Jock McLean's article in the August issue contained a number of interesting aspects that we would like to address. Although it was intended as a response to reader Ian Findlay's comments in the May issue, the article involves an area that we have studied extensively; hence our desire to reply.

First and foremost, we must emphasize that the significant question here is how much heat is carried away from the exhaust valve and valve guide by air and oil, not from the cylinder head itself. It is, after all, the valve and guide that are experiencing failure due to excess heat, not the cylinder head. Whether cylinder head heat rejection by airflow is 117,000 watts or one million watts, it matters little if that does not include sufficient exhaust valve and guide heat to keep the temperature of these components in check.

Our investigations have shown, as has Lycoming's S.I. 1479 (oil cooled exhaust valve guides for the Mooney TLS), that with normal cylinder head temperature, premature valve and guide failures are occurring due to excess heat. As an example, the Mooney TLS encounters CHT levels in cruise of 380 to 420 (F), indicating normal cylinder head cooling. And yet its engine routinely experiences compression loss due to exhaust guide wear at about 400 hours or less. Why? Because of the inability of the cylinder head to transfer sufficient valve and guide heat to the atmosphere while operating at a normal cylinder head temperature. Mr. McLean's view is that our recommended minimum oil flow to the rocker boxes has a minuscule effect in cooling the cylinder head, and we fully agree. However, when this oil flows directly onto the exposed exhaust valve and guide it has a very significant cooling effect on them. It is through the use of this concept that Lycoming solved the TLS problem.

Consider this simple, real-life analogy. If one held a safety pin with his fingers and heated its point red hot, it would be a few moments before enough heat transferred out of the pin for it to be safe to touch. On the other hand, if one merely dunked the red hot pin into a single drop of water, the pin would cool almost immediately. It would do this because it has a low thermal mass and water rapidly transfers heat from the pin into itself. In like manner, the thermal mass of the exhaust valve is very small compared to that of the cylinder head. While air in tremendous volume is required to cool the cylinder head, only a relatively small amount of oil making direct contact with the very hot valve stem and valve guide will effectively cool those components.

Based upon the above analogy, it is important to note that this limited oil flow most certainly is not intended to cool the entire cylinder head. Overall heat input to, and rejection from the head via air flow is massive, as Mr. McLean correctly states. Oil cooling of the guide and valve is minimal by comparison. However, please note that inlet oil makes its very first contact with a hot object at the guide and valve stem and immediately transfers heat away from these components. Since the valve stem is not very well thermally coupled to the cylinder head (the Mooney TLS problem in a nutshell), this additional oil becomes highly significant from the standpoint of cooling the valve itself. Think about it for a moment. If the problem had been caused by insufficient air flow through the cowling, as the article suggested, wouldn't Lycoming simply have told Mooney to increase air volume? Wouldn't the design of the air cooling system be faulted and then altered to function properly? Instead, Lycoming redesigned their own oil system to provide additional oil cooling of the valve and valve guide and did so at their own expense, not Mooney's. This alone is quite telling.

Second, as we have already written in your magazine, the mission of the aircraft is of paramount importance in determining whether or not a given engine will encounter valve and guide distress. Prolonged exposure to cruise power settings with leaned mixtures, even with normal CHT levels, contributes substantially to this problem. It was noted in the article that many flight school aircraft in Oz easily reach TBO and beyond, as is the case in this country also. However, from a thermal standpoint as seen by the exhaust valve and guide, the mission profile of a training aircraft is quite benign. High power is used only occasionally for takeoff and power-on stall practice and is immediately followed by a cooling cycle. Prolonged flight at cruise power and mixture settings for these aircraft is much less common. And yet it is this latter operation that bears directly on the valve and guide distress we have investigated. (Incidentally, we have discovered that in the O-320H engines used in many Cessna 172 aircraft, that the barrel type of lifter in that engine flows approximately ten times as much oil to the rocker boxes as do the mushroom style lifters used in most Lycoming engines. The O-320H, although earlier plagued with cam and lifter spalling problems, has a very good history of upper end longevity.)

A third area raised by Mr. McLean's article involves S.B. 388B, which is the infamous "wobble check." Although he believes that it originally came about due to guide wear in turbocharged helicopter engines, the current situation is much different. This S.B. now applies to every engine Lycoming manufactures regardless of horsepower, number of cylinders, turbocharged or normally aspirated, or airframe in which it is to be installed. Regarding helicopters, take the Robinson R-22 for instance. This normally aspirated O-320 engine operates at about a 350 (F) cylinder head temperature due to excellent shrouded, forced air cooling. Despite the low CHT and the normally aspirated induction system, this engine installation frequently requires valve and guide replacement at about 400 hours. It experiences this problem because of the prolonged heat soaking phenomenon mentioned earlier. As a helicopter engine, it of necessity operates continually at about 75% power, which takes its toll on guides and valves despite its comfortable CHT. Again, the problem is that the parallel valve cylinders, by their very design, are unable to adequately transfer exhaust valve and guide heat to the cylinder head in some operational situations without the added benefit of oil cooling augmentation.

And this leads to our final comments. As was stated in the article, Lycoming publishes that, "For maximum service life of the engine, maintain cylinder head temperature between 150 F and 400 F during continuous operation." Mr. McLean goes on to say that, "There is a message here." We agree, but would like to go one step further and decode it --

"The Lycoming parallel valve cylinders are marginal in their ability to shed heat from the exhaust valve and guide during some operational conditions despite normal CHT levels. If you operate in these areas, the boundaries of which we cannot define, you will experience premature valve guide wear and subsequent compression loss. If you own a Mooney TLS we will correct this problem by installing oil cooled exhaust valve guides to augment the heat transfer ability of the cylinder head. But if you own another aircraft with an engine with the exact same cylinders as the Mooney TLS and have the exact same problem, we have no solution. Our recommendation is to operate at a sufficiently low power and corresponding cylinder head temperature that the cylinder head can be expected to transfer sufficient heat from the valve and valve guide to prevent their early failure. CHT levels between 150 and 400 F ought to do it, but we cannot be certain of this fact."