A Clever Tachometer for the Model A Which

Does Not Require ANY Modifications to the Model A

By: Andy Wiedeman

Member of the Rocky Mountain A’s of Colorado

September 2008

Tiny Tach from Design Technology Inc. Info at www.tinytach.com

· Why have a tachometer? Here in Colorado, we have lots of mountain driving. To get the most out of your engine, it is important to know where you are on the power curve. Max power for hill climbing is obtained at about 2150 RPM with the power falling off before and after this RPM. Shifting points on up hill grinds can be easily seen by watching the tach. The tach will prevent over reving and relieves you of guessing by ear at the engine RPM. The tach can also be used to set idle RPM, and can be used to correct for road speed for modified gear ratios and non-standard tire sizes.

· Les Andrews covers a Tachometer Installation in his Model A Ford Mechanics Handbook Volume II, but the installation shown requires a +12 V power supply, and has a rather ugly pickup connector on the coil wire. The Tiny Tach, is simpler, easier to install, more flexible in mounting, and much less expensive. Since it is self contained, it requires no vehicle power and can be used on either +12V or -6V electrical systems. The Tiny Tach was originally developed to record operating time of snow mobiles and other similar vehicles used for commercial purposes.

· This tachometer is shown below, requires absolutely NO modifications to the Model A. NO drilling of any holes, NO modification to the Model A wiring, and NO power from the battery. Installation requires less than one hour, including making a bracket. What more can you ask for? If you are going to show the car, the tach can be removed in minutes, leaving no sign of having been installed.

· Price is $36.95 plus shipping = about $42. Get the Tiny Tach over the Internet.

· RPM to 9,990 is indicated, updates each 2 ½ seconds and includes an engine hour meter which reads out when the engine is turned off.

· Here is how it works; the Tiny-Tach has an internal clock, which runs all of the time. When the engine is started the tach records the time of the first firing pulse that comes from an inductive pickup wire wrapped around the Model A coil output wire to the distributor. This pulse time is stored in the tach electronics. Successive pulses are recorded and the time between the pulses is calculated. Since the time between the pulses is known, and the relationship between the crank angle (180 degrees for the Model A) and the pulses is known, a calculation of RPM can be made and shown on the liquid crystal display. The pulses are averaged over a sampling period of about each 2 seconds or so to remove ambiguities. When the engine does not send a pulse for more than 2 ½ seconds, the tach assumes that there is Zero RPM and logs the time of the last pulse. Since the clock time of the first and last pulse is known, the tach updates the hour meter to indicate the total elapsed “ON” time of the engine. When the engine is not running the total elapsed time of the operation of the engine since the Tiny-Tach was installed or has been reset is shown on the display. There is NO “on-off switch” so you do not have to remember to turn it on to record RPM and engine hours. The Tiny-Tach requires NO MAINTENANCE after installation.

· No Vehicle electrical power is required, it doesn’t matter if you have a +12V or -6V system. The tachometer is completely self contained. Estimated internal battery life is 5+ years – but is NOT REPLACEABLE

· For the Model A Ford, select model TT226NR – 1C (hours not resetable) or

TT226R – 1C (recommended - allows resetting the engine hour meter) for an engine which fires once per 180 degrees of crankshaft rotation. A version for $65 has more features and updates RPM every ½ second, but requires +12V from the Model A. (See the website for more information)

· The Tiny Tach can be mounted with screws to a bracket on the steering column, or can be taped under the curved portion of the dash with double sticky back tape or mounted many other ways such as I did, above the Rear View Mirror. There are NO modifications to the electrical system or the Model A required. The wiring in the engine compartment is nearly invisible.

· Installation is simple. There is a 6’ long coaxial wire connected to the Tiny Tach. Route the coax through the junction box or any other firewall hole. Wrap the pickup wire around the coil output wire 3 or 4 turns, and ground the other wire. Do not cut the coax wire, coil it up and tape it out of sight. When panel mounted or the face painted, it is possible to have it appear as shown below depending on selection of the reset feature. I painted the face of the Tiny Tach black to eliminate the white lettering on its face.

· I mounted my Tiny-Tach above the rear view mirror, see Figure 1. To mount it in this position, make a single plate bracket (I used aluminum, but fiberglass would do fine), which has pre-drilled sheet metal screw holes for mounting the Tiny Tach, and two through holes for the rear view mirror bracket. Shape the bracket to the mounting foot of the Tiny Tach.

Figure 1

Then simply remove the rear view mounting bracket, attach the new single plate bracket on top of the Rear View Mirror bracket with the original rear view mirror screws. Then using sheet metal screws attach the Tiny Tach to the mounting bracket. The Tiny-Tach will then be just above the mirror. Route the pick-up wire and ground wire along the upper window channel space (along side the vacuum line to the windshield wiper) to the right hand (passenger) side window column, down the column, and across to the electrical junction box in the center of the firewall. Now refer to figure 2 for completing the installation. Route the coax pickup wire and the white ground wire through the electrical junction box. Attach the ground wire to any convenient ground point near the junction box, and then wrap the red pick up wire around the coil output about 3 to 4 turns. This completes the installation.

Figure 2