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Discussion in '1974 - 1978 Mustang II Talk & Tech' started by Addicted_2_II, Jul 7, 2005.
Yea the engine is 10.8:1 err was
With a stock type camshaft, 8:1 compression tests somewhere around 130 psi. 9:1 tests around 150 psi. 10:1 will be 170 psi. 11:1 will be 190psi. 12:1 and higher, the pressure gauge will tell you to buy better gasoline or alcohol.
Oil present in certain cylinders at the time of the test can cause a compression fluctuation, since oil helps seal the rings.
So aren't the results within the 20% tolerance? They ranged from 151 to 179, and 20% of 151 is 30 psi and 20% of 179 is 36 psi. The difference between the highest and lowest numbers was 28 psi.
I did squirt 2 shots of oil in each cylinder... Hmm maybe the 179 got distributed better than the 152's???
Maybe I will try again today with two more shots of oil in each cylinder just to make sure!
Thanks for everyone's help on this!
One last thing... Could it be my valves leaking causing the big differences? I don't see how my situation could have messed up the valves but again I am no expert!
Maybe they got burnt or something?
Yeah, if you go by the industry's so called general rule, you're right... i think... as long as you've figured it right...
However, i don't. Personally, if the difference between the lowest cylinder and the highest is anymore than 10-15 psi, it gets torn into.
Yes- bent or badly carboned valves can cause compression loss but usually its like a quick bleed and won't be able to 'hold' the reading on the gauge for any period of time. (aka, excessive leakdown)
Based on what you have posted and the events, I dont think your hosed. I was expecting an 80 or a 0 . That would be a broken ring or holed piston. Time to pull the pan and look at your bearings. I think you'll be suprised how little damage there actually is other than the pump and Rod.
Working at a wrecking yard, I do at least two compession tests a day on engines. Your numbers look just fine to me. After running out of oil, everthing got really hot. The engine was likely really dry. The one cyl that was really high, probably had some oil left in it. Shooting more in it made the number shoot way up. I have cranked over engines dry and got 80 psi, shot a couple of squirts of oil down the cyl and it jumps to 300psi! Oil does not compress well, so it can really throw off the numbers! Even though the cars I work on are practically new, they can sit for 6 + months before I dismantle them. Like a car that sits your car is probably just thirsty for lubrication.
I would squirt a couple of shots of oil down each cyl with all the plugs out. Then I would crank the engine over for a minute. Then I would leave it over night and perform another compression test. Chances are you will get more uniform numbers. Good Luck!
Remind me not to buy used engines from you....
You're a dismantler now?? I thought you were a parts counterperson? Last time I checked, doesn't matter how much oil you squirt in the sucker, It's only gonna compress the amount the chamber and piston are designed for. unless you decrease the chamber size with a huge amount of oil. To get 300 psi, you had to of dumped a quart of oil (or not far off) in the damn thing...
2 squirts, 4 squirts - about the same readings. Really no more than 2 is necessary. Just check each cylinder 2-3 times in a row and mark down the average.
His higer pressured cylinders are right for his compression ratio. The lower numbers have me worried. Could have scored cylinder walls, stuck rings, stuck valve(s) or even a leaky head gasket. Tear down is the only 'for sure' way of knowing what you don't know. Cause as I see it, you have 2 pistions firing as high as 10.8:1 and 2 as low as 8.5:1 and I'll tell ya right there, that's gonna cause problems, if not now, later.
I have an update!!! (Good news too)
I did some more reading on how to DO compression testing and the results came out much better!
I shot 3 shots of oil in the "TOP" of each cylinder so the oil would run down and seal the rings completely! I then turned the engine over about 10 times and then tested compression...
I feel much MUCH better about these results! Just goes to show you a well lubricated engine does make a big Difference!
Thanks for everyone help!
Excellent!! now for the bearings...