[screamingchief]

Originally Posted by 73T/A

Ken,I recently had a similar issue with a Melling SPC-8(041) cam.We were degreeing the cam with Cam Logic equipment and also checking the cam specs.The lobe lift was just as advertised .313 but the intake and exhaust durations were not as advertised.We then picked up a second new Melling SPC-8 and the readings were the same as the first one.I then called Melling tech and told them about the issue.They stated that I could mail them the cam and they would check it.I never did send it though.It was my understanding that CMC(Camshaft Machine Company) in Jackson,Michigan ground most if not all of the flat tappet cams be it Melling,Comp,Edelbrock etc.Both of the Mellings had CWC(CMC upside down on them) stamped on them.

Just a note on this deal,understand that the factory cams were not rated at the same lifts as many "modern" cams are rated at.

Most modern cams are rated per current SAE guidelines,typically giving us the duration numbers @ .006" lift at the valve (which is usually around .004" at the cam,then multiply by the OE rocker ratio).

So .004 x 1.5 = .006

Now the '65 to '76 OE cams were typically rated @ .002" on the opening side,and .008" to .009" on the closing side,at least that's what was stated in the HO racing HPED&BPA book (page 78 first paragraph).

Which might explain some of these discrepancies.

So understand that how and where one is measuring the cam can make the duration numbers come out ever so slightly different than those OE cam numbers were.

Just where the cams duration is being rated needs to be known to accurately compare the numbers from one cam to another cam.

Crane had a bulletin about that a while back,I'll include the text here:

Originally Posted by Crane Cams

Why Crane Cams Measures Advertised Duration at .004 Lifter Rise

We are frequently asked why Crane Cams measures the Advertised Duration of hydraulic lifter camshafts at .004 lifter rise on the cam lobe (this applies to both flat-faced and roller hydraulic grinds) when several competitors measure their advertised duration at .006 lifter rise. The answer is that we attempt to comply with the SAE (Society of Automotive Engineers) J604 standard.This standard states that hydraulic automotive camshafts should have their duration measured at .006 lift at the valve.On pushrod engines, lift at the valve is the product of (cam lobe lift x rocker arm ratio). Most popular pushrod engines use rocker ratios of 1.5, 1.6, or 1.7.When you divide .006 lift at the valve by any of the popular ratios, you come up with .004 when the answer is rounded to three decimal places. Using .006 lift at the lobe results in at least .009 lift at the valve.That is quite a difference from the SAE standard of .006 lift at the valve.

This difference in measurement of advertised duration does make the intensity of Crane Cams lobes appear to be less aggressive when compared to the competition.Camshaft Lobe Intensity is frequently defined as the difference between the advertised duration and the duration @ .050 lifter rise.It is generally accepted that the lower the number of degrees of difference between these two figures relates to the greater the amount of Camshaft Lobe Intensity (or aggressiveness in the lobe design).Camshaft Lobe Intensity is only valid, however, when the advertised duration of two camshafts is measured at the same point.At first glance, it would not appear the .002 of lifter rise measurement would make much difference in the perceived Lobe Intensity.The fact, however, is this is the first part of the lift curve where any clearance in the valve train is taken up,and the lift rate at this initial point is at its slowest point.

Consequently, .002 lifter rise difference can create a significant misrepresentation of actual Camshaft Intensity. When comparing lobe profiles, it is best to compare cam lobes at exactly the same points.

Keep in mind that the discussion above is about hydraulic lifter camshafts in pushrod engines.For overhead cam engines (OHC) where there is no rocker ratio, Crane Cams measures duration at .006 valve lift.That creates different issues when comparing duration (and related power ranges) of camshafts used in pushrod engines as compared with OHC engines. See Crane's Engine Builder Newsletter #31 dated February 22, 2007 @ www.cranecams.com.

So who is right and who is wrong here?At Crane Cams, we are only claiming to comply with the SAE J604 standard.That way, Crane Cams can be evaluated with other cams that comply with the accepted world standard.

Hope that helps.

Bret P.