Precision Timing the Model A Ford

Simple, accurate, and reproducible!

For a smooth, powerful, and reliable running engine, there are two essential adjustments:

  • Proper combustion mixture
  • Timely spark ignition

This article discusses spark ignition.

Ignition of the gases in the cylinder, progressive burning, and subsequent expansion are the sequence of gas events causing the engine to run. Spark too soon expands the gases against the rising piston. This results in loss of power and possible mechanical stress problems. Spark too late causes low gas pressure so the energy heats the engine and also passes out the exhaust.

Extensive design theory and laboratory tests during the 1930s established that the gas mixture burns completely in a fixed time period-approximately 2 milliseconds (0.002 seconds). There is major dependence on the fuel mixture and geometry of the combustion chamber as well as location of the spark plug.

In a 4-stroke engine, the crankshaft rotates at 6 degrees per millisecond for each 1000 RPM. So at idle (500 RPM), the crank turns through 3 degrees in 1 millisecond. But the gas burning time is essentially fixed (2 milliseconds). The spark should thus start the burn process 2 milliseconds early - 6 degrees of crankshaft angle before piston Top Dead Center (BTDC) to best use all the gas energy.

At 2500 RPM (top speed), the crank turns at 15 degrees per millisecond. The spark should now start the burn process 30 crankshaft degrees BTDC to best use all gas energy.

The optimum spark timing should thus be varied dependent upon the engine RPM/vehicle speed. All cars since the early '30s do this automatically and many since the mid '20s - only Ford stuck to manual control during that period.

The "conventional" method for timing is well documented in the Model 'A' Instruction Book as well as other sources. The usual method is awkward, inaccurate and the result is not reproducible. There is no measurement of the timing you believe you have set. Most importantly, the actual timing with the engine running is not established.

A well tested method, tools, and instructions for accurate and reproducible timing is now available. It measures the timing set under engine running conditions. This method has been extensively tested over the past 20 years. Tests on hundreds of cars at 'A' meets showed timing variations from 25 degrees ATDC to 20 degrees BTDC with all the resulting problems of:

  • Overheating
  • Hard start
  • Mediocre performance under load
  • Low top speed

The test tools consist of a crankshaft degree indicator, cutaway distributor cap adjustment scale, and cam adjusting wrench. A conventional timing light (available from many sources) is required.

The crankshaft degree indicator can be either temporarily clamped in place permanently mounted. The test procedures are part of the kit instructions.

To properly set the timing, it is necessary to accurately know when the spark occurs in relation to the piston/crankshaft position. A timing or "strobe" light produces a flash of light each time a spark "trigger" occurs. If the flash illuminates both a reference dot on the crank pulley and a stationary degree indicator, the timing is then accurately and reproducibly known, This is the best timing method - TDC piston # 1 reference dot on the pulley, a crankshaft degree scale, and a strobe light triggered from a spark plug.

Timing lights are found at Swap Meets for 6 volt and plug power, and mail order and auto stores for 12 volt, spark plug, and 110 volt AC power.

Detailed instructions within the kit contain the procedure to set the retard/advance, idle and top speed settings, and how to adjust them to achieve the desired running conditions for that car and driver. Suggested settings are also contained.

Auxiliary diagnostic tests can be readily made with the tools and procedures. They pin-point problems found with cams, distributor shaft, points, and rotor which cannot be, and are not, found or even apparent with the "conventional" timing method/ instructions. These test procedures are part of the kit instructions.

A remaining ignition problem previously described in the Vol.1 #8 issue of MODEL A TRADER concerns reversed ignition coil polarity. A tool, the SPARKLITE, detects this fault and was fully described in the article. The wrong coil polarity requires approximately 5000 more volts to jump the spark plug gap as compared to the correct polarity. Under cold, low battery, or a marginal plug condition, the spark may not jump the gap - hence no engine start and misfires at high RPM, if it does run. Tests of 40 cars showed 21 with "reversed" coil polarity.

Tests of hundreds of Model A's at Model A events have revealed approximately half showed "reversed" coil polarity.

The Precision Timing Kit and SPARKLITE were designed for and are being produced by Nu-Rex.