Low Even Compresion..When to rebuild?

[jedimaster]

At what point would you consider reboring, sleeving, rings, valves, when your compresion is tested even but low? eg 100psi, 90 psi, 80 psi etc....
Is there much power loss with a motor with even compresion but in the low 100's, 90's, 80's? Everyone talks about even being fine if its low why is that?

 

[CWKboat]

Re: Low Even Compresion..When to rebuild?

I've always heard that as long the holes are within 10% of each other that even at under 100 they will run OK. Definitely try and decarb the engine with some Seafoam or equivalent product and those readings may come up as they have for numerous others on here.

 

[wilde1j]

Re: Low Even Compresion..When to rebuild?

There is a point beyond which a motor just will not run (as a guess < 50 psig), but low even compression causes somewhat reduced power. OK values vary depending on the specific motor design. Most are OK over 90 to 95 psig all around.

Most people run them until they won't run any more, then take action. A well cared for 2S motor will run for 1000's of hours before major surgery. The most typical scenario is one or two holes bad, others OK on the bigger motors.

BTW, sleeving is typically done only when max oversize cannot be done because grooves are too deep to clean up.

 

[CATransplant]

Re: Low Even Compresion..When to rebuild?

Is this just a general question, or are you talking about a specific engine you're testing? If it's your engine that concerns you, we need to know what engine it is and how you did the compression test.

If cylinders are even, but low, the best advice is to go get another compression tester and compare readings. If the engine's running fine, then it's likely that either the tester or the person using it are inaccurate. It's rare for a multi-cylinder engine to wear evenly over a long period of time.

Low compression overall will usually cause difficulty starting and very poor idling performance. At higher RPMs it's less evident. You can see this with an automobile engine with a burnt valve. It will idle poorly, but you won't notice the problem as much when the engine speeds up.

So, please let us know if this is your engine you're worried about and what that engine might be.

 

[R.Johnson]

Re: Low Even Compresion..When to rebuild?

This is where you want a leak-down test, at 10% difference you may want to consider a over-haul. You can not go by a compression test, as that simply is not an accurate guage. I have at least three compression testers, they vary by 15 Lbs. You can simply change the valve stem in these testers, and change the reading. If the rings' are free, no scuffing in the cylinder walls, and you can see the cross hatch from the last honing job on the cylinder walls, leave it alone.

 

[jedimaster]

Re: Low Even Compresion..When to rebuild?

More general info, rather than specific to one motor or another. Just that I often see, people saying 90's is ok if its even...Just wondering why such low compresion is still ok in a 2 stroke outboard, when in a 2 stroke motorcycle or 4 stroke car it would be cause for an engine rebuild.

 

[JB]

Re: Low Even Compresion..When to rebuild?

Too many thing affect psi readings to set any rule, even rule of thumb.

I have tested engines that the owner claimed made 45psi, but even, and gotten 90psi, and even. That engine ran just fine.

If the engine starts and runs well don't worry about psi. If it doesn't, do a compression check to see if one or more cylinders are waay out of line with the rest. Your chances of being hit by a meteor are greater than the chance of all cylinders going bad the same amount at the same time.

 

[jedimaster]

Re: Low Even Compresion..When to rebuild?

Mine were in the low 100's but after doing a sea foam cleaning and running a tank with seafoam the compresion has gone up to 118psi exactly even on all three. so I am not too worried about it, but there must be some point of failure. I know it makes a big difference on how you do the test, open throttle closed throttle, warm engine, cold engine etc...
In the Cklymerit says

While minimum compression should not be less that 100psi per cylinder the actual readings are not as important as the differences....a variation of more than 15 psi indicates a problem etc...

Having never seen an engine with less than 100 just wondering what they run like. Or even what a brand new engine runs at for psi, and how it relates to performance of the engine. I would imagine an engine with 150psi and even would be more powerful than an engine with 100psi and even, but by anything noticable?

 

[freddyray21]

Re: Low Even Compresion..When to rebuild?

a lot too depends on the motor. Some motors are low compression motors and will not have over a 100. My old 3hp has 90 in each cylinder and even.

 

[R.Johnson]

Re: Low Even Compresion..When to rebuild?

An interesting thing about compression! I have a 24" flywheel wrench, and a piston stop tool which I made for doing a leak-down test. If you try to hold a flywheel with that 24" wrench, then apply 100 lbs. of air pressure to a 3.5" dia. piston, it will take all you can do to hold on to that wrench. 100 lbs. per square inch is one heck of alot of pressure. That's why the piston stop tool , that 24" wrench could really ring your bell.

 

[Evinrude Boater]

Re: Low Even Compresion..When to rebuild?

My V-4 reads 85psi on my compression gauge yet it runs well. As said before, it's more important that they're even. Uneven compression is like having a weak or injured leg; you walk with a limp.
Search "speedifour" on YouTube and you see two videos of my motor running; one idling and one pulling a load near WOT. I think it sounds healthy