How do you determine what the advertised duration is as cam companies use different measureing points

Well thats a tough one.... as most cam mfgs use different checking heights depending on the cam design...especially mechanicals.

Heck just a "few" of Crane's mechanicals are :
.010
.012
.014
.015
.016
.0162
.0195
.020
.0205
.0225
.024
.0245

With no doubt a few I've simply forgot about.

Many use the end of the CV ramp ( if one exists ) as a checking point but NOT ALL mfgs do that on all thier lobes.... so unless you are familiar with lobe designs and can plot the lobe the best thing is to simply call the mfg and ask them....
Keeping in mind that some cam mfgs have a bad habit of changing grinds and keeping the same part number...so a part number 123456 from say 1975 may be a different lobe from part number 123456 from 2003.Same part number but a different grind.... this is especially true of a couple well known old name cam mfgs that have been around since the 1940's and 1950's.... Although some newer ones are guilty also.

They all have different theories as to where and how to measure their advertised durations.... on some lobe spec sheets they dont even tell you where their advertised durations are at and others tell you more info than you can shake a stick at.

Best thing is to call them....and if you have some really old cam try to find someone at that mfg who has been around awhile.....As an example a guy I work with had an old Engle cam that he lost the cam card for.... he used this cam in the late 60's to early 70's and wanted to reuse it but forgot the lash settings.... He called Engle and due to the cams age they didnt have any info on it.... It just so happened that Jack Engle was in the shop that day and he was able to speak with Jack Engle (Jack designed the lobe ) and Jack happened to remember that lobe and gave him the lash setting.... but if Jack wasnt there he may never have really figured out the correct lash setting.

Good luck.

To be technical, the advertised duration is whatever the cam company advertises it as. What you are really asking about is the seat to seat duration. The .050" duration came about due the the wide varity of lifts used by various companies. GM was the worst using .000" lift and advertising durations in the 340-350º range. Most companies use .004 or .006" for hydraulic cams. .004" is the SAE standard and is suppose to account for valvetrain compressiblity. So as the lifter rises, at .004" the valve should be just starting off the seat. Using .006" makes the cam look slightly smaller on paper than it actually is. How much depend on how the cam is designed.

Solids, on the other hand, have lash which has to be accounted for. Even at .050" tappet lift, the valve will not be as far open as with a hydraulic cam since a portion of the tappet lift is used up taking up the lash.

The formula for determining the correct tappet lift for seat to seat duation is "lash / rocker_ratio + .004". This means the seat to seat duration is heavily effected by the lash and rocker ratio. Changing lash causes the duration to change. I know of no companies that spec the duration using this formula. It seems they pick a tappet lift that is close for as many cams as possible. .020" is very popular. However, this tappet lift is only accurate for .024" lash with 1.5 rockers or .027" lash with 1.7 rockers. For all other lashes the seat to seat duration will be somewhat longer (less lash) or shorter (more lash) than the advertised duration.

Of course, you can do what most everyone else does; not worry about it and just use the number the company provides.

This is what I've done.....


When I was designing at UltraDyne, I used .0043" on the opening side, and .00477" on the closing side, for hydraulics, and called them both .0045" to have an easy-to-remember number. The SAE standard is .004", and I figured that was close enough. Besides, back then all my ramps were designed on a Texas Instruments pocket calculator, and when I got close to what I wanted, well, close was good enough.
On solids and rollers, I designed several good ramps that came out at .01734". and that times 1.5 was .026" valve lash, plus .004" for deflection, gave .030" total, and divided by 1.5 equalled .020" for seat timing. I kept the .020" number for 23 years, even when I designed NR rollers(Inverted-Flank stuff) with .0094"-high ramps.
Most(all, so far...) of my Holley stuff is at .01066667" for solid and rollers, giving .016" lash with 1.5s, and rated at .015", because I want to compare to other cam companies rating their cams at .015".
There is about a 7° difference between .015" and .020" in cam duration. The cams would be 7° SHORTER if rated at .020".
Ain't life funny?

UDHarold

And just to mix more water with the oil some cam mfgs suggest that thier lobes ... oh lets say a .400 lift lobe with 300 deg @ .020 will have a .026 lash if used with a 1.5 RR and .030 with a 1.7 RR.... Yet others will say to use an additional .002 lash on the exhaust side vs their intake side and still others will use the same lash no matter what the RR is.

And to make some things even worse there are published spec's by big name companies who show on certin families of lobes to be rated @ .016 but have an .012 lash ! Others have published adv dur spec at the same as thier lash settings.... For many decades it was normal to have the adv dur point somewhere under the normal lash setting.IE advetised @ .014 with a lash of .022 or adv @ .020 with a .028 lash ect ..... These days those rules have gone by the wayside , makeing it much more difficult to compare lobes...... IE buyer beware..
You may have thought you were compareing apples to apples but sometimes they aint apples.