Well, I ran my annual compression test this year just before doing the winterizing thing.

Here is the results - 2008 is in RED.



Overall, the numbera were a little lower. I am thinking that maybe I did not warm the engine enough. #4 is better than before, but still looking like a problem. So, I am thinking I might try the leak down testing.

What say you experts about the numbers?

The engine runs fine, but I do the check every year, just to get early intervention on any problems.

Could be a reduction in combustion chamber carbon causing the reduction. Maybe 10% ethanol fuel has cleaned up your engine a bit? I have read somewhere that it likely reduces combustion chamber deposits.

Harbor Freight has leakdown tester on sale really cheaply (<$30) right now; I think it would be a good investment. http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=94190

OK, thanks Dave. Also, T-Squared offered to loan me his tester. It would require shipping though, as he is in the mid-west (I think). So, maybe at 29.99, it is worth getting my own.

Did you hold the throttle wide open? Those numbers are just fine, BTW.

I fail to see any "problems", and I think you are looking for ones that do not exist.

Especially your 2008 figures, are all within just a few % of each other.
There is virtually no way in the world to get them all exactly the same.
Variations in port runner shape, volume, as well as manifold differences will alter each cylinder filling.
Combustion chambers are NOT all identical in volume, unless someone with a Dremmel, a burrette, and a sheet of Lexan has made them the same.

The standard tollerance is "within 10 to 15psi of each other".
Your charting shows an aproximately 120psi "center", giving a normal good range of 110-130psi. Most of your readings are much closer than this.

The best information you have, from your data, is that your 2005, 2007, and 2008 data are all very much similar to each other, showing no real issues, basically equal pressures with each cylinder. Both #1 and #4 are low spots, but may easilly reflect either a manifold quirk or a port runner quirk, not a mechanical problem.

Your 2006 data, is so far different from your other 3 years of data, it's validity is suspect, and should have been re-done then, to attempt to verify it's validity, or discarded. Discarded now as it is refuted by 3 other tests, 2 more recent.

The average 10psi pressure loss, over a span of 4 years running, is again, not a "problem" per se, IMO.

Since you started this exercise with an aprox. (hard to detail read the small graph) pressure average of 126psi, and have lost aprox 5 to 7 psi, over the 4 years, What is the magic "pressure figure" that you are going to call "worn out" and slate the engine for overhaul?
110psi?
100psi?
75psi?

When one cylinder is suddenly below 100#, or flirting with 80, or lower (or 2 ajacent cylinders),THEN, you have a problem and the motor needs to come out, the heads tested for valve leakage, head gaskets, or the cylinders deglazed and re-ringed.

When you get enough blow-by thru the crankcase breather (in the top of the valve cover) that you get oil onto the top of the valve cover, there is another clue, BUT, My 4-year old motor, does a tiny bit of this as well. The oil mist inside the valve covers of a big block at and above 5,000RPM is crazy, and some WILL get away, it just should not be very much.

That'll only take 20 years or so.

Put the wrenches away and enjoy the boat.

Your motor is shot. Unbolt it from the drive unit and send it to me. I will see that it lives out the rest of its life in a nice, warm garage.

Seriously from your high comp to you low comp in any one year your numbers are all within a proper range. Enjoy the boat!

I know the engine is 'fine'. I'm just curious about #4 cylinder, and a leak down test should tell the story. It may be fine as well.

+1 to Lowrider's excellent analysis.

The cylinder with a change this year was #3. I might have run that one again to make sure it was not an error. In any case, it is not something to worry about -- just watch.

You might laugh, but I have both compression and leakdown testers, and have never used either on our boat's 15 year old big block. It runs well and I have no intention of pulling it until it stops doing so. If I tested it, I KNOW I would start worrying.