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Compression Test Advice Needed


mach1
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Hi,

 

I'm planning to compression test the 351w in my 93 MasterCraft tomorrow and am looking for some guidance. Are these steps correct?

 

1) After engine is warm, remove all spark plugs

2) Disconnect coil wire from center of distributor cap and ground to engine block. (Is there a particular place on the block that I should be doing this?)

3) While in neutral, push the throttle handle all the way back so it is wide open (allowing unrestricted flow through carb).

4) Measure compression of each cylinder and compare.

 

What kind of values should I expect during the compression test? I'm also planning to measure the oil pressure with a mechanical gauge. What should I expect for oil pressure. The boat has 1,150 hrs on it btw.

 

Thanks in advance!

 

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  • Baller

If the difference between the lowest and highest cylinder is within 10% you're good.

A specified value is not as important as that also measures the current state of the battery and starter.

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  • Baller_
Also make sure you check each cylinder the same way, same number of strokes per cylinder. Inlike to watch the first one as a guide along with the peak. If you have the option, a leak down test will tell you a lot more.
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  • Baller

If you're looking for actual values, remember that atm. pressure at sea level is about 14.6psi. So if you get readings of 145 psi, then you're compression ratio in that cylinder is roughly 10:1.

 

FWIW :)

 

rpg

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I like to use a jumper wire to ground coil wire, need to attach to clean ( unpainted ) ground - bolt, bracket, ground cable away from carb./ fuel. You can also diconnect coil feed wire ( + ) to stop spark altogether. If fuel injected disable fuel supply, unplug injector, fuel pump, fuse, ect. Block open throttle so there is good air flow into intake. As @DanE said they should be even, under100psi would be getting worn but will still run.
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  • Baller

I like to crank until the pressure stops going up on the compression gauge. It usually takes 4-5 cranks. Then compare all the values, +- 10% is good in my book.

 

Leakdown is far more useful since it'll check rings, valve seals, guides, etc. Need an air compressor and a slightly more expensive tool for that though.

 

In all my years of racing I've never bothered grounding the coil wire. Just take it off. Don't lick the coil terminal while cranking.

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