Don Terrill’s Speed-Talk

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

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.

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.

Can a cam without proper decel ramps be reground just to add the ramps? It is the cam I posted about in this thread earlier.

This cam was very powerful but very noisy. Turns out it had no decel ramps at all. I was able to return it for a refund. I see they are now selling it as used at a discount.

This is a DOCH inverted bucket cam. New cams of the same type call for 0.006 to 0.008 valve clearance. Could this cam be reground just to add the decel ramps?

How much would something like this cost?

You can't just grind ramps in. Whole cam must be re ground, you'll need a profile designed with ramps.

It'll take about .010" to .020" off base circle diameter just to put the ramps you're looking for in , with cam shape being close or equal

I first saw this thread the other day. It started out with

This cam is made by well known Italian cam maker Colombo & Bariani for the Fiat/Lancia twincam engines. However, it is not a grind I can find on their website... I set the clearances per cam maker recommendation, 0.016". There is barely any decel ramp..."

This doesn't make sense.

I used Colombo and Bariani cams in Alfa Romeo fast road engines for many years. I always understood that they were an OE supplier of camshafts to Alfa Romeo. They were ground on what looked like the factory's forged steel blanks. And they shared the long conservative opening and closing ramps that the factory used. Valve clearances were usually Alfa's own ~ .425mm and exhaust .475, but I have C&B application sheets where intake lash was opened to as much as .55mm to shorten the long ramps. Those long opening and closing ramps were ridiculed by Alfa club guys. C&B made--continue to make--cams for a variety of European engines. I suspect that there was a measuring error, compounded throughout the thread.

Colombo and Bariani had a system for identifying cam profiles with codes on each shaft. More than one system. They changed that system several times, but the company was always helpful.

--Carter

Carter, I appreciate your insight.

There was no measurement error. I profiled this cam multiple times both in and out of the engine as I was corresponding with the vendor. This cam has no decel ramps and was extremely noisy.

The lack of a ramp will cause a shock force during opening and closing but will not affect float or bounce.

Float is a function of valve acceleration, valve mass, spring force, and rpm.
The existence of float will lead to valve bounce since the valve has lost contact with the cam lobe and will
crash into the seat, ramp or no ramp.