Model A Ford Garage
Distributor Advance Operation
The 1932 to 1934 Model B and Model 40/46 four cylinder engines used a distributor similar to the Model A, except it was equipped with a centrifugal advance weight mechanism as part of the rotating shaft assembly. The following is the technique I used to verify the geometric theoretical maximum design advance using a camshaft degree wheel on an NOS Model B advance shaft mechanism.
These static measurements are just the measured geometric theoretical maximum advance of the mechanism, and are not the dynamic advance curve for the rotating assembly.
The actual dynamic advance curve is a function of the rotating masses, counterweight geometry, spring rates, and rotational speed.
In actual fact, the Model B distributor advance will not 'max out' mechanically (geometrically) at engine speeds. I have run a Model B distributor with an NOS advance mechanism on a Sun distributor test machine and on an engine on Bill Stipe's dyno to confirm this.
The purpose for performing this measurement was to determine the geometric theoretical maximum possible advance in the mechanical assembly. The only published Ford advance curve showing the four cylinder ignition is in the March-April-May 1933 Service Bulletins.
That published graph is primarily an exhibit of V8 ignition curve information, and the four cylinder seems to be included as an additional reference. The graph shows a very 'questionable' initial (static) advance point of 15 crankshaft degrees and a maximum of 29 degrees advance for the four cylinder. There is no logical way to confirm the 15 degree number from the data available and the actual Model B initial static timing point.
It is a fact that the initial static timing point for the Model B ignition is 19 crankshaft degrees before top dead center (BTDC) on number one cylinder. This is based on the timing cover pin location design and the confirmed Ford Model B timing instructions. It is not clear where the static 15 degree BTDC figure in the 1933 Service Bulletins graph came from. It cannot be derived from the actual design geometry. I believe it is a published error by Ford.
Pictured above is the cam degree wheel and the NOS distributor advance assembly used to make the measurements.
The shaft assembly was held stationary in the vise and the degree wheel was attached to the upper cam assembly. A slot was cut in a piece of sheet metal to form the fixed reference point for the degree wheel.
This pic shows the degree wheel attached to the point cam on the upper advance assembly.
Pictured above is the shaft assembly fully retarded and the zero arrow of the degree wheel set at the zero slot in the improvised indicator.
In the pic above, the upper advance assembly has been manually rotated to the maximum advance position, with the lower shaft assembly held stationary in the vise. This shows the theoretical maximum centrifugal advance of 8-1/2 distributor degrees (17 crankshaft degrees).
The measurements above show that the initial static Model B ignition timing point of 19 crankshaft degrees BTDC plus a maximum mechanical centrifugal advance of 17 degrees equals a theoretical geometric maximum total advance of 36 crankshaft degrees. The slots in the centrifugal flyweights limit this travel.
Compare this to a Model A distributor which has an initial timing point of 0 degrees TDC plus an actual manual maximum advance of 40 crankshaft degrees. The breaker plate arm slot in the bakelite distributor body limits this travel.
The Model A ignition uses a zero crankshaft degree (TDC) initial timing point so that the engine can be easily started by hand crank, however it should never be run under load in this "retarded" position. The advance lever should always be advanced 1/3 to 1/2 travel (13 to 20 crankshaft degrees BTDC) on the steering column quadrant after starting the engine, and should generally not be run more than 3/4 advanced on the column under any normal circumstances.
The Model B is a higher compression (cylinder pressure) engine which uses more initial spark advance, and lower maximum advance compared to a Model A. This is consistent with the operation of the B engine and the need to protect it from the effects of running too far retarded or over-advanced.
More Distributor Timing web pages on Ford Garage:
- Model B & Model 46 Distributor Design Details
- Model B Pertronix Ignitor Ignition Installation and Timing
- Model A & B Engine Cam/Valve and Ignition/Spark Timing Discussion
- Model A, B, & Model 46 Camshaft Gear Timing Cover Differences