Since I’m the one who volunteered to fly N142CD’s cylinders from Hayward to the engine shop in Visalia and back, I thought you might be interested in knowing some of what I learned while there.
Lycon not only does engine overhaul work but also is involved in a fair amount of interesting testing in their on-site cell. Some of this is failure analysis related, which means they intentionally run parts to destruction now and then.
They pointed out a number of things I thought were quite interesting:
On a test of a Lycoming O-360 with one exhaust valve blocked to remain open .060 inches, the engine only lost 2% of its rated power. BTW, .060 inches open easily translates to zero compression on a static test of this 4 cylinder engine. However, in dynamic operation at rated RPM, things are happening so fast that much of this static compression loss is not seen and therefore has little effect on power output.
Related to that is their view that you can probably still get full rated power from an aircraft engine with all cylinder compressions in the 30s.
The static compression test should be looked at as a health indicator of cylinder components and not as an indicator of power. For instance, any cylinder in the 30s on a compression test would be unairworthy regardless of the power output of the engine to which it was attached. Severe wear and/or other distress would clearly be occurring somewhere.
On Continental fuel injected engines in particular, fuel flow setup in the airframe can be dramatically different from that on the test stand. They noted a number of cases where installation in the airframe led to immediate max EGT readings when takeoff power was applied. This was with engines they had built up and had set to factory specs on the stand. For that reason they demand that the engine installer go through the full fuel injection system setup after the engine is put back in the airplane. Only a few inadvertent full power takeoffs at peak EGT will cause valve burning, in their experience.
For every engine they overhaul, they do a “max EGT spread” test. On the stand at full power, they will intentionally lean the engine to peak EGT (this takes only seconds and does not harm anything). They want to see at least a 200 degree F rise in EGT on every cylinder when doing this. This amount of EGT spread assures them that the fuel flow system is set rich enough at full power to provided the needed extra fuel cooling for the cylinder heads in general and exhaust valves in particular. This is a test that can also be performed in cruise flight at cruise power settings. If you note that one cylinder on your engine has a much lower EGT spread between full rich EGT and peak EGT as compared to the others, you are alerted to a possible problem with it in the future.
This assumes, of course, that you have an all cylinder EGT/CHT system installed and preferably one that shows all of them simultaneously like the GEM. This is probably the single best thing you can get to really know what is happening in your engine.
- They also reiterated what I learned many years ago and still find that lots of pilots (AND mechanics) don’t know:
A. You lean to the leanest cylinder, not the “hottest.” The leanest is the one that reaches peak EGT first as you move the mixture back from full rich. On a fuel injected engine, it is commonly the same cylinder flight after flight. On a carbureted engine, however, it has been known to change from flight to flight.
B. The “hottest” cylinder is meaningless absent an answer to this question – are you talking about the cylinder with the hottest CHT or with the hottest EGT? They almost always occur on different cylinders because they are determineded by different causes.
- And finally, although they saw definite signs of excessive temperature on both the pistons and the exhaust valves of three of N142CD’s cylinders, they have no way of knowing when or how the damage was caused. In sharp contrast to that finding, TCM looked at the other three cylinders from the same engine and concluded that there was no evidence of overheating. Will just let that discrepancy simmer…
Hope you learned something. I always do.