The Model T Ford Ignition System Spark Timing

1 With the possible exception of the planetary transmis-sion, no part of the Model T ford s original design is morecommonly misunderstood than the magneto ignitionsystem. The purpose of this article is to simply explain howthe Model T ford Ignition System and associated sparktiming actually work. The authors have been discussing this subject for sometime, each approaching the project with differing was interested in optimizing Ignition Timing for racingpurposes and Ron wanted to write an article that wouldclearly explain the original Model T Ignition System . Ourpeer reviewers challenged us. It was a convergence ofinterests and we all had great fun learning the truth aboutwhat we believed we already we discuss the details of Spark Timing , letsreview the unique features of the various components of theoriginal Model T Ignition 1 is an original ford timer case and roller. Thisis important because there were many different manufactur-ers of timers other than ford and they vary significantly inmechanical design and construction.

2 The ford timer casecan be usually identified by the word ford in scriptstamped into the main case near where the pull lever isaffixed and the roller has similar markings on the springloaded roller support arm, see Figure 2. The timer case hasfour electrical conductive metal segments mounted in a non-metallic insulator at equally spaced intervals around theinside circumference of the case. Each conductive segmenthas an insulated screw that extends outside the caseperimeter forming a screw terminal with thumbnut wherewiring to the coil box can be connected. The timer roller ismounted on the end of the camshaft, indexed with a pin andretained by a locking nut. The roller rides on the innersurface of the timer case connecting engine ground to eachconductive segment as it rotates thereby completing theelectrical circuit to each coil. CautionUsing this procedure with other than ford timers mayresult in mis-adjusted Timing , possible damage to theengine and injury to the 3 shows the timer mounted on the engine and theoriginal ford adjusting gauge used to set the timer at Model T ford Ignition System & Spark TimingFigure 1 ford Timer Case and RollerPhoto Courtesy of Steve ConiffBy Ron Patterson & Steve ConiffFigure 2 ford Script on Timer Case and RollerPhoto Courtesy of Steve ConiffFigure 3 Timer Mounted on Engine Using Original FordAdjustment GaugePhoto Courtesy of Steve Coniff4 Model T TIMES, NOVEMBER-DECEMBER 2003inches from the adjacent front plate mounting bolt.

3 With thespark lever fully retarded (all the way up) the timer pull rodis appropriately bent to insert the end of the rod into thetimer lever hole without changing the As the steering column Spark lever is movedthrough its quadrant, which consists of 28 notches, the timercase moves through its range of movement advancing thespark at a rate of approximately crankshaft degrees pernotch. This relationship is not exactly linear because of theangle of the pull lever on the end of the Spark rod. The construction of the ford low-tension magneto iswell understood and will not be described in further detailhere. For those interested in the evolution of the flywheelmagneto used on the Model T please refer to Edward Huff,Henry ford and the Flywheel Magneto by Trent Boggess,Vintage ford Magazine Volume 31 Number 2 March-April1996. For the purposes of this discussion it is important tounderstand the basic electrical characteristics of themagneto output.

4 The magneto output is an AC signal ofvarying voltage, frequency and current with eight completecycles for each crankshaft revolution. The voltage variesover the normal range of engine speed from a low of 4 voltsto a high of over 30 volts with sufficient current capacity tooperate the coil. The sixteen positive and negative peaks ofthis signal are separated by degrees of crankshaftrevolution. The significance of this last point will beexplained later. Figure 4 shows a portion of Model T flywheel drawingT-701-C (viewed from the transmission side). If you studythis drawing carefully you will see the magnets are mountedon the flywheel in such a way that the magneto output isadvanced from Top Dead Center (TDC) by 7 degrees ofcrankshaft rotation. (11 Degrees 15 Seconds minus 4 Degrees 15 Seconds equals 7 Degrees) The significance ofthe magnet mounting will be explained later. The Record ofChanges for the flywheel drawings indicate the magnetmounting was unchanged for all Model T construction of the Model T Ignition coil is wellunderstood and will not be described in further detail those interested in the evolution of the Ignition coil usedFigure 4 Flywheel Drawing T-701-C (partial) From the Research Center Collection at The Henry ford Museum and Greenfield T TIMES, NOVEMBER-DECEMBER 2003in the Model T please refer to the three part article entitled The Model T Ignition Coil by Trent Boggess and RonaldPatterson, Vintage ford magazine Volume 34 Number 4, 5& 6 July to October 1999.

5 For the purposes of thisdiscussion it is important to understand that properoperation of the coil points will affect Ignition Timing andhence engine performance. Additionally, the Ignition coiloperates entirely different when running on battery asopposed to magneto. When a properly adjusted Ignition coilis running on battery and the roller grounds the timersegment, the coil will vibrate continuously and providevoltage to fire the Spark plug. When a properly adjustedignition coil is running on magneto and the roller groundsthe timer segment the coil points respond to the individualcurrent pulses of the magneto output to fire the Spark reliably and consistently respond to the individualmagneto current pulses and maintain accurate ignitiontiming the coil points must be carefully adjusted. The onlyway correct point adjustment can be attained is by using ahand cranked coil tester which simulates conditions verysimilar to that which occur in the Model T Ignition systemwhen running on 5 is a simplified diagram of all the variousignition components integrated together as a lets discuss the detailed Spark Timing measure-ments we took.

6 Figure 6 depicts the method used to take the annulardegree measurements. Remember, wherever the word degree or degrees is used hereinafter it is understood tomean crankshaft degrees of rotation . A degree wheel wasmounted on the end of the crankshaft with a stationarymarker pointing to the scale. TDC was located on cylinderone, the degree wheel indexed so the marker pointed to 0 degrees and fixed so the inter-relationship would not changeas the crankshaft was rotated. Figure 5 Simplified Diagram of Integrated Model T Ignition ComponentsIllustration from George W. Hobbs and Ben G Consoliver The GasolineAutomobile 3rd Edition McGraw-Hill Book Company, Inc, NY 1924 )Figure 6 Figure 6 Degree Wheel Mounted on Crankshaft with PointerPhoto Courtesy of Steve Coniff6 Model T TIMES, NOVEMBER-DECEMBER 2003 Figure 7 is a diagram depicting the first set of measure-ments. The engine front plate was checked for concentricitywith the end of the camshaft.

7 A reground ford camshaft,good original ford timer case and NOS timer roller wereused to take these readings. The timer case was positionedusing the ford gauge. By manually rotating the crankshaft itwas determined that the timer segment was grounded at after TDC and remained grounded for 87 the timer case was moved through its range of the sparklever quadrant it was found the total advance of the timercase was 80 previously mentioned, when operating on battery,the Ignition coil will vibrate continuously firing the plug aslong as the timer segment is grounded. As you can see inFigure 7 the coil will continuously fire the Spark plugstarting at degrees after TDC and cease at after TDC. As engine speed is increased and thespark lever advanced the coil will fire the Spark plugthroughout the degree range of the timer case movementuntil it reaches maximum advance of degrees beforeTDC.

8 Figure 8 is a diagram depicting the second set of mea-surements. The test set up is unchanged and the intervals oftimer segment and timer case movement are the taking these measurements a properly rebuilt ModelT coil was electrically connected to the timer, magneto andspark plug and the engine rotated at 600 RPM. Anautomotive Ignition Timing light was connected to the sparkoutput to read the position of the degree wheel pointer todetermine exactly where the coil fires in relationship tocrankshaft position. The magneto waveform depicted in Figure 8 is repre-sentative only and does not reflect the shape of the magnetowaveform when viewed on an oscilloscope. But it doesFigure 7 Ignition Timing Diagram-Battery OperationCAD Drawing Courtesy of John ReganFigure 8 Ignition Timing Diagram-MagnetoOperationCAD Drawing Courtesy of John Regan7 Model T TIMES, NOVEMBER-DECEMBER 2003reflect the Timing relationship attained by advancing themagnets on the flywheel by 7 degrees as indicated onflywheel drawing T-701-C.

9 The Spark symbols show thepoints where the Ignition coil will fire on the magnetocurrent output with respect to piston travel. Each of theselocations is logically separated by degrees. When starting the Model T engine on the magneto withthe Spark lever fully retarded, the coil initially fires at after TDC. This is the nearest location based uponthe timer case ( Spark lever) position where sufficientmagneto current output is available. As engine speed isincreased and the Spark lever advanced the coil will initiallyfire at one of the additional locations where sufficientmagneto current output occurs. These locations are 4degrees after TDC, degrees before TDC, 41 degreesbefore TDC and possibly degrees before TDC. Thespark depicted at degrees before TDC Spark may notoccur depending upon the travel of the Spark lever past theend of the quadrant. Figure 8 helps explain why it is easier to start a ModelT on magneto when the Spark lever is advanced a fewnotches.

10 By advancing the Spark lever the coil will fire at 4degrees after TDC. If the magnets were not advanced 7degrees the coil would initially fire at degrees 8 also helps explain why the engine sometimesspeeds up when switching from battery to magneto. Thereare locations of Spark lever setting when running on batterythat will result in more optimal Spark Timing when switchedto you can see in Figure 7, when running on battery,linear Spark Timing can be obtained by moving the sparklever. This would appear best for optimum engineperformance, but Model T coils do not work well on the 6volt battery at higher engine speeds. As you can see in Figure 8, when running on themagneto, Timing advance is not linear. The Spark lever is aselector that, when moved, determines the magneto currentpulse where the Ignition coil will supply Spark voltage to authors would like to formally thank BruceMcCalley, Trent Boggess and John Regan for their review ofdrafts of this article.