HEI to factory Tac reads tooo Hi - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**frankinstein** · 05-17-2017, 08:31 PM

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.

**HO455** · 05-17-2017, 09:07 PM

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!

MARKDTN · 05-18-2017, 08:34 AM

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

RichardJ · 05-18-2017, 11:14 AM

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.

davepl · 05-18-2017, 12:22 PM

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?

**Already Gone** · 05-18-2017, 03:44 PM

Quote:

Originally Posted by davepl

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?

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.

**mattfranklin** · 05-18-2017, 04:01 PM

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.

**'68OrangeSunshine** · 05-18-2017, 04:08 PM

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.

**FirstOwner69** · 05-18-2017, 04:55 PM

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.

05-17-2017, 08:31 PM	#1
frankinstein Senior Member Join Date: Jul 2016 Location: Anthem AZ Posts: 206	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.

05-17-2017, 09:07 PM	#2
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 10,804	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! __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban the WMB,1991 S(stink)-10 Blazer,1969 GTO, 1970 Javelin, 1952 F2 Ford 4X4, 29 Model A, 72 Firebird. 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

05-18-2017, 08:34 AM	#3
MARKDTN Registered User Join Date: Jan 2011 Location: Chattanooga, TN Posts: 2,131	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 __________________ '83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project

05-18-2017, 11:14 AM	#4
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,413	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. Attached Images __________________ '67 GMC 2500, 292, 4spd, AC

05-18-2017, 12:22 PM	#5
davepl Registered User Join Date: Aug 2006 Location: Redmond, WA Posts: 6,332	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? __________________ 1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible

05-18-2017, 04:01 PM	#7
mattfranklin Senior Member Join Date: Dec 2011 Location: Moraga, CA Posts: 801	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. __________________ (Very) Slow-Going Build Thread: Stock 1970 Short Step with Stock 1970 LT-1 http://67-72chevytrucks.com/vboard/s...d.php?t=567340

05-18-2017, 04:08 PM	#8
'68OrangeSunshine Senior Member Join Date: Feb 2002 Location: Tucson, AZ USA Posts: 7,045	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. __________________ Every 25 years I like to rebuild that 292, whether it needs it or not. Last edited by '68OrangeSunshine; 05-18-2017 at 04:18 PM.

05-18-2017, 04:55 PM	#9
FirstOwner69 Senior Member Join Date: May 2010 Location: Wisconsin & Arizona Posts: 4,839	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.



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