Increasing Compression

[leetbeef9999]

Hi 5.7 L OMC Cobra here again (1986 26.5 foot Chris). Our compression is fine on 7 of 8 cylinders, but we're only getting 100:1 on the 8th cylinder as opposed to 135:1 on the other 7. Any ideas on increasing compression on the 8th cylinder? Should it be done for all cylinders? Thanks.

 

[Don S]

Re: Increasing Compression

First off, you are looking at 135 psi, not a 135:1 compression ratio versus a 100:1 compression ratio. You obviously took a compression check and had 135 psi on 7 cylinders and 100 on number 8. Did you take the compression test properly? Here is how a compression check should be done:<br /> Compression Check <br /><br />A leakdown test is the next order of buisness to determine if you have leaky valves or rings or ????<br />With the maximum at only 135, I would say your engine is getting a bit long in the tooth and you may want to look at either an overhaul, short block, long block or a new engine.

 

[RatFish]

Re: Increasing Compression

Don S,<br /><br />Welcome. You have given me and others good advice over at BOC. Your knowledge and experience is highly valued.

 

[leetbeef9999]

Re: Increasing Compression

Don S,<br /><br />While we were out in the Chris we smelled something burning...Apparently, the intake manifold gasket was blown and oil was bubbling onto the manifold. The gaskets on both sides were replaced, but for some reason the other side is now bubblimg oil...Any ideas on why the new gasket on the other side is not doing the job?

 

[Don S]

Re: Increasing Compression

The gaskets on both sides of what were replaced, the intake manifold???? They should be replaced.....as a set? And where is the oil coming from. There are no oil ports on the intake or exhaust manifolds. Do you have a valve cover gasket leaking oil onto the exhaust manifold?<br />Sorry, but I don't follow your explanation.<br /><br />PS: Thanks RF

 

[ron7000]

Re: Increasing Compression

Originally posted by leetbeef9999:<br /> Any ideas on increasing compression on the 8th cylinder? Should it be done for all cylinders? Thanks.

If it's worn rings, one solution may be using a higher viscosity oil such as 15w-40 diesel oil or a 20w-50, if not just a few oil changes if the motor has been neglected. There's also the stuff called RESTORE, can be found at any parts store, walmart, kmart. It's been around for many years, I've used it in my old truck with no ill effects although I never did a compression check to see if it actually works. But for $10 that might be worth a shot.