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#1 |
Senior Member
![]() Join Date: Jul 2016
Location: Anthem AZ
Posts: 206
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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.
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#2 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,104
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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!
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Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 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 |
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#3 |
Registered User
Join Date: Jan 2011
Location: Chattanooga, TN
Posts: 2,149
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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
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'83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project |
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#4 |
Registered User
Join Date: Apr 2010
Location: So Cal
Posts: 1,437
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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.
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'67 GMC 2500, 292, 4spd, AC |
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#5 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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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?
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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 |
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#6 |
70+ ( Old Skool Club )
![]() Join Date: Nov 2003
Location: Saskatoon, Saskatchewan ,Canada
Posts: 9,117
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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.
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1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021 |
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#7 |
Senior Member
![]() Join Date: Dec 2011
Location: Moraga, CA
Posts: 812
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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.
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(Very) Slow-Going Build Thread: Stock 1970 Short Step with Stock 1970 LT-1 http://67-72chevytrucks.com/vboard/s...d.php?t=567340 |
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#8 |
Senior Member
![]() Join Date: Feb 2002
Location: Tucson, AZ USA
Posts: 7,255
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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.
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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. |
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#9 |
Senior Member
![]() Join Date: May 2010
Location: Wisconsin & Arizona
Posts: 4,844
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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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