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The theoretical compression pressure of an engine is almost never achieved due to the many variables affecting the reading - engine temperature, ambient temperature, cranking speed, valve timing diagram, accuracy of the gauge and accuracy of use...
Atmospheric multiplied by compression ratio is the theoretical pressure but mostly the compression gauge is used only as the first step in diagnosing misfires, something it isn't very good for unless there is a massive compression loss. A cylinder leakage test is much more practical use.
As long as the gauge reading is above 100psi, preferably around 120 I would be satisfied. The main value of a compression gauge is to compare the cylinder pressures, which on a petrol engine should ideally be identical but in practice there is likely to be around a 10% variation. More than that and the engine can be classed as past youthfulness. In every case if a single cylinder has a low reading there should be a cylinder leakage test carried out.
normal compression readings will be in the 150-165 psi range
There is a permissible 10% differences between adjacent pistons
anything below 120 psi is considered worn engine
really low readings on 1 cylinder indicates
loose valve seat insert
low readings between adjacent pistons indicate a head gasket
When performing a compression test on each cylinder they must be within 25% of each other or this will cause the engine to run bad. However, if you have a compression reading of say 120, 125, 115, and 120 PSI then you have something else causing the engine to run bad. If you need further assistance go to my website below. Have an awesome day! Auto Repair
ok more info... good.
ok , compression is #1 , no amount of spark or fuel makes a dead engine good. (the 120 is low x4 but i bet you didnt do it WOT like the
instructions stated. most cars can do 150 easy. , all my 1.6L do 180.
the reason i ask is the cam belt slips after 50miles and all 4 jugs
drop from 180 to 120 or lower. so did you block the throttle open.
my wild guess is compression really is good and the belt never slipped. yet. (interference engine they are) all of them
so backup and read my post, did you check spark yet.
compression readings will be the same as a natural aspirated engine readings Lowest will be 120 psi the highest around 170psi but what ever you get there should not be any more than 10% difference between readings. The turbo doesn't come into it as the compression test is done with no plugs in and at crank speed
+reason it will not start is because compression is low caused by bent valves no doubt ,but check it out first as some engines are valve free like the opel corsa from around this year and the fiat punto but i do feel the compression is low ,remove all the plugs then crank it up and what compression you have ?
I think with the lower pressure reading that the knocking will be coming from a broken compression ring or cracked piston. There is only 10% difference allowable between pistons in a compression test.. However check that the valve train is not causing you problems with a failed rocker or cam shaft bush gone.
you need to do a compression test on all cylinders. All cylinders should have about the same amount of compression with a minimum of 120 psi. if there is a major drop of compression in a cylinder that means there is a malfunction in that cylinder(bad valve, broken rings, etc if there is a major drop in compression between 2 cylinders then that means you have a bad head gasket. if all cylinders read 0psi then you have a broken timing belt If all cylinders read about the same then you need to check the timing and make sure the cam(s) and crank shaft are still at top dead center (TDC)