I have my 85gt 2.8 it run's strong an I'm not having any prob's at all, but I am going back home to key west, fl. From south california in a few so I'm trying to cover all my base's if you know what I mean. I know that if a out board motor has poor compression then it's just a matter of time till it's dead because low compess mean's bad bearings not just bad ring's. So this is my deal, what is the norm compress for the 2.8 an how much can you learn form checking it?
A compression check is more for comparing cylinder to cylinder to see if there are any low ones. The low cylinders (20% down) indicate a wear issue somewhere in that cylinder (scored bore, worn rings, leaking valve seats, bad head gasket - normally 2 low cylinders side by side). A leak down check can help confirm if the compression "leak" is in the head or in the cylinder.
In an automotive engine, compression has nearly zero to do with bearings... unless the piston is down about 1/8" in the bore because the bearing is completely gone in the rod - but there would be a very audible knock, which will be more of an issue than concerns about compression.
As long as the car idles smoothly and doesn't pulsate or feel like there is a slight miss, I wouldn't bother with doing a compression check before a long trip.
If the bearing's are going then the rod is lose so it can't compress fully so the compress is low, at less that's the theory. It sounds a bit off but I do know when I check out board's for compress and is bad an thay still run it it allway's blow's the motor. Iv had only one motor that had no compress it was a ford bronco 289 an it just stoped running . That's why I'm asking. I have no reson to think my motor is bad but it does have some miles on it an key west is 3000 mile's of driving.
Thank's, you all have cleared up what I was thinking. I have no leak's with the head's an it run's smooth at 850/900 idel an if I step on it it jump's an has no stammer to the motor at any rpm. I most likly just a bit worred of the long dive!!
If you do a compression test (called a "dry" compression test) and find one or more cylinders have a variation in the readings, then you can move on to a "wet" compression test. Squirt about a tablespoon of lubricating oil (normal engine oil) into each cylinder and then repeat the compression test. If the pressures increases appreciably, then there is likely piston ring and/or cylinder and piston wear. If after adding the oil, the pressure does not go up on the cylinder(s) that have reading variations, your valves may not be seating properly (worn/damaged seat or valve).
[This message has been edited by Neils88 (edited 01-28-2014).]