Quote:
Originally Posted by Huck
A few years back, one of the members (an engineer) of web site Colorado K5 did an analysis of factory tach's hooked up to both stock ignition and then to HEI. He came up with the signal wave length of the HEI was different then the stock ignition and he recommended some kind of filter.
AND NOW A TASTE OF REALITY---I HAVE PERSONALLY INSTALLED DOZENS OF FACTORY TACH'S IN BOTH STOCK AND HEI IGNITION AND THE DIFFERENCE IS SO SMALL YOU CAN'T REALLY TELL. I have also tested them against a new fully electronic tach for a test of accuracy---the factory tach is not as smooth or as accurate when installed in either original ignition or in HEI ignition. The factory tack is not fully electronic---it has a mechanical component that is influenced by temperature, dirt, vibration etc.
So if you want to run a new dash cluster with an old styple tach--if the tach is good, it will work fine with HEI ignition (relatively speaking). It's accuracy will be close enough for most drivers--giving rpm within a very small % or accurate (probably less then 3%deviation).
I have sold probably 30-40 Tach Dash's with never a problem due to relative accuracy. Further note---the factory had at least 4 different tach designs installed in their 67-72 trucks --I know because I disassemble evey one and inspect it before I sell it.
If you want to pursue a filter for greater accuracy, go to ColoradoK5 and do a search on Tach accuracy and perhaps the writeup is still around. One more thought, the wiring harness has basically nothing to do with the tach connection which comes directly off the distributor coil. When you change over to HEI, you do require a heavier gauge wire from the ignition but assuming the new harness you are buying is correctly built for HEI---you should be good to go.
Huck
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Huck,
The tach harness I have has the coil connection. Are you saying that the harness designed for the old style coil connection may have a different gauge wire than one designed for an HEI? Or, are they the same, just possessing differnt end terminals?