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Cam lobe decel ramp - valve life?

General engine tech -- Drag Racing to Circle Track

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olds63
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Re: Cam lobe decel ramp - valve life?

Post by olds63 »

CamKing wrote: Wed Sep 09, 2020 10:31 am
You want the velocity at the lash point to be under .0008" per degree. If it's over .001", I would worry.
Mike, what is considered a safe velocity at the lash point for a classic pushrod actuated engine with solid roller cam?

Sorry for the hijack
Nut124
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Re: Cam lobe decel ramp - valve life?

Post by Nut124 »

FYI only, in case anyone is interested, I swapped out the "bad" cam and installed my old Alquati 40-80 intake cam.

I adjusted the clearances and checked timing. C/L is 105ATDC. Target is 107-108. Will adjust tomorrow.

BTW, this cam, as installed now, producdes 190psig cranking compression. the "bad" cam only produces 165 or so - much more duration. Or the rings seated between now and then.

This cam has about 243deg duration at @0.050 cam lobe. 234deg valve lift @50 as installed.

Timing data is attached for curious souls. I did two sets of measurements at valve open and two at valve close. The results are surprisingly consistent. I set the timing wheel with a piston stop plug and checked after done.

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David Redszus
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Re: Cam lobe decel ramp - valve life?

Post by David Redszus »

Following is Cam Dr data for an exhaust closing ramp showing cam angle, velocity (in/deg), velocity (in/sec), and valve spring seat force.

Cam...Lift....Velocity....Velocity....Springforce
angle..in......in/deg......in/sec......lbs

65....0.05332.....7.170....129.1.......127.7
67....0.03994.....6.160....110.9.......120.4
69....0.02876.....4.980.....89.6........114.3
71....0.02002.....3.740.....67.3........109.5
73... 0.01380.....2.540.....45.7........106.1
75....0.00974.....1.575.....28.4........103.4
77....0.00726.....0.945.....17.0........102.5
79....0.00574.....0.590.....10.6........101.8
81....0.00486.....0.415......7.5.........101.2
83....0.00407.....0.370......6.7.........100.8
85....0.00332.....0.375......6.8.........100.4
87....0.00332.....0.375......6.8.........100.0
89....0.00187.....0.355......6.4..........99.6
91... 0.00116.....0.355......6.4..........99.2

Note that in this example, the maximum ramp height is only about 0.005" and lasts for about 10 cam degrees.
During this period, the velocity is almost constant. Also note that the spring seat force becomes less as the valve settles down on the ramp. It is very doubtful that valve velocity nor spring force could have an adverse effect on the
valve closing.

Closing, or opening velocities are not usually a cause for concern. The problems of float and bounce lie elsewhere.
If we were to examine the entire lift curve, we would be able to determine where a possible problem might
develop due to the valve acceleration and instant spring force.
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