question for duke willams

[Jeff C.]

I have a 1963 340hp car. I have unreliable history from previous owner. He bought it on e-bay. It is definitely a solid lifter cam but which one? Might be the correct one or one of the other GM cams or a after market. I do not have the equipment to determine lift and duration. So what is your recommendation that I set the valve clearances?
Thanks,
Jeff

[Duke W.]

Re: question for duke willams

...not enough info for me to make a recommendation, but some additional data will give a clue.

Measure and report vacuum @ idle speed. Report the number on the VAC and use a vacuum pump to verify that it meets the start and stop vacuum specs, and also report total idle advance. That would be with the VAC connected.

Then the final step would be to measure current valve clearance. Use the indexing scheme in the Hinckley-Williams valve adjustment paper and measure/report the requisite valve clearances at each 90 degree interval.

Measuring gross lobe lift gives the best clue and dial indicators are not that expensive. You set the pin on the rocker pushrod socket in line with the pushrod with the lifter on the base circle using the Hinckley-Williams indexing scheme, set clearance at zero, then rotate the engine until peak lift is achieved. Measure at least two inlet and exhaust valves and record the data. If the data is not consistent, measure more lobes.

Actually I look at the TDB every day, usually in the morning.

Duke

[Ed H.]

Re: question for duke willams

... set clearance at zero...

Duke

Do you mean adjust the dial indicator to 'zero'??? (I think).

[Duke W.]

Re: question for duke willams

Both. When the dial indicator shows you are on the base circle, zero the indicator and set clearance to zero because the lobe lift dimensions are gross, which includes mechanical lifter constant velocity clearance ramps, and with an unknown cam we don't know how high they are.

Of course the zero clearance is ONLY for measuring gross lobe lift, and if you go around two complete crank revolutions and record lift about every ten degrees you can pick out the tops of the clearance ramps within a couple of thou by plotting the data. With a mechanical lifter cam there will be long constant slope lines at the beginning and end of the lobe. The slope is velocity, and once it begins to inflect up, that's the top of the clearance ramp, which allows you to determine proper setting clearance of 1.37 (the actual rocker ratio at lash takeup) times the clearance ramp height.

If you use two indicators, one on the valve retainer and one on the top of the rocker pushrod socket, you can compute rocker ratio behavior, which will be, for a small block, about 1.37 at low lift and about 1.44 at max lift with a 0.3" lobe.

Duke

[Jeff C.]

Re: question for duke willams

Thanks, Duke
I am going to "good old harbor freight" to get a dial indicator. I have been using your 097 specs for the lash. I did check vac at idle some time ago and it seems to me it was high 12 to 14 inches. as soon as I fix an ignition problem I check it again. More info to come.
Jeff

[Ed H.]

Re: question for duke willams

...The slope is velocity, and once it begins to inflect up, that's the top of the clearance ramp, which allows you to determine proper setting clearance of 1.37 (the actual rocker ratio at lash takeup) times the clearance ramp height.

...
Duke

OK, I'm working to get my head around this, so let's start here. "The slope is velocity, ...". That's the first thing I don't understand.

[Duke W.]

Re: question for duke willams

It's junior high algebra. If a body moves from rest and you measure distance versus time and plot the data on an x-y graph the slope at any point, distance/time is velocity, say feet per second. If the slope is constant, velocity is constant, and the steeper the slope the higher the velocity.

The purpose of the long constant velocity clearance ramps on mechanical lifter cams is to take up clearance at low velocity to minimize valve train shock loading. When the engine is run hard, clearance, especially on the exhaust side can close up a few thou compared to cold or low load due to valve stem expansion. If there were no clearance to compensate for this the valve would not seat and rapidly burn.

Duke

[Ed H.]

Re: question for duke willams

Ah ha - a vector! Interesting explanation. Thank you!