Anyone ever heard of the 47.5% rule?

Ive been researching some camshaft stuff and I guess theres some guys at Isky that says for 1.5 ratio SBC's, the duration at .50 must exceed 47.5% of the total valve lift or your asking valve train problems. For example, take a Comp Cams Magnum 280H, with 230 duration and, 480 lift...230/.480 = 47.9% which exceeds 47.5% therefore would not pose a threat to components. Otherwise the lifters edge will dig into the cams surface and "scrape" the lope.

There has been alot of the spotlight on early cam failures recently and I was looking into it.
It seems to make sense that you can only get so aggressive with a ramp profile before you start "scraping" lobes off of camshafts with lifter edges.

What does anyone think?
Bogie?
Anyone?
Thoughts?

God bless
-Shred  

Yes, it's intended to keep the lift rate on the ramp from becoming too intense. Before the lifter digs (flat tappet)into the lobe, things are already stressed pretty hard. The unit loading between the lifter and the lobe is at the point where lubrication cannot be sustained and wear is going way up. The push rod is shaking in its' boots and lift at the valve is falling off because the rod is bending. Even the rocker is crying help as its surfaces are pushed to the maximum tolerable loads. That's why you get ball and cup failures, or trunion bearing failures on rollers, rocker tips get wiped out sometimes valve stem tip as well. Sometimes the rockers just busts in its beam section for no apparent reason as there no sign of contact anywhere else like the piston hitting the valve and slamming it backward into the rocker.

To shove the Chevy harder you've got to go to larger diameter flat tappets or to a roller cam. More expensive and exotic materials have to go into the components. The stud mounts have to be girdled together or replaced entirely with shaft mounted rockers. That's why a full up racing SBC starts around 30,000 bucks and just keeps going up. It may look a street SBC on the outside, but that's as far as it has anything in common with what we drive.

Bogie