Factory Tach and HEI Question

[198plus]

Just trying to figure out if there's any way to use HEI distributor with a factory tach ?????????

[Rich 5150 69]

Ron try here, Camaro boys using HEI with factory TIC TOC tachs..http://www.camaros.net/forums/showthread.php?t=130006

[ratskin]

I used HEI with my factory tach. Just had to run the wire from the tach to the HEI. Simple hookup. You need to be sure, though, that the tach is 64(?) to 66. The earlier ones need adaptors. I.E. TWO wire.

[SFTorange]

I thought I'd revive this old thread to bring up some of the issues I'm having with my repop factory tach from Classic Parts. I was comfortable when I bought the tach for use with my stock HEI distributor due to the note on the Classic Parts website that says "works with HEI". After installing the tach, it seemed to work great. Smooth operation, etc. until I started bringing the truck up to speed. The tach reads about 400 rpm too high across the board. I used the RPM calculation (mph X 336 X rear end gear ratioX trans final drive / tire diameter = RPM) to figure out where I should be RPM wise at a given speed. I installed my old column mounted tach, and its readings match what I should be getting. I'm getting my exact speed from a GPS unit, so I'm confident that it is accurate as well.

Any ideas why the factory repop tach is reading too high with HEI? I called Classic Parts and was told that others have had the same problem with HEI.

[brap999]

Some aftermarket tachs (maybe all?) have a way to set the tach for a 4, 6, or 8 cylinder motor. I have no idea if this is the case for your reproduction factory tach, or if the reproduction is intended for only one size motor? Are there any pins or switches on the back that allow you to set the number of cylinders?

[LostMy65]

I was under the impression that our generation truck's tachometer could not be used with an HEI distributor.
But I'm wondering if the mixed opinions is because the two different tachs.used through 60-66.
The 64-66 tachs are connected directly to the coil/distributor.
So I'd like to know if anyone is using a 64-66 stock tach with HEI?

[Rich 5150 69]

The originals used a control unit, not compatible with HEI as I was told, ( but not verified sense it only went to 5000, I stopped there ), the new units are HEI compatible as the ones sold thru ECE, shown below...

[chevyrestoguy]

GM put tach filters on all of their tach-equipped HEI models back in the day, and there was a reason why They are an RC filter that is designed to remove high frequency noise from the tach line. They smooth out the spikes in ignition system. I would try that first.

Here is an ebay ad for a factory GM filter:
http://www.ebay.com/itm/1978-TO-87-T...df7b56&vxp=mtr