Compression on a 350 Magnum??? Anyone know?
 

Hi Everyone,
I was hoping to find out what the compression should be on a 350 Magnum engine with the alpha drive. The year is a 1996.

I went to check out a boat today and the compression was all over the place. On the left hand side they were 150, 140, 150, 140, but on the right side they checked out at 120, 120, 80, and 80.

Sounds like problems await. My guess was that they should have been more in the 140 to 150 area.

Anyone know what the compression should really be on this engine???

Re: Compression on a 350 Magnum??? Anyone know?
 

don't buy it , it has two bad holes, I personally don't like more than ten lbs variance 15 in low compression motor.

Re: Compression on a 350 Magnum??? Anyone know?
 

Mercruiser says the lowest cylinder should not be less than 70% of the highest cylinder, with a minimum of 100 PSI.

150 * .7 = 105 PSI

The two cylinders with 80 PSI fail both the calculated and absoulte minimum. Do a leak down test to determine if it just needs a top end rebuild or a complete engine rebuild.

Dave

Re: Compression on a 350 Magnum??? Anyone know?
 

Thanks for the replies guys.
That is what I figured. Seems like the cylinders varied in pressure too much to be a clean engine.

What exactly is needed to do a leak down test? I will only need to do this type of test a few times so what should I expect to spend for a decent gauge/tester? Also, how exactly is this test done?

Re: Compression on a 350 Magnum??? Anyone know?
 

Back in "my day" :rolleyes: we would take compression readings then do it again after squirting a little oil in the cylinders. After squirting the oil you spin the motor to distribute the oil around the rings. If the compression jumped up radically then we knew the rings in that cylinder were bad. If there's no change then the valves are bad. Or possibly a holed piston.

Re: Compression on a 350 Magnum??? Anyone know?
 

Head gasket? Maybe it was replaced with an automotive head gasket at some point?

Either way... I would pass on it.

Re: Compression on a 350 Magnum??? Anyone know?
 

Back in the day? We still do it to this day! :D

Re: Compression on a 350 Magnum??? Anyone know?
 

Could I interest you in a leakdown tester? :D

Re: Compression on a 350 Magnum??? Anyone know?
 

Very similar readings to one I looked at with a blown head gasket, which was the result of losing an impeller and the subsequent overheating

Re: Compression on a 350 Magnum??? Anyone know?
 

with 2 cylinders side by side reading low the gasket between them is likely gone.

My 454 read ~80 on 1 cylinder this fall. A teardown revealed a cracked head.

So, either pass on the boat or buy it at a price discounted to allow for a rebuild. Keep in mind a nice 383 long block is relatively cheap!