Date: Thu, 12 Nov 1998 22:25:25 -0600 (CST)
From: Bill Marvel
Subject: Valve problems
Although I intentionally do not participate in the Grumman Gang, or for that matter in any AYA events anymore, I do occasionally get a message forwarded by Stew Wilson. As you know, I have been working with Bill Scott extensively for the past three years uncovering the core problem in the design of Lycoming engines.
I just read Bill's response to you and pointed out to him one thing he omitted in his remarks. This is very new and is unknown to practically everyone. In fact, we were the first ones to point this out to most of the editors in the aviation media. It is a major item, however, and involves valve cooling in both the new Cessna 172 and 182. If you want to share this with the gang, feel free.
First a brief background before the punch line of all this. As Bill Scott correctly pointed out, there are two oil flow paths to each valve. One is via the hydraulic lifter to the pushrod and then to the rocker arm. This path has practically no possibility of providing any significant valve cooling. The second path is via the tappet body and case boss clearance, down the pushrod shroud tube and then onto the exposed valve and guide. Our testing has proved that this latter flow path is the one that has significant capacity to augment valve cooling with oil. It is also this flow path that explains why most valve problems occur on the copilot side of the engine. Oil flow through this second path is, for a given oil temperature and viscosity, directly proportional to oil gallery pressure. Due to a design characteristic in the Lycoming engine case, gallery pressure is ALWAYS greater on the pilot side than on the copilot side. Thus the copilot side cylinders receive a lower volume of cooling oil to the valves than do those on the pilot side, accounting for their higher failure rate.
With that explanation, the significance of the engine modification on the two new Cessnas will be clear. We had a chance to see both the new 172 and 182 side by side and uncowled. Even from 50 feet we saw something that caught our attention big time. In the two new aircraft, oil pressure for the instrument panel gauge is taken from the forward end of the copilot side oil gallery, which is the lowest oil pressure point in the engine. This is in stark contrast to where oil pressure is taken in almost every older Lycoming engine -- from the oil pump, which is the highest pressure point. That, however, is not the punch line. The real significance is that the two new planes have upper oil pressure limits on the gauge of 115 psi rather than the 100 psi that is typical of most of the older Lycoming-powered planes. In normal operation, there is about a 10 psi oil pressure drop from the rear of the engine to the front due to losses from lubrication requirements. If you take this typical 10 psi drop and add it to the new 115 psi red line taken from the front of the engine, it is equivalent to a 125 psi red line at the old oil pump location. This is a 25% increase in allowable maximum oil pressure compared to the 100 psi it was before. This translates into a 25% increase in allowable operational oil pressure at all times, which in turn provides additional oil to the valves via the tappet body and case boss clearance flow path I mentioned earlier. Recall that flow through this path is directly proportional to oil gallery pressure. Increase the pressure by 25% and you increase the flow by that amount.
There is simply no plausible reason for this alteration of a long-established oil system design, other than to secretly address the fundamental valve cooling problem that Bill Scott and I have identified. With no fanfare and still no admission of the problem, Lycoming is using this scheme to alter oil flow volume to the valves for greater cooling. As I said, almost no one knows about this and had it not been for Bill's and my knowledge in this area, no one would know of it today.
Is Lycoming actually setting up higher oil pressure in their engines to fulfill our explanation of this process? Since they do not reveal information to anyone, let alone us, we cannot obtain confirmation from them. And since no one knows (yet) about this change, there is little reason for one to be suspicious of different oil pressure readings on a new engine from the factory. One who was suspicious, however, is Stew Wilson. When Stew got two factory reman engines from Lycoming for his Cougar, he swapped out two high-time engines that were running fine but were at TBO and had experienced the usual valve and guide problems. They ran with mid-green oil pressure. Stew was quick to notify me, however, that the new engines, which used the same oil pressure gauges in the plane and the same hoses and fittings, ran at upper red line oil pressure in cruise flight with hot oil. This is, in our view, no coincidence. It is the result of Lycoming's intentional plan to try and quietly resolve a problem that has existed for over 30 years and that to this day they refuse to admit. And they most certainly could not reveal that it was discovered by Marvel and Scott with nothing more than perseverance when they had not identified it themselves.
Thus comes the missing hint for engine longevity. Crank up the oil pressure to the maximum you can, subject to the red line limitation on your engine installation. Although it is true that Lycoming is increasing the red line on the new Cessnas, you cannot legally do the same on your existing aircraft. You can, however, operate with increased pressure, so long as it does not violate any of the engine's limitations. Do a reality check here. Who set the green arc and red line in the first place? Why did they set them at the limits they did? Were those limits realistic at the time they were set, based upon their understanding of the engine at that time? Since Marvel and Scott have subsequently found the correlation between low oil flow to the valves and increased valve and guide distress, were these limits realistic in the first place, given the new information? Since Lycoming is now increasing the upper red line by 25% and thus increasing the allowable operating pressure by the same amount, why should you not do as much of the same as you can within your installation's limitations? Do what you want, but the picture here is pretty darned clear.
Since this information on the new Cessna engines has not yet been in print anywhere, you are reading it for the first time here. No one who we have told about it, and this includes most of the country's engine experts, has any explanation for it aside from ours. Take it to heart.
HERE'S A THOUGHT --
Control what you can control..
Influence what you can influence...
Life's easier if you can learn to live with the rest....
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