Out of complete curiousity, has anyone tracked compression loss versus mileage on a high mileage engine? My Ford Escort has just reached 245k miles. I do a plug/wire change around every 50k mi and started the habit of doing a compression test while I have the plugs out. Here's what I noticed:
200,000 mi:
Cyl 1 - 170 PSI
Cyl 2 - 170 PSI
Cyl 3 - 170 PSI
Cyl 4 - 180 PSI
245,000 mi:
Cyl 1 - 155 PSI
Cyl 2 - 155 PSI
Cyl 3 - 150 PSI
Cyl 4 - 170 PSI
Those are all dry compression numbers. I read that Ford specs 180-190 PSI new on these engines and 100 PSI is the rebuild threshold. If there's any linearity to the drop I can estimate around 380k miles before I hit the rebuild PSI. It currently burns a quart of oil every 4,000 miles.
200,000 mi:
Cyl 1 - 170 PSI
Cyl 2 - 170 PSI
Cyl 3 - 170 PSI
Cyl 4 - 180 PSI
245,000 mi:
Cyl 1 - 155 PSI
Cyl 2 - 155 PSI
Cyl 3 - 150 PSI
Cyl 4 - 170 PSI
Those are all dry compression numbers. I read that Ford specs 180-190 PSI new on these engines and 100 PSI is the rebuild threshold. If there's any linearity to the drop I can estimate around 380k miles before I hit the rebuild PSI. It currently burns a quart of oil every 4,000 miles.