Murray on Cams



"Random Speedster Notes"

by Murray Fahnestock

The following are excerpts from the above article which is reprinted in "Model T Speed Secrets - the Fast Ford Handbook"

When one installs a 16-valve cylinder head, or in some other way greatly increases the area of the valve openings, then the engine is no longer a regular Ford engine, and the usual Ford timing will not give the best results. Under such circumstances, it is more logical to compare the Ford engine with some other engine, such as a Stutz, which has the same 16-valve construction. Thus the valve timing of the Stutz engine (which is well known to be very fast becomes the logical valve timing for the 16-valve Ford engine.

The Laurel high-speed cam shaft is designed to give the same valve timing as the timing of the Stutz 16-valve racing car, as follows: intake opens ten degrees past top dead center and closes at 55 degrees past lower dead center. The exhaust valve opens 55 degrees before the lower dead center, and closes at 10 degrees past upper dead center.

Now let's compare this with the regular Ford timing, which is as follows: Inlet opens at 12 degrees, 40 minutes past top dead center; and closes at 50 degrees, 49 minutes past lower dead center. Exhaust opens at 37 degrees, 52 minutes before lower dead center, and closes at top dead center.

Thus we find that, in addition to the higher lift, the inlet valve is held open for about 7 degrees more of crank shaft rotation, thus allowing more gas to be drawn in.

But the difference is far more striking in the case of the exhaust valves, for the racing cam shaft not only opens the exhaust valve 17 degrees sooner, but holds the exhaust valve open 10 degrees longer, thus making a total of 27 degrees longer opening for the exhaust valve.

Since these special cam shafts have larger cams than the regular Ford cam shafts, it is necessary to cut a small groove in the bearing retainers to allow the special cam shaft to be installed. Proceed as follows: To remove the regular cam shaft, loosen the two set screws that hold the cam shaft bushings in place. Turn the crank shaft so that all the valve tappets can successively be brought to the ‘raised’ position and nails or cotter pins placed through the tappets, so that the tappets will not drop down out of place while the cam shafts are being exchanged.

Remove the cylinder front cover plate, and draw the cam shaft forward, and replace with the high-speed cam shaft. This is practically the same as changing a regular Ford cam shaft, save that it is necessary to chisel slight grooves in the first and second bearing retainers at 90 degrees apart, in order to allow clearance for the high-speed cam.

The proper clearance, between the valve stems and plungers should only be about .010 inch, as compared with the .018 to .022 inch or more that is usual with the regular Ford cam shaft. Set the gears of the high-speed cam shaft on the regular Ford gear mark.

Additional Notes:  Cloths pins also work well for holding up the tappets while exchanging the cams.  In this article, Fahnestock does not actually state that larger tappets are required, but it is obvious from the above drawing.  Based on our research, the stock cam requires a valve clearance of 0.025 to achieve the stated opening and closing angles (see Stock Cam).