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OK..I have noticed a few members ask what the correct compression in PSI is for the 1.2L Spark engine is. With the compression ratio at 10.5:1 the correct compression is 152-PSI at sea level and about 148-PSI at elevation of 1,000ft ASL. Average is 150PSI at 500ft ASL. Of course ambient temp and elevation play a fairly small part in the PSI #. As you can see the difference between sea level and 1,000ft does not produce much of a difference..the ambient temp variation is even less. Surprising as it may seem, all engines of all types have the exact same PSI at the same compression ratio. For example: a 6L-V8 with a compression ratio of 10.5:1 also has the same PSI as the 1.2L Spark engine...This is all calculated with a universal formula that takes into consideration the compression ratio, above sea level altitude and ambient air temp...so all engines with the same compression ratio have the exact same PSI at piston TDC. With the Spark 1.2l engine, it was determined that the critical PSI # is 70psi. At 70psi the cylinder, valves etc are considered damaged to the point of engine replacement..thus, during the PCV fiasco GM ordered the engine to be replaced if one or more TDC cylinder readings were at or below 70PSI.

Just though I would share this info since I have seen this question pop up not only here but on many car forums. Although the formula is fairly simple, you need to know the pressure (PSI) at your altitude..If anyone wants it here it is (atmospheric psi (ASL) x compression ratio (ie:10.5) divided by 1= your engine PSI at TDC. You will rarely have all cylinders exactly the same PSI but the variation should not be much ie:7lbs for the Spark and more on big V8 engines. Usual symptoms of a big variation is lots of vibration at idle.

NOTE: Some mechanics will tell you the cam angle, specially on a dual cam variable phasing system (DCVP-VVT) the compression PSI will vary..although true, obviously the PSI pressure when valves are open or partially open is not the PSI pressure that should be used. Others will tell you the PSI should be about 18-20 times the compression ratio..also incorrect. Engine size (displacement) and volume of air in a cylinder also has no bearing on PSI as we are dealing with pressure, not volume. The age old standard for a PSI reading is piston at TDC with all valves on that cylinder closed and the only way to get that reading is to have the engine rotate 5-6 times using a locking gauge that will lock the reading at the highest PSI. This is done with the fuses for ignition and fuel pump removed..we want the engine to crank without ignition and without fuel flying into the gauge or all over the place. A big engine with the same compression ratio as a small engine will have more HP only because it has a greater volume of air at the same PSI as the smaller engine. Sorry, can't get more detailed than that..'Nuff said.

Edited by Retired old Gearhead
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