I have taken the measurements and reported the results with the last image. But the numbers reported do not tell all. There is more to it than that.
For one thing, the duration and overlap readings do not account for total valve opening duration; nor do they account for total overlap period. The valves are open for more degrees of rotation than those recorded. That extra opening time is at opening values less than 0.020in; ie the valve is barely open. BUT, it IS open for that longer time. That does have an effect; how much of an effect is also a subject of considerable debate and disagreement.
How much longer are the valves actually open? The T cam valves are open for about an extra 18 degrees of rotation on each side of the measured duration. In other words, the T cam valves have a total duration of 36 degrees longer than the duration recorded. The A cam valves have a total duration that is 52 degrees longer than the recorded duration: 26 degrees more on each side of the lobe.
The discrepencies between recorded duration and total duration can help explain some of the conflicting listings of cam specs: depending on where you start recording duration, the values assigned as duration will vary. If I had used the "regular" method of measuring duration from valve lift points of 0.050in, then my results may be more acceptable to some. But using the 0.050in standard for these cams would zero out some of the characteristics of these cams: namely, the characteristics of the clearance ramps; and the transistion of lift point I'll be bring up momentarily. I'll stick with the 0.020in lift duration point. I believe it to be more accurately indicative of the camlobe's action and operation.
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Sidenote [2-23-04]: I think there is something to the clearance ramp profiles that contributes to the durability and reliability of this valvetrain. I will investigate this further as I set things up to measure some other camshafts: the B, D, and V cams. Suffice it at this point to say that I will continue to quantify the volvo SOHC cams based on the 0.020in lift duration. Why? I suspect that the clearance ramp profiles are designed to 'set the stage' for the mushroom effect at the 0.020in lift. Right now, this is only a suspicion on my part; further measurements of the other cam profiles, and a re-examination of the A and T profiles to measure and observe that area of the profile, should give me evidence to either confirm or deny my suspicions. If I do find a consistency to the clearance ramp/0.020in lift/mushroom effect operation on all the volvo cams, I will then be very curious to see if the aftermarket cams offered for the redblocks also do that. It might be a tempest in a teapot; or it might be insightful as to why certain profiles do what they do. But to ignore what goes on before the "standard" 0.050in lift duration measurement point is, in my opinion thus far, to ignore something of importance.
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Whether I use my 0.020in lift, or the 'standard' 0.050in lift point to measure duration, one thing is for certain. There is a total duration of more valve opening than either lift point duration method would show; and that "extra" duration has implications not usually recognized. One of those implications is in just how the cam lobe interacts with the tappet.
How the lobe interacts with the tappet is what we will examine in this and the next several images.
To a person familiar with pushrod motors [engines that use pushrods and rocker arms to transfer cam lobe commands to the valves], the cam lobe profiles in the first two images of this gallery would appear to be a bit strange. The cam lobes shown look rather flat sided, if compared to the flat tappet cam lobe profiles normally seen in a pushrod motor. That observation would be quite correct.
The T and the A cams are also what is known as flat tappet cams. But there are two differences.
Volvo SOHC cams are "direct-acting" camshafts: the cam lobe works directly on the tappet; which then works directly on the valve...no rocker arm or pushrod in-between. And, these cams interact with the tappets as if the tappets were "mushroom" tappets. And mushroom tappets are not normally seen or found in pushrod motors.
Mushroom tappets, when used with a cam lobe profile designed for them, increase the speed of the valve opening or closing: ie the valve moves faster to get open or to get closed.
This image shows the point at which the clearance between the cam lobe and the tappet [the "lash"] has been taken up: contact has occurred; the valve is about to start to move.
