I have seen it mentioned somewhere in one of these boards that you must "recalibrate/rework" the old style factory Buick tachs to work correctly with an HEI distributor, can anyone with experience verify that info? And do I need to wait to connect the tach until that modification has occurred? Here is what I have: An original console tach for my '67 GS with a complete GM HEI unit on the '71 455 sitting in it, I just need to connect the brown wire. Thanks for any info you can provide, Couch. o No:
The OEM tachs are frequently innaccurate no matter what ignition they are connected to. Some are worse than others. I know plenty of guys running an original tach off an HEI. Nothing is needed, just hook the brown wire to the tach terminal.
Do not run an Hei dist with stock tach!!!!! You will fry it, mine worked for a little while, but after I got it back from the body shop, it was fried. I ended up installing an aftermarket unit, will someday send mine in to get it repaired and hei ready. Kevin
Kevin, I know lots of guys running the stock 70-72 tach with an HEI. No problems whatsoever. Your tach fried for some other reason.
http://www.v8buick.com/showthread.php?t=25732&highlight=stock+tach+fried Larry, this is the thread that I started when I fried my stock fried. Hope the attachment works out, otherwise do a search for "stock tach fried" Kevin
Yes, I even posted on that thread myself. Guys suggested some alternatives to the stock tach, including the Autometer that I use. But the stock tach will work just fine with an HEI. My friend Ron Mooney (Tufbuick on this board) has his stock tach hooked to the HEI he uses in his 72 Stage1. That's the way he bought it 5 years ago, and it's still going strong. It was just time for your tach to go. Mine dropped dead too, and it was attached to a Stinger S-4. The stock tachs aren't the most durable or accurate for that matter.
I'm still using my stock tach with HEI. I also have a hood tach that worked for about 3 years, but the hood tach stopped working one year after I propped my hood up in the garage while I freshened up the motor. When I put it back on it never worked. But all that was with stock points. Now I run fine with HEI. Oh, and I was amazed that both my tachs read the exact same... when they worked.
On the dyno... My hood tach was only 300 rpm off, dash tach 100 off what the dyno equipment read. All 3 were read & compared at the same time, up to around 5,800 (no, it was not my engine, probably a ford). I suspect they begin inaccurate #'s once they have dust/crud/corrosion inside. Yes, I had completely & professionally cleaned, and repaired them before the dyno, even tested with square wave signal for needle movement and accuracy.
I fixed my tach once, then put an HEI on it. It lasted a year. Coincidence? Maybe, but I think HEI's develop too much voltage for the stock tach. When mine fried, I put a 'scope on it to find out why. I saw about 40 volts on the tach wire with the HEI. With the stock ign, I was seeing about 25 volts when the points opened. The turns ratio for both coils is about the same. So when a voltage is impressed across the coil secondary, a voltage is reflected across the primary that is the secondary voltage divided by the coil turns ratio. So it stands to reason that if the spark voltage is higher, the tach input will see more voltage. The fix is a simple resistor voltage divider that would divide by the voltage in half. PS Have you ever grabbed the tach lead with the engine running? You'll feel the 25 or 40 volts !! Some of you may like that kinda thing but be careful.
