Why Cams Break
15 June 1999
Dear Hank,
After speaking with you on the phone yesterday, I have been giving thought to your Question as to why the Yamaha FJ-1200 engines have a problem breaking intake camshafts. I have considered all of the possibilities that come to mind, and the one that seems most likely to me is that there is a large torsional vibration at a frequency close to the natural frequency of the camshaft. Obviously, it would require an engineering study to confirm or deny this supposition, but the evidence points in that direction.
One of the things we have observed here at Megacycle is that we have no greater instance of failure with much more aggressive cam profiles than the stock Yamaha used in the Legends cars. In fact, if anything, we see fewer failures with radically increased cam profiles running against much stronger than stock valve springs at higher than stock RPM's. For this reason, I don't think the failure is a simple over stressing of the camshaft due to normal bending or torsional loads. The stock intake and exhaust camshaft castings, although not exactly identical, are very nearly so. If the failure was caused by a normal load condition, one would expect to see approximately as many failures on exhaust cams as intake cams. Both cams use the very same profile, and all of the valves use the same spring force. The difference in weight between intake and exhaust valves is negligible for this comparison, so all of the ordinary loads are essentially the same intake to exhaust. The question then becomes: If all of the above conditions are so similar, why does the intake cam fail but not the exhaust? I believe the answer is that the cam drive chain goes directly from the crankshaft to the exhaust camshaft, with a relatively long run of chain between the two. This allows the exhaust cam to be driven relatively smoothly. However, the intake cam is driven downstream of the exhaust cam, with a relatively short run of chain between them. Therefore, the intake cam sees a much less smooth driving force than the exhaust. Combined with the normal torsional reversals associated with driving a camshaft with only four pair of lobes on one shaft and relatively little overlap from cylinder to cylinder, I think it is very likely that a torsional vibration is set up which kil1s the intake cam. By switching from the stock cast iron camshaft to a Heat-treated steel camshaft, we will not substantially change the load condition. However, the steel cam is several times stronger than the original one, and therefore much less likely to fail. From a horsepower production standpoint, by using the original profile, there would be no difference in performance.
Sincerely,
Jim Dour
