That's right - the original movement has a lot of mass which was made worse by the factory adding counterbalance weights to improve the meter's linearity. So no matter what you do with the electronics the mass will always be the limiting factor. You have to either overdamp the response to reduce 'bouncing' but then it will be sluggish. Underdamping improves the response but then it will 'bounce' and takes a long time to settle.

The tach I used was called a 'Pro Tach' made by Equuis. They run about $30 or so. I mount the circuit board on the back of the VDO housing using the original standoffs and other parts off the old tach. The original VDO needle fits on the new movement's shaft. It's heavier than the new tach's shorter needle so you'll lose some response, but it's not that much and I wanted to retain the stock appearance. You need to drill or punch a couple new holes in the tach's faceplate (bezel?) to mount the new mechanism. The original small holes are not noticeable once it's in the car. I suppose you could epoxy the movement to the VDO bezel and no holes would need to be drilled. The new tach has to be recalibrated to the old meter scale using the trimmer pot on the circuit board. In my opinion this is the only way to go if you want a stock appearance tach and also much better performance.

Are you guys saying if you have a 6 cylinder, you just have to recalibrate the pot, without adding an additional resister or whatever? Thanks

You don't need to recalibrate or change resistors. Once calibrated, you just move the 3-position switch to 4/6/8 cylinders according to your engine.

Peter, just to be clear, 914GT is talking about using the board and movement from a Equus Pro Tach, not the original VDO.

OK, so the Pro Tach is available in 2-1/2 and 3-3/8 versions. The smaller version is the cheaper one, so I'm assuming it will work fine with the metal VDO needle. I would bet dollars to donuts that they both have the same internal board, as the installation instructions are the same for both versions.

Can't wait to get one and break it open!

I've used the smaller version. The plastic ring around the bezel will snap off, just be careful. Also be careful prying off the needles, using equal force on opposite sides to not bend the shaft. One other thing - if you want the needle to stop at the '0' rpm mark you need to modify the movement or add something like a resting pin on the VDO meter face so the needle doesn't go too far counterclockwise. I rigged a piece of copper house wire on the movement as a stop. Once you get it apart you'll understand what I'm talking about. The reason for this is because the VDO 'zero' mark is in a different position than the Pro Tach 'zero', and the VDO did not have good 'resolution' below 1000 rpm.

914GT, I've started doing the math for a 555-based tachometer calibrator. Maybe you can help me out with a couple questions (which are at the very end of the buildup. . . )

For a 6-cylinder 911, the dwell period is 38 degrees. So the points are closed 38/360, or a duty cycle of 10.6%. That is the charge time for the coil regardless of point gap, it's the measured spec. (Obviously in a CDI ignition the dwell period doesn't matter much, but we're assuming kettering ignition for this example)

As RPM increases, the pulse width, or amount of time that the points are closed, decreases. Here's my reasoning.

* Crank is turning at 1000 rpm;

* At 1000 rpm, the time it takes for the crankshaft to rotate 360 degrees is 60,000 microseconds. (1 / (1000 / 60) ) * 1,000,000;

*In a four stroke engine there is one ignition pulse per cylinder (6 of them) every two revolutions of the crankshaft or 720 degrees. 720 degrees divided by 6 cylinders is a pulse every 120 degress of crank rotation, or three pulses per crank revolution;

* Since in a single rotation of the crank (60,000 uS) there are three pulses, the total time the points are open and closed for each pulse must be 20,000 uS.

* A 10.6% duty cycle means that the points are closed for 6360 uS and open for the remainder of that period during the ignition window, or 13,640 uS.

Now here's where the question comes up in selecting the resistors and capacitors for the 555.

The duty cycle of a 555 is established by the relationships between the two resitors, because the external timing capacitor charges (output high) through Ra and Rb and discharges (output low) through Rb. The equation from the literature is duty cycle = Rb / (Ra + 2Rb). With that in mind, there are a bunch of combinations of resistors you can use, anything that gives you 10.6% as a duty cycle percentage.

So if we choose a 14.8K ohm resistor for Ra and a 2K ohm for Rb, we get a duty cycle of 10.597%, close enough. To choose a capacitor we need a charge time equal to 6360 uS, and the equation is .693 (Ra+Rb) * C where the resistors are in ohms and the cap is in farads. So a 4.6 uF capacitor should work.

Working the math for 8000 rpm, with the same resistors, you need a cap of .570 uF.

John,

Unfortunately it's been a couple years since I was working on this stuff, and the schematic I drew up has long since disappeared. I remember I used a few 10-turn pots on the board to set each frequency with a single fixed timing cap. I was working toward a dwell of around 28-32 deg. for a Chevy V8. I'm sure I didn't put as much thought into it as you have, and had to play with the component values a little until I was satisfied. If I were you I'd build it up and scope it to see what it looks like. Buffer the 555 output with a transistor to drive some inductance. You can use an old relay coil for some inductance. If you really want to strive to get a waveform duplicating a real ignition, I suppose you could play around with the circuit and a real ignition coil to make it look like the real thing. I found that my little cobbled-up circuit was close enough though, and didn't put a lot of time into it. It was great to have something to check or cal tachs on the workbench, and it having a built-in 12V supply for power.