HEI to factory Tac reads tooo Hi
 

What is the way to go to hooking up a factory Tac to an HEI or should I. I have hooked up 2 factory TACs to my HEI and they both read about 30% higher than they should I tried adjusting one and it would not adjust enough I did however hook up an original looking after market one I purchased from one of the after market suppliers and it read correctly. I have called a couple different places and I have been told that the HEI's will burn up these TACs including the after market one. Does MSD make a distributer that will work. I would like to avoid going back to points. Input welcomed.

Re: HEI to factory Tac reads tooo Hi
 

There are lots of posts concerning factory tachometers with HEI's. Just enter factory tach with HEI and select 67-72 then search. My aftermarket HEI (I think its an Acell) and factory tach play well together. I don't believe all HEIs are the same inside (electronically).
Good luck!

Re: HEI to factory Tac reads tooo Hi
 

Do you have the inline filter? You may have issues without it. They can be bought from the Corvette and F-body restoration places.

http://www.zip-corvette.com/75-91-ta...FduNswoddJoFhg

Re: HEI to factory Tac reads tooo Hi
 

It's a factory item on many '80s GM vehicles with tachometers. I have one on my truck that I took from a Z24 Cavalier in a wrecking yard.
It is shown on the lower right of this schematic.

Re: HEI to factory Tac reads tooo Hi
 

I have this too, though only at low RPM I thought, so I assumed mine was from the multispark feature of the Petronix.

Anyone know if this inline filter will help with the Pertronix III reading high?

Re: HEI to factory Tac reads tooo Hi
 

Yes, i'd like to know if that filter will work with a Petronox as i'm having the same problem with the tach 800/100 rpms to high at 60/65MPH.

Re: HEI to factory Tac reads tooo Hi
 

The V-8 Vega guys used to have a trick where they'd change resistors (or capacitors?) depending on how many cylinders were in the car the tach came from and what the new motor was. Sorry I don't have a link.

Re: HEI to factory Tac reads tooo Hi
 

You have a Factory tach, in-dash type? They came in V8 and L6 types, no ''Number of Cylinders Switch'' like on aftermarket tachs. If you have a V8 engine and a V8 tach, it should work if the tach lead is clipped to the TACH terminal on the HEI cap.
So is it a GM HEI or aftermarket like PerTronix?
A friend of mine has the PerTronix III on a big block, and can't get the tach to function. He called their tech line, and was advised to get the Ignitor II.
I had an Ignitor II on a 292 L6 and an AutoGage tach with no problems. When the PerTronix burned out, I went to points for a year, then a GM HEI and still had a working tach each way.

Re: HEI to factory Tac reads tooo Hi
 

I've been running a factory 8 cylinder tach with a GM HEI distributor and no filter for 800 miles without any problems. It reads about 2900 RPMs at 60 MPH if I recall correctly. I have a 4.10 axle.