Cam specifications

[Bruce B.]

The attachment from the "62 Corvette Owners Guide" contains "theoretical valve timing" information on the 62 solid lifter cam used in the 340 and 360 HP engines.
Can this info be used to determine LSA?
Is this type data still used to define cam parameters?
Thanks,
Bruce B

MVC-028S.JPG

[Duke W.]

Re: Cam specifications

These data represent the duration measured from the top of the clearance ramps. Add 180 to the opening and closing point numbers and the result is 287 degrees. And since the Duntov cam lobes are symmetrical, the POMLs can be derived from the data. They are 108.5 deg. ATDC and and 112.5 deg. BTDC, and the LSA is just the arithmetic average of the POMLs, 110.5 deg.

Today, an SAE standard specifies "total duration" as the span between the .006" valve lift points, which would be .004" above the tops of the clearance ramps assuming the 1.5:1 theoretical rocker ratio. (It's not actually 1.5 - more like 1.37 at the tops of the clearance ramps and 1.44 at full lift.) By this measure, assuming 1.5:1 rocker ratio, the Duntov cam has 276 deg. duration on both sides. Incidently, the lobes are identical other than the exhaust side having .004" greater clearance ramp height - .012" versus .008" on the inlet side.

The aftermarket has long used the duration at .050" lobe lift, however, you cannot use this method to compare hydraulic and mechanical lifter cams because part of the .050" above the base circle on a mechanical lifter cam is clearance ramp height, which varies from cam to cam, and no valve motion occurs until the clearance is taken up. The "equivalent" .050" lifter rise for the Duntov cam - what could be compared to a hydraulic lifter cam - is measured at .058"/.062" lifter rise and is 220 deg. for both lobes, which is the same as the L-79 cam, but the Duntov has more effective overlap because of the narrower LSA. (The L-79 cam LSA is 114 deg.) If the clearance ramps are not taken out the Duntov cam, the .050" lobe lift durations are 228/231 deg.

By the same measurement method, the LT-1 cam is 231/239, the 30-30 is 239/239, and the L-78/72/71/LS6 cam is 231/231. Why do these cams share numbers? Because the LT-1 cam uses the big block lobe on the inlet side (with a slightly smaller base circle) and the 30-30 lobe on the exhaust side, and both of these lobes are asymmetrical - the final closing phase is slower than the early opening phase. Without taking clearance ramps into account, these cams' .050" lobe lift durations are 242/254, 254/254, and 242/242 respectively.

Despite longer duration, the wider 116 deg. LSA of the LT-1 cam provides about the same effective overlap as the Duntov measured in square-inch-degrees. The longer inlet duration and 114 deg. LSA of the 30-30 cam yields about 50 percent more effective overlap than the LT-1 and Duntov cam, which is why the 30-30 is so weak down low.

In fact, all these cams have too much overlap for best torque bandwidth. High overlap is only effective with headers and open exhaust (or a very low backpressure muffled exhaust), which can harness wave dynamics to create negative pressure at the exhaust port during the overlap period over part of the rev range. Manifolds and mufflers prefer low overlap (as is the case with modern Corvette cams) because they can't take advantage of wave dynamics to create negative pressure at the exhaust port during the overlap period to begin the inlet process early. With manifolds and mufflers there is always positive pressure at the exhaust port, so the inlet process cannot begin until the exhaust valve closes.

One of the problems with "cam specs" is that they are often measured using different methods, and the specific method and full context of the measurements may not be stated making comparison with other cams meaningless. Also, the "cam specs" that you typically see are very high level, and two cams with the same or nearly same "specs" may yield very different engine performance characteristics and require different valve spring designs depending on lobe dynamics that includes velocity, acceleration, and jerk, which is the rate of change of acceleration and is associated with shock loading.

Duke

[Bruce B.]

Re: Cam specifications

Duke,

Thanks for the information and the education.

Bruce B