Quote:
Originally Posted by Studebaker
Before you say anything, I know this post is a months old. I have had numerous failures with Compcams camshafts in the last 10 years, I no longer use them. They always have some excuse not to warranty anything. I have had real good luck with Isky and Howards brand cams. I do not believe the ZDDP excuse. If this was true, small four stoke engine camshafts (Briggs& Stratton, Honda, Yamaha, Etc) would also be having record number of failures, but this is not the case. Yes, there is the arguement that these small engines do not have high valve spring pressures but neither do most mild SBC and BBC rebuilds. You be the judge, I know what my verdict is, I will never use Compcams products again.  |
The Root Cause of the problem with modern fast lift flat tappet cams is just that; "fast lift". This works really well for making power by getting rid of the extremely long ramps used so often on the engines of muscle cars back in the day and are still popularly made and sold by Howards, Engle, Isky and even Comp and Lunati among others. We get into what Harvey Crane calls "Hydraulic Intensity"; that being the amount of lift per degree of duration expressed as a ratio. The more aggressive the lift per degree the more important ZDDP becomes as such lobes require a lot of spring pressure to keep the lifter tracking the lobe shape. This causes the interface between the lobe and lifter to squeegee the oil from their interface. But the attractive advantage of the fast lift rate (actually the fast settle rate in closing the intake) is the reduction in the loss of mixture mass from the reversion forces of the rising piston (the essence of DCR calculation). This builds much stouter bottom end torque reducing the need for low ratio gearing and it keeps the power peaks from jumping into considerably higher RPM bands than the older slower rate cams, while retaining all the power a long duration gives at the top RPMs. But the big downside is wear, this is actually a two sided coin where spring pressure is concerned where the recommended high pressure springs wipe lube from the lobe/lifter interface right at a point in history where ZDDP is being greatly reduced (ZDDP provides a constantly renewing dry lubricant into the interface that is not easily squeegeed by the forces in that interface); the flip side is where a lot of people get into trouble not using the comparable kit for these high rate cams which then lets the lifter bounce on the lobe when the valve is suddenly closed. The lack of spring pressure may even allow the valve to loft going over the top of the lobe then slamming the lifter back into the lobe after said lobe passes out from under the lifter. These are not Briggs and Stratton kinds of problems. The OEM addressed this issue back in the mid 1980s with a near universal change to roller cams, only trucks with very mild cam timing and lift soldiered on with flat tappet cams for a few more years all of these gone to rollers by 1996.
Best advice I can offer is use a retro roller kit in anything but a stock truck engine rebuild. Second best advice is if you just gotta have one of these fast ramp flat tappet cams along with it, put in the thrust snubber button used for old fashion roller cams in these early blocks, these at least relieve the lobe/lifter interface (and the distributor gear) of having to keep a preload on the timing gear to block thrust faces. A little less for the lobe and lifter to do and it positively reduces and lateral excursions of the cam to .005 inch or a bit less so there isn't any (much) side to side rubbing of the lifter face against the lobe allowing it to spin in its bore without being dragged or pushed side to side at the same time.
Bogie
05-10-2012, 05:54 AM
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