185 psi compression in a 1970 60hp?

[reelfishin]

A few years back I cleaned out an old OMC dealer who had closed up back in 1972.
In the many parts were several factory new power heads, all for electric shift motors. Most were 55hp and 60hp three cylinder models. They sat on a shelf for a few years but I finally found a use for the one 60hp power head, a 1970 60hp Evinrude Triumph that came from a freshwater only area which had a bad middle cylinder.

I did the swap, with new everything, and it runs great, however, I decided to check compression after its first run on the water and was surprised to see a reading of 185 psi on all three holes? Thinking my brand new Snap On gauge was acting up I dug out two others and all agree within a pound or two. Next I got out the other five power heads of this type and ran a compression check on those, and got 180 to 195 on all of the 60hp versions and 165 and 175 on the 55 hp models.

The old motor had 155-100-160 top to bottom, the middle cylinder was scored up pretty bad and the piston looked like swiss cheese from detonation.
The old motor had been sitting for years and the last owner told me he rebuilt it using spare parts from other motors and had spent a lifetime in saltwater.

What was normal compression for these motors? 175 psi plus sure seems pretty high to me.

I was also wondering if there's something about these motors that might give a false or too high reading?
The first thing that came to mind after seeing the high compression was that the starter didn't seem to be straining to crank over the motor.

 

[JB]

Re: 185 psi compression in a 1970 60hp?

I don't think compression can be too high. The psi you measure say more about your technique and guage than the engine, except that reading that high, as long as they compare withing 10%, indicate a good seal.

 

[Chris1956]

Re: 185 psi compression in a 1970 60hp?

I am skeptical of those compression readings. You might do the math to see the max compression, and see how close to that you are.

 

[reelfishin]

Re: 185 psi compression in a 1970 60hp?

I am skeptical of those compression readings. You might do the math to see the max compression, and see how close to that you are.

I'm not sure what you mean?
To check compression I did as I've always done, pull all the spark plugs, disable the ignition, hold open the throttle plates and crank the engine at normal operating speed till the gauge stops climbing.
On my 1989 110HP the same gauge reads 145 on all four cylinders, it reads 137/135 on both cylinders on my 55hp twin, and it's consistent on all other motors as well.
I have never had an outboard give me this high of a reading. Most are on the low side compared to other types of engines. My concern is with detonation or just plain excess wear on the crank, rods, and bearings at such a high compression.

 

[Chris1956]

Re: 185 psi compression in a 1970 60hp?

Reel, Every motor has a compression ratio. You can calculate it or get it from the specs, but it will tell you the max compression you could have. If your compression gauge reads higher PSI than the motor can have according to the compression ratio, you know the gauge is wrong.

 

[reelfishin]

Re: 185 psi compression in a 1970 60hp?

The gauge isn't wrong, I own five compression gauges, one from Matco Tools, one from Craftsman, one is Kent-Moore, and two Snap On gauge sets, with one of the latter being a brand new Snap On Motorcycle and small engine set. They are all within a pound or three.
Knowing the factory compression ratio spec won't tell me anything, without looking I'd guess it to be somewhere around 6.5 to 7.0 : 1 or so, but things like head gasket thickness and combustion chamber volume also change that spec.
My concern is that this thing has too much compression to survive. Its more common for a 2 stroke engine to give a lower than expected reading when it comes to compression since the compression doesn't begin to build till the piston passes the exhaust port where as a 4 stroke begins building compression from the bottom of the compression stroke.
What I need to see is what is 'normal' for one of these motors when brand new. As with any motor, there will be a normal test reading, regardless of compression ratio or any volumetric calculation. For instance if you have 5 of the same motor, and four give a compression reading of 145psi on all cylinders, and one has readings in the 100psi range, then it's likely there's an issue with the motor with the odd readings.

 

[JB]

Re: 185 psi compression in a 1970 60hp?

Trying to predict psi compression in a 2 stroke with a gonkulator is futile, mainly because a good portion of the stroke is with open ports.

Quit worrying about your high readings as long as they fit within the 10% rule. When I was racing karts I sometimes got as high as 210psi from my air cooled engines.

There is no such thing as "normal". Variations in guages, air temp, engine temp, amount of lube in the cylinder, operator technique and speed of rotation all vary the readings. That is why Manuals almost always hedge on psi and specify 10-15% span.

 

[jbjennings]

Re: 185 psi compression in a 1970 60hp?

Reelfishin',
My nephew is bringing a johnson '70 model 60hp to me very soon that he was given and wants me to get running for him. I'll try to post the compression numbers I get as soon as he gets it here. However, I suspect that will be as late as a couple of weekends. I have lots of motors, and I do get very similar compression readings from them if they're in good running condition. I usually get about 85psi on my '50s 10hp motors, 115-120psi on my '50s 18hp motors, around 125psi on my late model johnson 25 or 30hp motors, and my '98 90hp yamaha gets 120psi in all 3 cylinders---very similar to what everyone else has when I did some internet research. So I understand what you're asking.
There was a similar question here about this a couple of weeks ago. I think someone might have mentioned that a different head gasket thickness might be the reason for the high compression, and that it wasn't good for the motor to be running with that high compression as you suggested. I have no idea if that is correct or not. I'll look for it and see if I can find the thread and post a link if I do.

Link: http://forums.iboats.com/showthread.php?t=473532&p=3205044#post3205044
Good luck,

JBJ

 

[reelfishin]

Re: 185 psi compression in a 1970 60hp?

Because the ports in the cylinders most two stroke engines read low on a compression test, not super high.
That other post pretty much conveys the same thoughts I have on this thing, but to the best of my knowledge, these power heads are all original factory replacements from OMC from way back when. They all came from a closed up dealer that we cleaned out a few years ago.

I really would have expected closer to 145 psi on all three, which is what I usually see on later triples, but I just never have seen an older triple, at least not one that's still healthy or new.

My mid 80's 70hp has 145 on all three holes, its tested the same for years and did when I replaced the power head four years ago. It actually gained a few pounds after it was broken in, it read 140-141-139 when I first installed the reman power head. That power head was an aftermarket rebuilt power head, it was not a new replacement as I figure these 60hp power heads are.

 

[Big flop]

Re: 185 psi compression in a 1970 60hp?

Go to the top of the page and in the
advanced search put in
(thicker head gasket to lower compression)
It covers a lot of points about older motors
with high compression