Tach it Up Part II



Check Out – Part 1

Spin the clock back to the last issue. We talked about Sun’s tachometers (then and now). In those formative years, when someone would show up on the street in the drag strip staging lanes with a Sun tach strapped to a steering column, it meant one thing: The car was serious. That evolved quickly, though. It didn’t take long before that Super Tach was superseded by a Jones-Motorola mechanical tach strapped to the dash top (plenty of folks sold those Jones Motorola tachs – many were private labeled). When the car had a mechanical tach, it was even more serious. But the real significance here is that the tach found in the car always seemed to tell the story. And that’s not the end, either:

Obviously, times have changed. So have tachometers. Mechanical tachs aren’t that common anymore. There’s a reason for this, too: For those who were there, how could you forget the labyrinth you had to deal with when you installed a mechanical tach? It wasn’t so much the tach body. It was the cable. You had to chop a good sized hole in the firewall for the cable to pass through and then you had to deal with dropping the distributor in just right so that the cable was aligned. Upstairs, you had a choice between back drive, bottom drive, or side drive tachs. If you were lucky, the big drive cable could be routed through something like a defroster vent. And that’s not even taking into consideration the grief racers with big block Chryslers and most Fords had to deal with (the cable could be four feet long or more).  Not exactly fun or even possible with something like a rear engine bracket dragster. On the other hand, mechanical tachs aren’t bad news. They work pretty well, but one of today’s modern electronic tachometers is simply a whole bunch easier to live with.

A big benefit today is the reliability of electronic tachometers. Instead of being more or less accurate, modern electronics have made tachs into precise and reliable instruments. Of course, today, there are literally hundreds of different electric (or electronic) tachometers available. Features vary. What we’ll look at here are some of the important tach characteristics available to racers today. Something important though is the fact that this article isn’t focusing in on the latest and greatest when it comes to tachs. Instead, we’re looking at a rock solid choice for the sportsman drag racer.

A perfect example of a rock solid tachometer for the little guy racer is Auto Meter’s Pro-Comp 2 tachometer. This tach incorporates a two-stage shift light. This allows you to input a pair of shift settings –  one for the fast revving first gear and a different RPM shift point for the remaining gears. They also include another useful piece of technology named “Peak RPM Memory Recall”. This bit allows the tach to display the highest RPM achieved during a race.

So how do the basic features work? When it comes to Auto Meter’s Pro Comp 2 tach, the face has a “function” panel on the right hand side of the face. The setup goes like this:

  1. To enter the setup mode, apply power to the tach with the engine off (no RPM signal to the tach).
  2. Press and release the “SET” button. The pointer will move from 0 RPM to 1,000 RPM. This is the indicator for the Low Set shift point.
  3. Press and release the SET button to review and/or set the Low Set shift point.
  4. Using the “erase” and “recall” buttons, move the pointer to the desired RPM for the Low Set shift point.
  5. Once the pointer is positioned at the desired RPM, press & release the SET button to exit low set mode.
  6. Press and release the “erase” button. The pointer moves from 1k RPM to 2k RPM. This is the indicator for High Set shift point.
  7. Press and release the SET button to review and/or set the High Set shift point.
  8. Using the “erase” and “recall” buttons, move the pointer to the desired RPM for the High Set shift point.
  9. Once the pointer is positioned at the desired RPM, press & release the SET button to exit High Set mode. To store the settings you must push & release “recall” until the pointer reaches zero.

NOTE (1): If the Low Set shift point has been changed ,the High Set shift point will also be changed to the same value.

NOTE (2): If you prefer to use only one shift point, do not connect the blue wire (more below) as shown in the diagram in the wiring section and only set the High Shift Point.

In order to accomplish the change in RPM shift points, something has to tell the tach you’ve shifted gears. The tach has five wires protruding from the back of the case: Blue (switch); White (dash lights); Red (ignition switch +); Black (engine ground); Green (coil negative -, or “tach terminal” on an electronic ignition box). In addition, the shift light connector protrudes from the back of the tech case, but wiring is a simple matter of plugging the female connector into the existing male connector at the back of the tach. For shipping purposes, this two-wire harness is left unplugged.

The Blue wire mentioned above is probably the only uncommon wire at the back of the tach. This wire is routed to a normally open momentary switch (on the shifter). This allows the driver to manually switch from high to low RPM shift points on the tach. When the momentary switch is closed, the Low Set mode on the tach will be in effect. When the switch is open, the High Set model will be in effect. It’s common for many racers to use a transmission-mounted switch. Here, when the car is in first gear, the switch is closed which in turn places the tach in the Low Set mode. Once first gear is “pulled” or air-actuated, the switch returns to the open position, and the tach reverts to the High Set mode.

What about the recall button on the face of the tach? That’s easy too: It’s the tach tell tale. It will tell you how high you went on the shift point. It might not be something you care to see later, but it’s important. Before each pass down the racetrack, press the “Erase” button. This clears the member from the previous lap and you’re ready to record.

Something you should ponder with electronic tachometers is “radio noise”. What we’re talking about is radio interference and inductive crossfire. Sure those things can ruin performance, but they can also raise havoc with your tachometer. Radio frequency interference (RF Interference) produces a false triggering effect on crank trigger and electronic distributor ignitions. Since an electronic tachometer counts the number of times the ignition fires in a crankshaft revolution, additional random, false triggering causes erratic and inaccurate tach movement.

Auto Meter tells us their tachs are internally protected with circuits that are engineered to filter out a majority of the “noise” associated with racing conditions. When this noise reaches a high level, however, the tach sees it as legitimate ignition firing and responds accordingly. Auto Meter states that your racecar does not always demand full capacity of the ignition system. If radio interference is present, it will most likely show up at the top end, in top gear where the most horsepower and high ignition load is produced. In addition, tachometers with memory features will give indication of this condition with abnormally high memory readouts. In the end, you have to ensure that your ignition isn’t “leaking” through the wires. Use quality components such as spiral core ignition wires, wire sleeves and be sure that wires to ignition components such as the crank trigger are properly installed.

Of course, companies such as Auto Meter offer a plethora of different tachs with a wide cross section of features. Select the tach that suits your program and install it correctly. Next issue, we’ll look at a mounting solution you can copy. Watch for it.

Don’t take the RPM range of the tach lightly. A tachometer that reads to 10,000 or 11,000 RPM might be perfect for a high winding, small cubic inch Competition Eliminator motor, but it certainly isn’t practical on a stock-rod 454 bracket motor that will never see the other side of 7,000 RPM.
“Shift lights” are common today. The reason is it’s tough to watch a needle as it sweeps across the face of the tach on a fast revving combination. There are a number of different shift light setups available today, but the LED jobs are feature packed. Aside from being much smaller in footprint size than regular shift lights, the LED is extremely bright. The yellow cover you see is removed for racing.
Today, two stage shift lights are common. Here’s why: Your racecar uses mechanical “leverage” in the form of gearing to move forward. Due to the RPM drops between gears, this regularly means that the car will run quicker and faster if you short shift first gear. In most combinations, the 1-2 shift point is often 300-400 RPM less than the 2-3, (and/or 3-4 and 4-5) shift points.
The shift light can be positioned just about anywhere on the tach face.
The hookup for a modern tachometer is a bit different than older models: The tach has five wires coming out of the back (not counting the shift-lite connector). The shift-lite connector only goes on one way and the main wires are all color-coded. The text provides more info.

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