cylinder compression

[Mr.Stuart]

I thought I'd throw this in here for what it's worth.

I'm also a member of a farm tractor forum, my tractor broke down and I've got it tore apart and am in the process of fixing it.

I was doing a compression test on the tractor engine (a Briggs and Stratton Vanguard) and had posted my compression readings in the tractor forum, and had made the comment that (to my knowledge) most older outboard boat motor manufactures, (at least Evinrude and Johnson) don't list anything about compression readings in regards to what they were (or were supposed to be) when the outboards were new, I ran into the same thing with that Vanguard motor, I couldn't find any info on what the recommended compression readings were supposed to be.

I got a comment from a poster about that, what he said didn't really make any sense, but thought I'd share it anyway.

he said "Most small engines have some form of a compression release that cause a false reading, your Vanguard doesn't seem to, but I believe that's the reason that small engine manufacturers don't use them.(compression readings)"

I've had the cylinders apart on at least one of my outboards, and I've never seen anything that would cause a compression release, that in turn would cause a false compression reading.

but, on model airplane motors, (OS MAX is one brand) some of the older engines used what they called a dike ring set, where it didn't build compression until it was running at a high rpm, turning the motor over slowly (on the OS model airplane engines) would produce very little compression.

so, I guess this leaves me with the thought and question, why didn't the outboard manufacturers (in my case Johnson and Evinrude) ever provide a recommended compression reading in any of their manuals or motor literature?

 

[Crater]

Re: cylinder compression

Here's a guess:
Because unless you were rebuilding a motor to it's OEM specs the compression would differ from the manufactures, even in the break in period there will be a difference in commpression until it's fully broken in. From there, the're will only be a natural decrease in compression as the rings wear down. However even at lower compression the motors may run fine, just not as much hp as new.
So as you can see, unless you buy a motor that has allready been broken in by the manufacture you wouldn't even have a constant commpression reading. And I don't think the manufacture's break in the motors so they wouldn't know either, and by putting a reading in print would make them liable for that reading.
That just a guess though.

 

[JB]

Re: cylinder compression

The main reason for no specs is that measuring compression is very inexact.

Among the things that can cause differences in readings on the same engine are:

Different guages.
Different engine temp.
Different amounts of residual oil on cylinder walls.
Turning engine at different speeds while measuring.
Measuring for different numbers of strokes.
On some 2 strokes and all 4 strokes different throttle position.

Just those 6 factors can make as much as a 50psi difference in measurement of the same engine, but as long as they are the same for all cylinders a good judgement can be made.

I have never checked a 2 stroke with even compression that didn't run well. Most 2 strokes with one or more cylinders slightly out cleared up with decarb. EVERY 2 stroke that had a bad cylinder or two was waaay out of balance.

 

[Mr.Stuart]

Re: cylinder compression

Thanks for your comments Crater and JB, both of those statements make sense.​

While this isn't exactly outboard related, I may try to do a decarb once I get the tractor back together, one cylinder read 140 psi, while the other one read 130, so I may try a decarb to see if I can even out the compression.​

The people over here at iboats seem to have a good understanding of motors, and I find that the different theories about the lack of documented compression readings are interesting, and the theories seem to differ between the different forums. but I'm going to lean towards what you guys said​

Thanks again for your comments.​