posted January 19, 2014 Or a second comp gauge with wot ? Quote Link to comment Share on other sites More sharing options...
BAMF87 Posted January 19, 2014 Author Report Share Posted January 19, 2014 Mine got a fresh bore, new pistons and rings. One dune trip on them. I bought the gauge at oreilly auto parts Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 19, 2014 Report Share Posted January 19, 2014 I had a no name compression checker and it read about 15 psi compared to my craftsman one. Its best to do a compression check when you first rebuild so you have a baseline to read. Quote Link to comment Share on other sites More sharing options...
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