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Unread 05-04-2013, 01:04 PM   #16
ratmonkey
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Quote:
Originally Posted by StrateLOSS View Post
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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what?
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.

Quote:
Originally Posted by Candymancan View Post
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.

#2. 173-175
#4. 185-188
#6. 185-188
#8. 185-186

#1. 175-175
#3. 175-179
#5. 179-180
#7. 170-173.


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.

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Unread 05-04-2013, 03:38 PM   #17
Candymancan
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Quote:
Originally Posted by ratmonkey View Post
what?
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.



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.
Yea my gauge was 30$ i stayed away from the crappy one at harbor freight.. So 165 is new and it can be + -15 so max 180 ? i guess this is why my engine still pings a little every now and then when the rpms reach 4k+ the compression gets too high and the gas is bruning from that.... i in no way dont have the money to have a shop clean the engine or to buy new heads.. Or the money to do it myself right now.. Id probly do it myself when i can afford the tools ect.. Maybe itll clean itself up a little now that it isnt burning oil anymore.. Ill use better gas then the cheap stuff and try getting some bg 44k cans and see if that will help some... Im almodt inclined on getting a 5 gallon bucket full of wanter and running a extremely tiny line from it to the pcv valve with a t fitting and drive it on the next georgia trip at the end of the month that will be around 1500 miles.. My jeep has been city driving while burning oil for three years almost... I just checked my manifold and ts dry now so my catch can is working good
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Unread 05-04-2013, 05:28 PM   #18
ratmonkey
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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.
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Unread 05-05-2013, 09:07 AM   #19
LouC
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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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Unread 05-05-2013, 10:53 AM   #20
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Is techron what used to be called "white gas"?
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