Rookie Compression Question

[sanleonkid]

I'm an accountant so forgive my engine naiveté. I am in the hunt for another boat and when I sold my 16 Classic the buyer did a comp check and "thought" that there was an issue with one of the cylinders. I just asked the maint guy if it would affect the performance and he said not really so we stayed pat on the price.

I did not ask at the time what type of "numbers" you should expect? Is it a numeric value or a percentage? If I have a comp test performed on a engine(s) at what level/value can i start negotiating down with the price and at what point should I look for another boat?

Thanks.

[onesubdrvr]

I'm sure I'll be corrected here, BUT

CR is the X of times smaller the engine makes the air, for example, on a 9:1 compression ration engine, you should see (9*14.7) or approx 130 psi. or a little less on a stock engine, say 120 or so.

Anything less than 100 I'd say problem, or if a cylinder bleeds down pressure too quickly (drop down test).

I'd like to see all of them at +/- 2-3psi.

Ok, criticize away y'all

Wayne

[sanleonkid]

ok, call me slooowww. the 14.7 factor comes from "where"?

[mikev]

ok, call me slooowww. the 14.7 factor comes from "where"?
thats about the psi at sea level of the atmosphere so maybe thats where it comes from just guessing though

[Carl C]

Camshaft profile and cranking speed will affect the reading. Anything over 100 PSI should be OK. Perhaps more important is that all cylds. be within 10% of each other. If possible check with engine warm, all plugs out and throttle open.

[sanleonkid]

OK thanks..... now I have something to work with.

[MOP]

Last but not least if when you find the low one squirt a little oil in it and re-check it, if the compression stays the same it is a cylunder problem if it comes up in pressure it is a valve problem. Valves causing compression loss is very common, may just need a valve job.

Phil

[mikev]

Last but not least if when you find the low one squirt a little oil in it and re-check it, if the compression stays the same it is a cylunder problem if it comes up in pressure it is a valve problem. Valves causing compression loss is very common, may just need a valve job.

Phil
i think you have that backwards if you squirt oil in it and it comes up the rings are bad because the oil helps seal the rings if it stays the same its the valves.

[mrfixxall]

[QUOTE=Carl C]Camshaft profile and cranking speed will affect the reading. Anything over 100 PSI should be OK. Perhaps more important is that all cylds. be within 10% of each other. If possible check with engine warm, all plugs out and throttle open.[/QUOT

Ignition off or BOOM!!!!!!!!!!!!!!!

[MOP]

Last but not least if when you find the low one squirt a little oil in it and re-check it, if the compression stays the same it is a cylunder problem if it comes up in pressure it is a valve problem. Valves causing compression loss is very common, may just need a valve job.

Phil

Scot keeps telling me not to post after 10:00!!! Backwards is the way my old grey head is going! Along with Cylunder!
Should have just said what I was thinking and have done a few hundred times, left it at if the compression don't change it is a valve problem, cause it will rise as the oil will seal the rings.

Dumb Old Phil