hei stock tach ,how far off is the reading? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tom71 · 06-22-2006, 08:23 PM

does any body know how to make a factory tach work accurately with a hei? how far off is it ? any one ? any one??

Oldtruckfanatic · 06-22-2006, 08:44 PM

There are a few places that will set up your tach to work with the HEI. I don't know how far off it is I have just read a bunch of posts on the subject on other sites. I have always wondered if a guy could wire in a radio shack potentiometer (adjustable resistor) and a temporary tach and adjust the potentiometer to match the factory tach to the temp tach. I have no clue if this can be done just something I have wondered.

Longhorn Man · 06-22-2006, 08:46 PM

it should read fine...assuming your 35 year old tach is acurate.
A tach is a tach, doesn't matter if you have points or HEI.

barepaint · 06-22-2006, 08:48 PM

longhorn is right just plug and play...just did it a month ago

byrd · 06-22-2006, 10:20 PM

Yep, all the repopers say their tachs won't work with HEI and I have personally tested well over 150 repop tachs and all have worked fine with HEI. I think the tachs are accurate enough but not perfect,and I haven't had the chance to hook two up at the same time to check them against a factory tach or another type of tach for that matter.

**Bob B.** · 06-22-2006, 10:26 PM

Don't know that HEI would cause a problem with either an original tach. or a reproduction, but an MSD or any other multiple spark discharge ignition will.

Blue85 · 06-22-2006, 11:37 PM

So is their anyway to make it work an msd style ignition?

tom71 · 06-23-2006, 12:33 AM

well a while ago i asked a speedo shop about it and they said they would have to make a" special " harness to work with the hei and the factory tach . is this special harness nothing more than a plug for the tach and a plug for the hei batt and tach (like the plug i nabbed today at the bone yard ??)oh and the speedo place said they would charge me "only 125 dollars " geee thanks for nuthin !

wibilly · 06-23-2006, 12:59 AM

so do the factory tachs last wired to a hei? i thought i read a post awhile back on here that they would not last without burning out but i don't know why but then again i know very little about electronics

**Richard** · 06-23-2006, 01:16 AM

I had always thought the pot that is accessable from the back of the tach is for adjustments

Never messed with it though.

byrd · 06-23-2006, 06:09 AM

Richard you are right, you can calibrate them some.

**StingRay** · 06-23-2006, 10:53 AM

I have run my stock oe factory tach with hei and msd. No problems. I checked it against a tach in my digital volt meter and while it was not killer accurate it wasn't more than a couple hundred rpm out in it's worst case. I have about 10 years of straight hei to fact tach with no problems and about 3 years of msd to factory tach with no problems. I believe that adjustment pot on the back is used to set the tach to zero isn't it?

Madbomber88 · 07-04-2006, 12:16 PM

Not sure if this helps, but my factory tach with HEI reads 400 RPM faster than it really is.

byrd · 07-04-2006, 12:19 PM

they you reach around to the back of the dash with a small flat head or phillips I can't remeber which one and turn the little screw in the back of the tach until you get the right reading. This is how they are calibrated

06-22-2006, 08:23 PM	#1
tom71 what happened to flint mi Join Date: Oct 2005 Location: san diego Posts: 437	hei stock tach ,how far off is the reading? does any body know how to make a factory tach work accurately with a hei? how far off is it ? any one ? any one?? __________________ 71 swb 2wd fleet frame off in progress 72 4x4 blazer daily driver 70 olds cutlass s mint condition owned by a grandma 56 gmc 2wd project waitng 97 fiance mobile

06-22-2006, 08:44 PM	#2
Oldtruckfanatic Truck junkie Join Date: May 2006 Location: Reno NV Posts: 697	Re: hei stock tach ,how far off is the reading? There are a few places that will set up your tach to work with the HEI. I don't know how far off it is I have just read a bunch of posts on the subject on other sites. I have always wondered if a guy could wire in a radio shack potentiometer (adjustable resistor) and a temporary tach and adjust the potentiometer to match the factory tach to the temp tach. I have no clue if this can be done just something I have wondered.

06-22-2006, 08:46 PM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Re: hei stock tach ,how far off is the reading? it should read fine...assuming your 35 year old tach is acurate. A tach is a tach, doesn't matter if you have points or HEI.

06-22-2006, 08:48 PM	#4
barepaint Senior Member Join Date: Mar 2006 Location: bay area, ca Posts: 305	Re: hei stock tach ,how far off is the reading? longhorn is right just plug and play...just did it a month ago

06-22-2006, 10:26 PM	#6
Bob B. Senior Member Join Date: Aug 2005 Location: Los Angeles Posts: 1,333	Re: hei stock tach ,how far off is the reading? Don't know that HEI would cause a problem with either an original tach. or a reproduction, but an MSD or any other multiple spark discharge ignition will. __________________ 1967 GMC CM-2500 Camper Cruiser, 351E V-6, NP 435 4 speed, Dana 60, and factory A/C. 2012 GMC K-3500 WT regular cab, 6.0L Vortec, 6L90.

06-22-2006, 11:37 PM	#7
Blue85 Oilfield Trash Join Date: Sep 2003 Location: Raymore, Sk, Canada, Earth Posts: 3,171	Re: hei stock tach ,how far off is the reading? So is their anyway to make it work an msd style ignition? __________________ 14 Dodge 3500 Mega Cab 08 Dodge 3500 Mega Cab 87 Chev 3+3 4x4 80 GMC K10 High Sierra bone stock 70 Chevelle 350SS long term project 68 GMC 3ton Grain Truck/Garbage wagon 50 Pontiac 2 Door Coupe Project "I've got a shotgun rifle and a 4 wheel drive and a country boy can survive" Hank Jr.

06-23-2006, 12:33 AM	#8
tom71 what happened to flint mi Join Date: Oct 2005 Location: san diego Posts: 437	Re: hei stock tach ,how far off is the reading? well a while ago i asked a speedo shop about it and they said they would have to make a" special " harness to work with the hei and the factory tach . is this special harness nothing more than a plug for the tach and a plug for the hei batt and tach (like the plug i nabbed today at the bone yard ??)oh and the speedo place said they would charge me "only 125 dollars " geee thanks for nuthin ! __________________ 71 swb 2wd fleet frame off in progress 72 4x4 blazer daily driver 70 olds cutlass s mint condition owned by a grandma 56 gmc 2wd project waitng 97 fiance mobile

06-23-2006, 12:59 AM	#9
wibilly Senior Member Join Date: Nov 2005 Location: northwestern wi Posts: 2,111	Re: hei stock tach ,how far off is the reading? so do the factory tachs last wired to a hei? i thought i read a post awhile back on here that they would not last without burning out but i don't know why but then again i know very little about electronics __________________ common sense isn't so common anymore

06-23-2006, 01:16 AM	#10
Richard Senior Member Join Date: Dec 1999 Location: Southern California Posts: 2,811	Re: hei stock tach ,how far off is the reading? I had always thought the pot that is accessable from the back of the tach is for adjustments Never messed with it though. __________________ Richard 1972 K10 Custom Deluxe SWB Fleetside My build https://67-72chevytrucks.com/vboard/...d.php?t=800746

06-23-2006, 10:53 AM	#12
StingRay Senior Member Join Date: Mar 2000 Location: Saskatoon,SK,Canada Posts: 2,476	Re: hei stock tach ,how far off is the reading? I have run my stock oe factory tach with hei and msd. No problems. I checked it against a tach in my digital volt meter and while it was not killer accurate it wasn't more than a couple hundred rpm out in it's worst case. I have about 10 years of straight hei to fact tach with no problems and about 3 years of msd to factory tach with no problems. I believe that adjustment pot on the back is used to set the tach to zero isn't it? __________________ Saskatoon, Saskatchewan Canada

07-04-2006, 12:16 PM	#13
Madbomber88 Active Member Join Date: Mar 2004 Location: Virginia Posts: 169	Re: hei stock tach ,how far off is the reading? Not sure if this helps, but my factory tach with HEI reads 400 RPM faster than it really is. __________________ 71 Blazer CST 72 K20 Capt USAF



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