Unique case, wearability
szicree said:
I'm going to Texas in a couple of weeks to build up a Superior O360. Originally I had selected flat lifters, but am aware they offer a roller version. Their website claims better durability, but gives no hard data. It also seems to imply that the cam on the roller engine is less "rampy" than the conventional model. This is the opposite from how things work on cars, so I'm a bit confused. Anybody have any factual data on the pros and cons?
My opinion, if starting from scratch today I would go for it. Factual data is a little thin right now except in the deep dark dungeons of Lycs R&D, but I did talk to them.
Lycoming service guys could not shut-up about roller cam. They said they wish (as a company) they had gone to it sooner and think it's a real improvement. What improvement?
Like all things "roller" it does free up some HP, but from what they said it seems that they love the way it wears and solves any and all existing/potential cam problems we now have.
Not that Lyc flat tappets are bad, but we all know cams and tappets wear. They make TBO all the time consistently, but the design relies on splash lubrication only as you know. With the cam sitting top center in the case, it drys out if it sits too long. The work around (besides flying often) for cam lubrication is the aftermarket cam oil squirter mod, but really the problem is still dry start-up's after a long periods of sitting. In GA planes those periods of non use are the problem.
I recall from what I was told, you can not convert a Lyc Non-roller case to a roller cam case. I'm not 100%, but I think its a unique case and conversion of older cases is not possible. Lycoming did the roller thing not to increase power but increase wearability. I think that is the main advantage, wear and may be a few HP as a bonus.
Aftermarket roller cams where widely coming out for cars in the late 70's. They had their issues with cam walking, especially with some engine blocks that where not perfectly square with respect to the lifter bores, cam and crank.
Rollers cams allow you to run massive valve spring forces which enhances the RPM potential,
lowering valve float at high RPM. That does not apply to Lycs, plugging along at 2,500 rpm with a red line of 2.7k. I'm not sure what you mean by "rampy", but assume more aggressive lifts and profiles. Ya, I don't think Lyc did anything radical, but the roller allows valve train and valve forces to be "smoother". The engine designers, engineers call it
valve train "jerk".
"Although high accelerations are needed to give rapid opening and closing, too rapid a change in acceleration - the 'jerk' or 'jerk rate' - will give rough operation due to the sudden changes in forces. For this reason cam profiles are designed not to give very rapid changes in accelerations."
It's the acceleration of acceleration if you will, inertial loads. Clearly not slamming the valve down would have some anti-wear-n-tear pay off, as well as opening it quicker, with less overall "jerk". Roller cams have more options for the designer of the cam profile. Again most of that is for high rpm applications, but I see some practical advantage for the whole valve train.
Is it worth it? Don't know, but if you fly rarely it would have a clear advantage. I have flat tappets and don't have roller cam envy. However building the dream engine, sure why not. It's only money.