Be The Camshaft Expert
We'll show you how, but you'll need to leave those old myths at the door.
February 24, 2009
By David Vizard
Photography by David Vizard, Johnny Hunkins

Intake Duration
Here's the intake cycle from start to finish. The duration (in this example 270 degrees) is indicated by arrow three. The degrees opening before TDC is indicated by arrow one, and the degrees after BDC to the closing point by arrow two. Number four indicates the intake centerline angle, which is exactly halfway between the opening and closing point

Lobe Centerline Angle Comparison
What's it worth to get the LCA right? In this graph the difference between three LCAs can be seen. The optimum for the 350 small-block mule motor was 108. A point to note here is that output was lost faster when the LCA was too wide (111) than when it was too tight (105). Although the 105 was a good match for output over the rpm range tested, it did lose idle quality and vacuum over the 108 LCA cam, hence the choice of the 108 as the best all-round deal.



Lobe Centerline Angle Determination Chart
To use this chart, first establish the number of cubes in the cylinder per inch of valve diameter. To get this number, divide the engine displacement by the number of cylinders, then by the intake valve diameter. Find that number on the vertical axis, then move across to the green line. At the intersection point, drop down to the base and read off the LCA required. Because big-block Chevys have angled valves, they need to have about 2 degrees less than this chart indicates.

302/347 Small-Block Ford LCA Cam Test
When used in the 302, the 276/280 roller hydraulic cam on a 110 LCA proved optimal, as even a 1 degree change either way produced worse results. Using a SCAT stroker kit, this engine was stretched to 347 inches and re-tested with the original 110 LCA cam. The stroker kit really helped both power and torque. When the 110 LCA was replaced with a more appropriate 108 LCA cam, the output made a further jump to the tune of some 20 lb-ft and 20 hp. The 108 cam in the 347 gave as much in terms of idle and vacuum as did the 110 LCA in the 302. Dozens of tests such as this show conclusively that the overlap and LCA--not the duration--are the first steps toward generating a cam spec

Output Versus Duration

Here is what happens when duration is increased, but all else is held constant. An important point to note is that there is no real big change in peak torque values, only that they occur further up the rpm range.

Output: Lift Versus Duration
Note how, for the same top-end output, the shorter-duration higher-lift combination made the same top end, but much better low speed power than the longer, lower-lift combo.

Spring Surge: What Is It?
Spring surge is when the spring can no longer fully control its own mass. This situation is caused by the spring being excited by a force that coincides with the spring's natural vibration frequency. Imagine a hammer striking the end of a spring. The impact (not unlike that of a valvetrain about to open a valve) will take time (be it very short) to travel down the spring. The effect of the impact is to close up the space of the top coil and the one immediately below it. This closing of the coils passes down the length of the spring until it reaches the bottom coil. There it is reflected and returns to the top. The point to note is that as it reaches the spring seat, the poundage the spring delivers is substantially reduced. In severe cases the lower coil of the spring can actually jump off the spring seat. When a spring experiences this sort of internal motion, it is in no way able to control the valvetrain.

Setting The Record Straight - Exploding Valvetrain Myths

Bump and Grind