Latest -- Increasing Oil Pressure in the New Cessna 172 and 182
by Bill Marvel and Bill Scott
Although Lycoming's official response to the discoveries we have published on this and other sites is that "they disagree," it surely does look like they have been reading and listening quite diligently. Witness the discovery we have made of the modified oil system in the engines installed in the new Cessna 172 and 182. Going out on a limb, we predict the same system will be in evidence on the new Cessna 206, which we have not yet seen.
A few months ago, we had an opportunity to view the two new Cessnas side by side, uncowled. From even 50 feet away, we saw something that to most people would not be notable, but to us caught our attention big time. What was it? A clear bit of evidence that Lycoming's eyes and ears have been focused on our work over the course of the past three years and that they have incorporated a subtle but meaningful change to implement its concept. That concept, of course, is the use of oil to augment exhaust valve and guide cooling.
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First a brief background and refresher. As we have previously pointed out, there are two oil flow paths to each valve. (refer to figure 1). One is via the hydraulic lifter to the pushrod and then to the rocker arm, where it terminates in an "oil squirt orifice." However, flow through this path is always minimal due to the fact that its source is the mushroom style hydraulic lifter. This lifter type cannot bleed down fast enough to provide a large volume of oil because to do so would compromise its primary function of maintaining a near-zero valve train lash.
The second oil flow path is via the clearance between the tappet body and the case boss in which the tappet body reciprocates in response to camshaft and valve spring forces. Oil from the main gallery squeezes past this clearance and flows in two directions -- 1. back into the case, and 2. down the pushrod shroud tube and 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 that we have previously pointed out -- that oil pressure is tapped from the right (co-pilot) gallery to lubricate the main and cam bearings (refer to figure 2) -- oil 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. (We should note here that the new Superior O-360 engine version for the homebuilt market is reputed to have an oil system design which addresses this pressure difference. We are told that in this engine, the right side gallery still provides lubrication for the main bearings but the cam bearings receive their oil supply from the left gallery.)
With that explanation, the significance of the oil system modification we saw on the two new Cessnas will be clear. When viewing both aircraft, the initial feature that caught our attention was the source for the oil pressure reading that is displayed on the cockpit gauge. On almost all of the older Lycoming engine installations, oil pressure for the cockpit gauge is tapped from the rear of the engine, at the oil pump. This location is the highest oil pressure point in the engine. In stark contrast, the source for the oil pressure reading in the two new aircraft is at the forward end of the copilot side oil gallery, which is the lowest oil pressure point in the engine. The steel tube that carries this oil pressure back to the transducer from the front of the engine is hardly an afterthought. It has 5 different 90 degree bends in both the vertical and horizontal planes to route it clear of other components on its way back to the rear of the engine (refer to figure 3a).
That, however, is not the punch line. The real significance is that both new planes have an upper oil pressure limit on the cockpit gauge of 115 psi rather than the 100 psi that is typical of most of the older Lycoming-powered aircraft. Figure 3b shows this red line on the gauge installed in a new Cessna 172. This is very significant. 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 taken at the oil pump location at the rear of the engine. This is a 25% increase in allowable maximum oil pressure compared to the 100 psi it was before. It 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 we mentioned earlier. Recall that flow through this path is directly proportional to oil gallery pressure. Increase that pressure by 25% and you increase the oil flow by a similar amount.
There is simply no plausible reason for this alteration of a long-established oil system design, other than to subtlety address the fundamental valve cooling problem that we have identified. With no fanfare and still no admission of the problem, Lycoming appears to be using this scheme to alter oil flow volume to the valves for greater cooling. When we pointed this new design out to several engine experts around the country, almost no one knew about it and had it not been for our knowledge in this area, no one would know of it now.
But there's more. Enter Robinson Helicopter. Their new aircraft all come with the 115 psi red line oil pressure gauges and older aircraft sent in for overhaul are also upgraded to this configuration. However, the oil pickoff point remains at the former oil pump location. Why is this? It is only conjecture on our part, but since both the new Cessna 172 and 182 have engines not previously used in these airframes, they were no doubt certified from scratch with the new oil pressure pickoff point and red line. Robinson helicopters come from an older, existing design, and one cannot simply change the oil pickoff point without additional testing and certification. Curiously, the limitations section page in the R22 helicopter's Pilots Operating Handbook (refer to figure 4) mentions this change and cautions the operator to abide by the limitation specified on the gauge in his aircraft rather than the one it will have after its next overhaul.
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, one cannot obtain confirmation from them. And since no one knows (yet) about this design change, there is little reason for owners to be suspicious of different oil pressure readings on a new engine from the factory. One who was suspicious, however, owns a Grumman Cougar twin and contacted us with an interesting story. When this owner bought 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. He was quick to note, however, that the new engines, which used the same oil pressure gauges in the plane and the same hoses and fittings, no longer ran in the middle of the green range for oil pressure. They in fact 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 would never reveal that it was discovered by Marvel and Scott with nothing more than a small budget, perseverance and working in our spare time when they had not identified it themselves.
Does this say something about how to gain additional exhaust valve and guide reliability until Bill Scott comes out with his new design of critical components? Most likely it does. Crank up the oil pressure to the maximum you can, subject to the limitations on your engine installation. Although 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 and ask yourself some questions. Who set the green arc and red line limitations in the first place? On what basis did they set them at the specified limits? Were those limits realistic at the time they were set, based upon their understanding of the engine at that time? Since we have subsequently found the correlation between low oil flow to the valves and increased valve and guide distress, are these limits still realistic for the current situation, given the new information? And finally, since Lycoming is now, without explanation or justification, increasing the maximum upper red line oil pressure 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? Draw your own conclusions and do what you want, but the picture here is about as clear as you can ever hope to see.
Since this information on the new Cessna engines/oil pressure systems has not yet been in print anywhere, you are reading it here for the first time. (Today's date is November 15, 1998). No one who we have told about it, and this includes many of the country's engine experts, has any explanation for it aside from ours. Take it to heart. And, as to the existence of this new information on our web site, tell a friend.
UPDATE!! In the above article we mentioned our prediction that the new Cessna 206 would also incorporate the same oil system modifications contained in the 172 and 182. We were correct. On March 1 we saw the new 206 for the first time and verified that it does indeed have this modification.