I was told by a cam tech that the duration figures from a hydraulic cam and a solid cam are not equal (I'm paraphrasing). In essence, his point was a solid cam needs higher duration #'s to equal that of a hydraulic cam. Example, on a big block street motor say the hydraulic had duration at .50 of .230 int and .238 exh. His point was that if you installed a solid with the identical numbers that it would be "smaller" as compared to the hydraulic.
I understand the fact that the solid cam's valve lash must be deducted to detremine an accurate valve lift amount. But I hadn't heard about the difference in duration between hyraulic & solid.
If true, then in my example mentioned above, how much should the duration of the solid cam be increased so it is "equal" to the duration of the hydraulic ( such as: +.05, +.08, +.10, any guesses?). In essence, does this mean you can/should run more duration in a solid cam (without it becoming too much cam for the motor)? Another camshaft mystery to me!!!
When the lobe comes around to the lifter it has to raise the lifter an amount to take up the slack of the rocker to valve gap. The figure that the cam mfg. provides are from the start of lift, not taking in to acount the duration the cam turns before the lifter takes up the slack and causes valve contact. Unlike the lift this measurement is not something that you can calculate out unless you know the ramp angle.
Measured duration is measured duration, whether the cam is designed for mechanical or hydraulic lifter ramps, but measured duration does not necessarily represent EFFECTIVE duration on the engilne. You can't compare durations at .050" lifter lift between a mechanical and hydraulic cam because this number includes the clearance ramps, which are higher on a mechanical lifter than hydraulic lifter cam. It's apples and oranges.
SAE standard J704D calls for duration measurement at .006" VALVE lift and this spec is what manufacturers usually use for "advertised duration". If you look at the info in the front of the catalog a statement to this effect will usually be evident.
Actual duration is probably a little less than specified due to variation in rocker ratio. Advertised duration is usually based on a constant rocker ratio of 1.5:1 on a SBC, but the rocker ratio is neither 1.5:1 on OEM rockers, nor is it constant. Based on my measurements, the OEM rockers start out at about 1.37:1 and peak at about 1.44 on cams with about .450" total specified valve lift.
Finally, mechanical lifter cams usually have more aggressive initial acceleration, so for the same .006" timing numbers, a mechanical lifter cam will usually make more top end power, and about the same low end torque as an identically specified hydraulic cam. Mechanical lifter cams typically have better torque bandwidth than a similar hydraulic lifter cam.
Bottom line is published cam specs can only be used to make gross differentiations between cams. If you want to make a detailed comparison, you have to take a lift-crankangle diagram, then compute the areas under the inlet and exhaust curves and the overlap area in order to accurately compare two cams and make a judgement on their comparative performance.
The old OEM high performance cams specified total duration at some lift off the base circle that GM never specified, but it included a lot of ramp on both mechanical and hydraulic cams, so it's impossible to compare aftermarket with OEM cams using total duration. The only exception is the Duntov cam where total duration is based on the point where clearance is taken up and the valve just begins to lift of the seat.
In order to make sense of all this I took lift crank angle diagrams of all the OEM mechanical lifter cams and computed the .006" valve lift points using actual measured rocker ratios and rocker ratio compensated clearances. This is the valve event data I use for simulation programs as most of them give best accuracy with the timing points specified at .006" actual valve lift. The specified 287 degrees duration duration for the Duntov cam corresponds to about 270 at .006" valve lift. The 30-30 cam is specifed at 346 degees duration, but actual duration at .006" valve lift is about 292/298 and the LT-1 cam is 277/292.
Hey Duke, when will you be ready to chassis dyno your car with the LT1 cam? Anxious to see it as I'm wondering if mine would run better with that cam and our builds are otherwise very similar.
Thanks very much for the great technical explanations. That is what I needed. I'll start comparing the advertised numbers instead of the .050 #'s. Thanks again!!!
Hey Duke, when will you be ready to chassis dyno your car with the LT1 cam? Anxious to see it as I'm wondering if mine would run better with that cam and our builds are otherwise very similar.
Don't hold your breathe. It's a ways off, but in my simulation reseach I have yet to find a cam that produces better torque bandwidth than the LT-1 cam.
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cam duration question
