Some of you will recall that a few weeks ago Swiss member Pat Patrik asked for help with his cams which did not seem to time up properly. He is new to Vincents and had enough sense to ask for help and advice. I cannot find the original posting so here is the latest. Pat set up a measuring system and sent me a value for the valve lift on both cams of his Comet at every ten degrees of engine rotation. We have had some discussions about this and the following graphs show what was found.
You will see that although the cams time up quite well there is something wrong at maximum lift on the inlet valve. I got Pat to check that the valve collar was not hitting the lower guide, the valve hitting the piston etc and he tells me that there is still plenty of potential lift left at this point. He asked a retired competent engineer and was advised that this would probably not make any difference to cylinder filling etc. It is hard to explain to some people that there is more to cam design that just making the valve open and close. For example one cannot just weld a bit extra on to the nose of an existing cam, blend it in and then expect to have a trouble free engine.
So the next step was to calculate the valve velocity and acceleration. The next graph shows the velocity.
Already one can see that there is potential trouble near the top of the lift on the inlet valve.
The next graph shows the acceleration figures.
Now look at the mess in that area. It will, of course, depend entirely on whether Pat intends to potter around at 3,000 rpm or whether he intends to vigorously overtake and go up to 6,000 rpm on occasions but if it is the latter then there is every chance of a valve head being pulled off. Perhaps those who advocate just going ahead and trying the cam would like to work out what the G force would be at 6,000 rpm For those who wish to do the sum note that the engine rotation figures are exactly that, not cam rotation figures.