Originally Posted by StrateLOSS View Post
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 View Post
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.
Originally Posted by ratmonkey View Post
premium is premium just about everywhere. shell adds a few extra detergents that many others don't. but almost every grade from every station has the detergent additive packages that used to be reserved for the "premium" fuels.
I agree with the thought that the high comp readings are due to carbon build up which decreases the size of the combustion chamber slightly and the carbon itself gets glowing hot and can cause preignition. The best thing would be to remove the carbon but you can keep running it if it does not ping on the highest octane gas you can find. I have a marine Chevy 4.3 with similarly high comp results and I have run it on 93 octane gas without pinging for a long time. I'd try running Chevron Techron through it right before you do an oil change each time.
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Is techron what used to be called "white gas"?
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Originally Posted by HighLonesome
lol...if life is as a simple as a 5.9 making you happy, I'd say go ahead and buy it on impulse. ZJs are the way of Zen. Wax on, wax off
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