I have always thought that one would be lucky if the engine was not ruined, and the chances of it running at all were slim. I have asked other shop owners about this, and in fact none of them had even seen a cam installed that way. One guy said he kinda suspected that he worked on a Shovel that may have been that way, but like all the rest of us, he could not be sure because the cam almost always falls out when you take the cover off[1].
Seemed to me that in order to determine under what conditions it might be possible to have an engine run without self-destructing in this situation, one would have to figure out the effect on TDC lift at the valves if a cam was off a tooth. I looked around the net to see if there was an equation, with no luck. Some of the simulation software will do it, but none that I own will. I am not good at math. The concepts just don’t come to me the way they do other people, so it looked as if I’d have to just work it out with Excel and logic as best I could
There are only two circumstances where the piston is at TDC. The first is at the end of compression, and the other is at the end of exhaust. In the former, both valves are closed and are not a concern. However, nearing TDC on the exhaust stroke the piston is rising, the intake valve is opening, and the exhaust is closing. The exhaust is “running away from” the piston, but the intake is “running toward” it. It is the intake valve that is of most concern, as a result. If we know the TDC lift on the intake, and we know the opening point, we can determine the average lift per degree up to that point[2]. Real cam guys might correct me here, but I think for our purposes, this average lift per degree is adequate.
I decided to see what would happen if I looked at an actual cam. I had the figures for a Woods W9B handy, so that’s what I went with.
[1] Note that this is not the case with twin-cams, and I am not saying that TCs cannot run with a cam installed wrong. Maybe they can, maybe they can’t. I don’t know. I don’t really work on them much
[2] Even though the lobe lift rate will almost certainly change as the cam turns, it may not mean too much to us at this point. By the time TDC arrives, the cam lobe is at least 30-35% of maximum lift, so the rate of lift has slowed, and may be relatively stable for another 20-30% or so, until the thing starts to prepare to go over the nose of the lobe