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Tech Question

Jon-

This is actually a very complex topic.

All OEM cams, as well as MP's offerings, are measured and rated using SAE procedures, which calls for measuring duration, in crankshaft degrees, from/to 0.006" valve lift. This results in the common "advertised duration" that you see in factory catalogs, service manuals, etc., as is the case for your cam which sounds like the immensely popular - but outdated - stock 3838/440 Magnum grind.

But, as camshafts became "wilder", the California grinders began to realize that the true "area under the curve" - which, to a fairly large extent, determines how much HP an engine can make, can be more accurately demonstrated by looking at the duration from/to 0.050" lift. By 0.050", the "shape" or aggressiveness of the lobe has made itself known. So they decided to use duration @ 0.050", which, of course, results in a lower number. As you can see in the chart from Comp Cams' latest catalog attatched, there's a world of difference between the numbers. But, depending on the lobe's rate of lift, this difference isn't linear or uniform, so, despite what you may have read, there can be no accurate conversion factor to go from one number to the other.

As roller cams, and fast-ramp flat tappet cams designed for the "Chrysler advantage" - the large 0.904" lifters - have begun to proliferate, even the 0.050" number becomes less significant. You also need to know the duration at 0.100, 0.150, 0.200...etc. In other words, you need to see the whole cam plot and a calculation of the area under the curve. Most engine builders now have a cam checker device that can do just that, even superimposing the graph of you old and potential-new cams on the same sheet. Then you can visually compare the overlap, lift, duration, etc., and hopefully, make an informed decision about which cam you need.

However, it takes years of experience, dyno runs, etc., to get this right. That's why, for most of us, either on of the popular cam-picking computer programs, or a call to a cam grinder's tech line, is usually the best plan. At any rate that tech line should be able to give you duration at 0.006" and 0.050" for any cam they manufacture. It is possible to get a cam with dead-stock advertised duration, lift, and overlap, and yet mke a LOT more HP!

Unless the cam you choose is very, very little changed from stock, you must check for coil bind and V-P clearance. Or pay the price later.

Rick