compression test question
re: Lull 225 Slant 6
The vibe I'm getting from AJ/FormS is that 145 psi, the highest compression reading I have, would be the benchmark for the entire engine, and numbers less than that in the other cylinders should be filed under "compression loss" (?) (And what about the +/- 25psi rule for general engine health?) And besides this forum, I've read that 125psi is an acceptable number for a stock 225.
This engine always starts right up, and besides the smoking at high rpm, it purrs like a kitten and runs like a top.I'll do another compression test to see if the numbers have changed since I've changed the stem seals and adjusted the valves. Thank you for the feedback.
Yes the 145 is the benchmark. But we don't know if that number is high or low. Here comes more confusion; The cylinder might have had more than 145 when it was fresh. If it had say 155 new then 1 - (145/155) =6.5% leakage from new.It also means all the other cylinders are worse than you thought; and really the engine could be called a core. This is why we do a leakdown test on an engine such as yours, in conjunction with the pressure test. Suppose you did a leakdown test on the weakest cylinder, and it showed absolutely just 5 % leakage. What are we to think ,then? Or if that high cylinder showed 10% leakage; what then? And these are distinct theoretical possibilities.The LD test is a snapshot of leakage at TDC;that's all. That cylinder with the low compression and theoretiocal 5% LD could have a wristpin gouge 3 inches long in the cylinder. That would be junk. But the LD says otherwise.
On the flipside, that cylinder with 145 psi and 10% leakage, could have 1cc or 2cc of carbon in the chambers, artificially decreasing the chamber size and thereby increasing cylinder pressure. But at TDC it is junk, according to the LD test.
So the two tests need to be studied and carefully analyzed. And sometimes additional tests need to be developed.
Now as to the 25psi rule.You list it as +/-, and this then allows 50 psi variance.This IMO is absurd. It would allow a typical slanty to have pressures of 75 to 125, and would be totally unacceptable.
But let's say you really meant a 25psi variance. This then would then allow 100 to 125 psi. The first thing is we know nothing else about the cylinder with 125psi.When new it could have had 135,or 145, or 155 etc. Lets say it hat 145! Well, again 1-(125/145) =about 14% leakage. That means that good cylinder is already worn out. So how about the one with 25psi less? Ok but let's do an LD on it. So we come up with 5% leakage. Well thats not too bad right. If all the cylinders were at 5 %, we would still have a smooth-running easy-starting slanty. It would just be tired, that is not very powerful. It would require a lot more throttle to do the job it used to do, and it would use a lot more fuel doing it, and it would require more frequent oil-changes. If it still had enough power to do the tasks it was called upon to do, then a guy could justify leaving it alone.
You see the interplay of compression test and LD test and Analysis?
Getting close to the end now;
Now, getting back to your slanty;with just the compression results to go on, 120-125-120-135-145-120, we also have to consider your other statements.Such as "This engine always starts right up, and besides the smoking at high rpm, it purrs like a kitten and runs like a top." In this case I would throw out that hi cylinder reading and call it an anomaly. Then I would average the rest =124 psi, and you are running on the money. Furthermore the difference from highest to lowest omitting the anomaly is just 9psi and the % difference is just 11%.So taking that at face value, your engine is in fine condition. Except for the smoking under full power.
Home-stretch now;
As to that smoking, there is another possibility; a too-high oil level. If the crank is whipping the oil into a froth and overloading the rings, it will smoke. Or if the engine is operated at a steep angle that would push some of the oil to one end, same deal; crank picks it up and smoke comes out the tp.
And done!
