OUtboard Cylinder Compression?

[_Dan_]

Hi All,

Just wondering whay is a good compression rate for a outboard motor.

I was thinking anything over 100 would be ok? Is that true?

Any info would be appreciated.

Cheers


Dan

[Roughasguts]

So far I have worked out Ball park, 120 psi to 135psi for the jap motors.
And 140psi to 155psi for the US ones.

[BaysideMarine]

Dan,

From the mid-late 80's or thereabouts outboards were lowered in compression to around 120psi in order to cope with the newly introduced Unleaded fuel at typically 89-91 RON (Research Octane Numbers) as opposed to the previously used Super (approx 95 RON).

The actual compression number itself is not that important (to a degree) but the evenness of the numbers is the most critical.

Generally speaking below 100psi is not desirable and above 150psi is getting dangerous in terms of low octane or stale fuel.

Most V6 outboards have tended to run comps close to 100 psi with the smaller engines being higher.

If you find your comps are too high then a hit with a can of powertune or engine tuner or decarb (depends on the manufacturer) can help bring it back to original specs. Although not all manufacturers publish compression numbers. Mercury for many years never did and probably still dont.

Cheers

[Kopey]

Depends on what outboard you are referring too.

A fourstroke Suzuki 115hp for instance has around 190psi a cylinder but a Mercury 115 two stroke has around 140psi a cylinder.

A typical petrol engine has around a 9:1 compression ratio whereas a diesel engine usually has about a 20:1 compression ratio (obviously relys on compression igniton) which obviously equates to higher cylinder compression.

Some people even shave say 2 thou off a cylinder head on an engine to make more compression to give better top end performance, a common pratice on 2 stroke motos.

Cheers Ads ;)

[BaysideMarine]

For the average boater, shaving cylinder heads is extremely unwise.

Higher compression and then mix that with some stale fuel and its a bang waiting to happen.

Quite some years back a gropu from Mercury wanted ot see if they could take a stock engine and modify it to the hilt and see what resulted.

They shaved the heads, altered the pistons and port timing etc etc. You name it, they did it.

What they wound up with was an engine no more powerful on the dyno than a stock product...

[_Dan_]

Cheers guys,

It was an older Johnson outboard.. I did a test and they were both 120psi which was suprising.

Cheers

Dan

[finga64]

good enough, both the same :)

[fishingjew]

BASIC OUT LINE FOR OUTBOARD COMPRESSION

120lbs to 140lbs----------Excellent

110lbs to 119lbs----------GOOD

100lbs to 110lbs----------fair

90lbs to 100lbs-----------poor

below 90lbs--------------TROUBLE ! ( some outboards will run even at 60 to 70lbs but I would not consider a fishing trip with it )