Originally Posted by StrateLOSS
compression tests are accurate it showing compression psi relative to the cylinder next to it.
meaning the "185" number may or may not really be 185, it could be 160 with a different gauge or 200 with another gauge.
everything is close enough that I'd be happy with those results.
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as long as the pressure is released from the gauge before moving to the next test or next cylinder it will show the actual pressure built in the cylinder. there's no "relative numbers" about it.
the only way it would be "different with a different gauge" is if you were using a seriously poor quality gauge set. but, if you pay more than 20$ for the gauge you're going to get one that displays accurate pressure information.
variation of a few psi or more per test is all due to friction and imperfect mechanical fit variations. that's why we average the numbers over a few tests on just about anything we want to measure.
Originally Posted by Candymancan
I havent ran a compression check in 2 years and the first time i did it wrong i didnt step on the throttle to open the valve body up..
So per what a few said to do since i fixed my intake manifold i did another compression check.. Engine was hot, Took all the plugs out disconnected the ignition coil and ran the check with throttle open all the way and checked with oil as well.. I should note i forgot to unplug the fuel pump so i think 1-3-5-7 are a tad lower then the other side because they were washed with gas
Here are the numbers first number is dry second is oil.
They are all within 10% of eachother (well the 185's and 170's are close) and they dont really go up with oil in them.. But the compression just seems really high.. Perhaps my engine is really gummed up ?? or my engine is in very good health... or both lol. I dont know what the norm is suppose to be
your numbers indicate an approximate dynamic compression ratio of something like 12:1. i know you've been getting these numbers all along and that's a hard ratio to get running on pump gas.
at this point, i'd be inclined to pull the heads and either replace with some engine quest heads if you've got the coin, or have them rebuilt, cleaned, and the combustion chambers polished by a local shop. or you can do it yourself, it's not that hard and most of the tools can be rented for valve spring installation and valve lapping.
both the 4.0 and 5.2/5.9 run the same compression ratios and the stock. factory fresh reading is 165psi. no more than 15psi high/low variance is acceptable and anything lower than 120psi should be rebuilt. 90psi is about the bare minimum for a gasoline engine to have a moderately complete combustion event. much below that and you'll not get any power from it.
you can get a rough idea for your dynamic compression ratio(which is affected by many factors other than the math that gets the mechanical compression ratio everyone usually talks about)by dividing your cylinder pressure by atmospheric pressure, 14.7psi.
oh, pressure gauges are calibrated so that 0 on most gauges is 14.7psi actual. that's why there's gauge psi and actual psi when working out math on many things.