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I recently bought a used (salvage yard) 3.9L engine for my 1997 Dodge Dakota. Spec'd 8.9 compression ratio. If the 8.9 number means atmospheres, that should be about 130 psi. Local Dodge dealer tells me compression should be no less than 100 psi per cylinder and vary no more than 25 psi between cylinders. Measurements on newly purchased junkyard engine range from 87 psi to 72 psi. I understand that engines will lose compression with wear, but are these measurements reasonable for a used motor?
there is always amin spec for compresion in an engine. As long as all the cylinders are above the min spec for that engine and do not vary by more then around 25 psi then the engine is good. As far as what the min spec is for that particular engine is im not sure
Also keep in mind that a used engine will sometimes have low compression readings from sitting, I've had a few of the low mileage import replacement engines that had valves not seating completely and had to be run to operating temp before they had enough compression to run normally.