Acceptable compression/ leakdown #'s

This post does not debate the value of a compression test versus a leak-down test. Rather, it answers a fellow poster's question regarding the meaning of 'dry' and 'wet' compression tests - and why they are so named.
A dry test is just the name for a compression test done with the engine just as you shut it off. That is, you have put nothing into the cylinders AFTER you shut it down. That means you are testing it in a normal operating condition. If it passes, things probably look healthy. And BTW, this probably has nothing to do with whether it burn oil. Its about the sealing capacity of the cylinder. Oil burning is another thread...
A wet test is so named because oil is added through the plug hole to WET (and therefore seal) the rings. With the rings sealed, the compression test evaluates the sealing capacity of the valves. A 'dry' test evaluates the rings and valves together. Thus, the logic is: if dry is good, all is good. If dry is bad, you still don't know which (or both) of the areas are leaking. BTW, a wet test is all but impossible on a boxer (108 degree "v") engine - for reasons you can now understand.

The 'operating temperature' bit is required so all the parts have expanded to their normal running dimensions.

rick #28133

 

That's interesting. So when you say that 355's with bad valve guides never smoke, do you mean that they don't smoke on start up, don't smoke on deceleration, and don't smoke on hard acceleration?

Likewise, are you saying that a 355 with bad valve guides will have no increase in oil fouling of plugs or rough running/idling? That they don't consume more oil than usual?

Or is it that they consume more oil, foul plugs, and/or run rough, but just that the cats do a marvelous job of burning up the oil on 355's with bad valve guides?

 

Typo, of course. Should read: 180 degree "v"

rick

 

My Europa have very short valve guides. To help prevent premature wear, they have no stem seals. Oil pools on the valve and capillary action pulls it down the stem, so on startup there is lubrication, and that wonderful puff of blue Lotus smoke. This isn't a design flaw, it's a feature!

The key is that the car doesn't smoke on deceleration or acceleration. I'm told that one means valve guides and one means rings, but I forget which is which.

Ken

 

Acceleration=rings (be careful not to confuse too much fuel for oil burning, smoke color can be the same, they just smell different)

Deceleration=valve guides (actually more noticeable at the start of picking back up on the throttle after extended off-throttle, high vacuum conditions)

Ah memories, Brian! I remember seeing my first Euro TR at the Annual Meet at Watkins Glen in 1985. It emitted a touch of blue smoke on cold start-up (first start of the morning) and people were freaking out. I asked them if they'd ever seen a P-car or a Boxer at cold start! <grin> Then, bought a brand new TR for a customer in the spring of 1986 and it did the same thing, for a lot of miles... No problems.

-Peter

 

A leaky intake valve will make for a really miserable running engine as it will have a seriously detrimental effect on manifold vacuum. Fortunately, intake valve problems are rare as they lead a life of leisure compared to exhaust valves. Exhaust valves are made of much higher quality (and costlier) heat resistant materials. Custom engine builders have to be careful not to inadvertantly use an intake specification valve in an exhaust position.

Exhaust valves and connecting rods are the real workhorses in an engine, especially a high rpm, high power to displacement application.

 

THAT'S where I know you Ken. I knew you sounded familiar - as does your sage advice. I was on the Europa list when I was rebuilding my '70 Europa. Sadly, I had to let the car go. It was a basket case and I just couldn't budget the time for ANOTHER full restoration. Sure had some great conversations on that list though and enjoyed researching the quirks of those great little cars!

Regards,

rick lindsay, previously from Tulsa, now in Houston
'79 308GTB
other stuff

 

LOL Hey Rick! Yeah, I slum it here because I love the passion of the Ferrari guys. Europa guys seem to distain passion because they take driving so damn seriously. And are all gear heads, by necessity, and that's the passion: keeping them running! LOL It's great when I need help but start talking about the love of the car and the marque and a *few* guys will start flaming you.

These guys here are more "gee whiz" about the cars as cars, as am I about mine, and I enjoy sharing that. Every now and then a general technical question lights a bulb in my head and I'm thrilled if I can contribute something useful.

ken

 

Not to pick nits, but from what I observe, you were not fooled. You seen a 10% loss and accepted it as normal loss.

I never did leakdowns, never really gave it a lot of thought, thought it was just a fancy way of doing a compression test. But I got working on airplanes and there is no way around it, it is REQUIRED by the FAA and has to be recorded for inspection by the FAA licensed inspector and logged in the log book on every airplane flying, every year, or more often in some cases. The procedure has to be followed by the book, and on an airplane it takes two people. One to run the leakdown tester, and one to hang onto the prop. We could all argue three ways from sunday about how to test the condition of an engine, but stick your nose into a hanger and you all will suddenly learn there is ONLY one way to test the condition of the cylinders on an airplane engine, and its the same way virtually world wide. The Airforce, NAVY, NASA, makes no difference, they all do leakdowns on piston engines and they all are done about the same way. It makes no difference if its a 50 HP Continental from 1936, an 18 cylider WW2 radial, or a turbocharged Lycoming from 2006, the test is done the same way, and the same rules apply. 10% (72/80) past the rings is getting close to grounding the airplane. No leakage past any valve is really acceptable. Any engine would test the same way, no matter if it were a giant ship engine, or a small model airplane or model helicopter engine. Our beloved Ferrari are no different, they still have rings and pistons and valves. And no valves on any engine are not supposed to leak. If they do they need a valve job.

You need a quiet shop so you can hear where the air is escaping. A leaking intake valve will be easily heard at the aircleaner. You can easily hear air escaping past the rings by listening through the oil filler neck. Air leaking past exhaust valves can be heard at the tail pipe. Valve leakage generally had to be dealt with before the plane could go back out. The reason is that if its leaking past a valve with only 80 psi, its really gonna blast through when the engine is firing, and the flame going past the seat will rapidly deteriorate the valve and seat. Of course an airplane is working hard constantly, while a car, even a Ferrari, is seldom at full throttle for a very long period. So a leaking valve in a car can run a much longer time, and the potential harm is much less than in an airplane. But the point is, loss past the rings could correct itself with a few oil changes and some hard runs if only the rings are gummed up. But leaking valves are never going to get better. And as they deteriorate the damage will accelerate rapidly.

I can see that a compression test, especially on a car like these with variable vale timing from belt service, could indicate a valve timing problem, but if it passes a leakdown and runs okay, I wouldnt worry over it. But 10% leakage on a Ferrari and I would be asking for some serious consessions.

 

I can accept 10% as normal loss on a eleven year old, infrequently driven car...

Other operational charactaristics affect relative power output and delivery far more than plus or minus 10 or even 15 percent leakdown. I agree that if one or more is far out (and I don't consider one reading at 5% and the worst reading being 10% far out, I consider that acceptable tolerance), you should dig deeper, but this is silly trying to establish a value deduction based on a test that has as much variability as the number of people administering it!

It's an old car... that's why it's less expensive than a new car. If the compression readings are in the ballpark, roll on. If not, dig deeper...

-Peter (won races in spec classes with far more than 10%, even 20% leakdown...)