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Discussion Starter #1
I am about to install a OER in dash tach in a 69 Camaro that has a HEI distributor and have read that it needs a in line filter to the tach. No one seems to know anything about them. I have asked around but can't find out what it is or who sells them. I know that a lot of guys just install their tachs without them but I want one. It suposedly filters high voltage at higher RPMs and eliminates gauge fluctuations. I can't be the only person in this situation and can't understand why I'm having such a hard time getting any info on something that should be offered with every new tach. Can anyone here point me in the right direction? Thanks
 

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I'll bet this one addresses your issue. http://www.camaros.net/techref/electrical/tachfilter/
I built my first tach filter over 20 years ago, when my 5" autometer tach would not work right with my new HEI.

Six years ago, I had to remember how all over again, when my new repro tictoc tach wouldn't work right with my electronic ignition.

The above post has been part of this site since. It's in the electrical basics sticky at the toip of the electrical forum. I've built and mailed dozens, and parheps hundreds of others have built thier own. (no, I don't build them for others anymore) THey work.

Tach filters are also sold by a number of ignition manufacturers, including MSD. Similar devices were used on lots of production cars, and they can be found in a junkyard if you know what to look for (note that I don't, so don't ask me)

Tachs are frequency counters. The pulses from the ignition system are converted to a DC voltage that is put thru a coil to generate a magnetic field. The strength of the field is proportional to the number of pulses. The condensor in a points type ignition is almost a tach filter. Electronic ignitions tend to put out a lot of high frequency noise, which can be seen by the tach as extra pulses. Various other things can happen to the signal as well that will "confuse" the tach.

Since electricity will always take the path of least resistance, the filter uses a capacitor and a resister. The resster feeds the tach and provides a constant (small) voltage drop. The capacitor's resistance (actually impedance is the proper word) varies depending on the frequency of the signal. To a low frequency, it is a very high resistance, so the signal passes thru the resister to the tach. Higher frequency's see the capaicotor as a low resistance, and seek ground thru the capacitor, bypassing the tach.

The filter I designed cuts off around 10,000 rpm. If you rev that high, your tach will go to zero!
 

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Discussion Starter #5
Finally...found someone who knows what I was looking for. Thank you very much. Since this link is so old, you show it with a coil system but, as mentioned in the text, can be used with a HEI. Thanks again.
 

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Discussion Starter #6
One more thing...if anyone needs one but doesn't think they canor want to build one, drop me a line. After I make the first one, I'll see what everything costs and work up a cost analysis to see if it would be worth my time and effort to build/mail these out. I probably have 80% of the parts already. I think installing at the tach would be the best option so I am going to order some harnesses to see what I come up with as far as a plug in, turn key setup. Thanks
 

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The parts cost less than $5. They take about 30 minutes to build. For a time, I was making them and sending them out for $10 including mailing, but it got to be too much and I don't do it anymore.

Several that I know of have mounted them under the dash, though that would involve "adjusting" the wiring harness.

The finished product is 3" long and 1/4" think. It basically looks like a fat spot in the wire. You could put an HEI correct connector on it and just plug it in.
 
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