Compression test

[revingtosh]

Hi,

Just one of my usual quickies... lol

Assuming a healthy engine with a 10.5:1 CR and a 73.5mm bore, what PSI should a compression test read? What is the acceptable level of variance between the cylinders too?

Cheers,

Mike

[Cooperman]

As a guide you multiply the comp ratio by 14.7 and that is the nominal cranking compression pressure you should see with absolutely no leakage. Do it with the plugs out after warming the engine. In the case of 10.5:1 you should see between 140 psi and 155 psi. Any lower than 140-ish would indicate poor valve seating or worn bores. To see if it's worn bores, run the engine warm with the oil filler cap off and it will 'chuff' out oil smoke at tickover if it needs a re-bore and new pistons.

[revingtosh]

How about cracked pistons? Would this show on a comp test do you think?

Mike

[Cooperman]

If the engine has a cracked piston(s) it will already be smoking like an old train. Cracked pistons are not common on a Mini unless it is a competition engine with a very high comp ratio which has been run into detonation due to too much advance or poor fuel.
Let us know what the compressions are for each cylinder and then it will be easier to be more specific about what, if anything, needs to be done.
If the engine is not already in the car, then the best advice I can give is to do a precautionary strip-down and re-assemble with new bearings, oil pump, etc, before re-fitting. Then you can inspect the pistons and see what the bores are like.