leakdown/compression specs history
 

Every so often the subject comes up of should a motor be freshened ,what is acceptable lkeakdown or what should the compression be. Something every person on this board should consider doing is after dynoing their motor,rebuilding it if not dynoing it or after buying a new to them or brand new boat (after break in) is to conduct a full compression test and leakdown test. Document each cylinder on each motor and put the info where you won't lose it so later on you can go back and use it as a reference pont to see whats going on in your motor. I would suggest doing it cold because as steve xsm has mentioned and is a good point is the fact it is fairly dificult to warm your motor then conduct the test in your boat. This info could be very helpful down the line for comparison purposes,I have been doing it on all the motors I build for anything for a while,Smitty

Re: leakdown/compression specs history
 

no way in the world to suggest that this is anything other than exactly correct

Re: leakdown/compression specs history
 

Seems like everyone would do this but every one I ever talk to on the phone who calls with these questions never bothered to see what they had to start with,plus I see it come up every year on here,Smitty

Re: leakdown/compression specs history
 

More solid thinking from Smitty. Keep it coming. Jas

Re: leakdown/compression specs history
 

I'll share my recent experience... A couple months ago, I pulled my motors with the intentions of just cleaning up the bilge. They were running perfect but I had three seasons on them. Since they were on the floor, I decided to do a leakdown test. On the port motor, cyls 1-3-5-7 all had between 8-12% leakage. I was pumped. Then tested the other bank... cyls 2, 4, 6 had 70-90% leakage, #2 leaking intake valve, #4 & 6 bad exhaust valve.

Starboard motor also ran perfect but found two dead holes, cyls 2 & 4 had bad intake and exhaust valves. Last night I pulled the heads on both motors and found a broken hyd roller lifter tie bar and a wiped out cam from the broken lifter. :(

I guess I should consider myself lucky because the last time I lost a lifter tie bar, the roller got loose and went threw the cylinder wall (at 5800 rpm) and exploded the motor.

At least I found these problems in February not the middle of July! :drink:

Kurt

Re: leakdown/compression specs history
 

I agree completely that this is a good idea. Assuming that an initial leakdown and cranking compression test are done and the results of both are good, would a leakdown be required on subsequent tests if the cranking compression remained the same? Just wondering because leakdowns are a pita!

Dave

Re: leakdown/compression specs history
 

Compression tests don't tell the whole story. I've seen plenty of motors that have acceptable compression readings but show excessive leakage when leak tested.

Re: leakdown/compression specs history
 

Lets say you have 1 or 2 seasons and only 60-150 hours on your motors,no intentions of rebuilding yet. Before you winterize you spend a day doing these basic test every year,wow,we got a cylinder thats just along for the ride,instead of not knowing and having problems in june or july a month into boating season you have a whole winter or off season to get things taken care of. It seems mandatory to me,I'd rather know now and fix it then when its time to go boating but countless people skip this simple test every year then chase losses of speed or oil consumption issuses or worse,Smitty

Re: leakdown/compression specs history
 

Biggus,I'm sure you already know this as you are a experienced mechanic as I am also but I will lay it out for the guys who are still learning or that don't do this for a lliving. You can short cut a leakdown test very easily to find the worst cylinder by paying careful attention to your compression test. When you do your compression test have all the spark plugs out,throttle wo,battery fully charged. Check every cyl,watch your gauge as you crank motor over and listen very carefully to the sound of the motor hitting compression. You will notice as the cylinder the comp gauge is screwed in comes up motor will make a different noise,count these noises as you watch the gauge rise. If you have cylinders that take 4 or 5 full cycles or more to bring the gauge to its highest reading compared to others that max the gauge out in 3 or 4 full cycles,do your leakdown on them first. You may only see 5-10 psi less compression in those cylinders but they will leak down signifigantly higher than the ones that pop right up. If you just blindly crank away until gauge is maxed out you may lull yourself into a false sense that all your cylinders are good especially when everyones been taught that a 5-10% difference is acceptable,thats what guys mean when they say a compression test doesn't tell the whole story. If you do the worst cylinders and they are hurt then you don't need to waste your time doing the good cylinders,you can spend that time tearing the motor apart instead:eek: ,Smitty

Re: leakdown/compression specs history
 

Articfriends and Biggus,

I'm used to doing leakdowns on OB's where I turn the crank to read leakdown throughout the cylinder stroke till you get to a port.
For 4-stroke engines do you only test at TDC? I saw an old thread on "How to" but niether of you weighed in on that one.

Any other tips.

Thanks for the info!!

Re: leakdown/compression specs history
 

On a 4 stroke, it should be done at TDC compression stroke so both valves are closed. It's a time consuming process.

When I do a leak down test, I pull the valve covers and pull the rockers so all valves are closed.

never thought of that... that makes things go faster... and only have to go 1 full rotation versus 2 or more to get all the cyl...

race 51 royal purple is SIMILAR in wt as 20/50W BUT it is actually closer to 35/50... just because its got 500 hp doesnt mean that it needs more oil thickness.... besides a lighter oil will cool the engine more and actually allow for better more consistant lubrication due to its abilty to get through the journals and tight tollerances easier than a heavy oil... ESPECIALLY when cold.... and oil in 145-160* engines to begin with doesnt get very warm..... THEN add in the fact that yu all run oil coolers....... oil temps around 120-160 arent all uncommon.... which means that the oil is THICK and may not slide through the .0025" clearences as efectivly as say race 41 (15/40) or even a true 10/40 10/30.. and when does all the wear occur in an engine.. AT STARTUP!!!! not on top or at peak TQ... its when CHIT is dry..... and lacking lube...