One of the things they don't advertise is that the cam installed in the motor may have the same advertised lift and duration as the catalog says, but the cam the race teams got was far different. If you made a graph and profiled a given cam lobe, say, #1 intake and exhaust valves, you would find some real surprises. The lift may start at the same place on the degree wheel, and the maximum lift may be at the same point, but the rate of lift on the race only cam is way different with the duration at .050" being way more.The cam profile in a cam like an NHRA Stock Eliminator engine may have the exact OEM lift specs, but the actual duration at .050" on up to max lift is way more. The limiting factor for a lot of Stock and Super Stock motors is the piston to valve clearance at .050" lift or so. These cams have a very brutal series of valve events between those two points, and it is not uncommon to have small block Chevy motors with valve spring pressures of 200 PSI on the seat with 400 PSI at maximum lift. That's what is necessary to control valve float and valve bounce when it closes. Another thing, a cam that is ground to be optimal for a Z-11 might not fit a 409 without piston to valve clearance issues because the longer stroke crank moves the piston a bit sooner and a bit faster away from both TDC and BDC. The fact is that the 2nd design 409 hi po cam was designed for the second design head with the taller springs and the deeper spring seats just for that reason. The #583 cam might say it's for a Z-11 and the factory second design 409 heads, but in actuality it was only a standard replacement cam and not very optimal. It would be nice to find an OEM Z-ll cam and install it in an engine, set up an accurate valve profile measuring device, and compare degrees of actual duration with a 7735 service replacement cam. They probably wouldn't be any where close. Another air flow enhancer for an engine with valve port size restrictions is the profile of the back of the valve. OEM valves tend to be rather thick and heavy for longevity reasons, where true race only valves would flow significantly more air. My NHRA legal 327/250 horse '68 Chevelle motor has .390" intake lift and .410 exhaust valve lift but it sounds like a jackhammer breaking concrete when it is running because the heads and intake were rather carefully matched to it to take advantage of the rules any way we could. It is funny to fire it up and drive to the lanes or back on the return road after making a run because it is quite loud for 9:1 actual compression. The valve train wouldn't last too long on the street, however. If I took the blueprinted NHRA stocker cam out and put in an OEM replacement cam meant for Grandma's Caprice, the engine would idle at 500 RPM in drive, lay down just past 5000 rpm, and would make about 265 horse power at 4800 RPM. Can't have it all, that's for sure.