I have been running the Dave Ray small cap HEI distributor ever since building my engine about 2 years ago. I have an original '71 In-Dash tach that I just installed after having it calibrated by an individual that has done this for other tachs before. It was working before but, I just wanted him to check and make it as accurate as possible which he says he did. Fast forward to the other day when I finally installed it into my dash and low-and-behold, it is reading 20% low all the way from idle up to 80 MPH. In doing some on-line research on HEI ignitions and original factory tachs, I find that some tachs are known to run erratically with HEI ignitions. Some recommend installing a tach buffer to smooth the RPM signal out. But, most of the problems give erratic operation of the tach and my issue is just the inaccuracy. The tach is smooth all up and down the dial. 20% low at all readings. So, all that said, has anyone else had this kind of a problem with interfacing a factory tach to an HEI system? I need to figure this out before putting my dash back together. (had to remove the dash pad, AC Duct, radio, dash cluster, and drop the steering column to get the tach in).
If it is at 20% low, that sounds like a voltage issue. Is there a resistor of some sort, even within the wire, that would cause that?
No resistor wire. When I replaced the wiring harness, I also removed the original resistor wire and replaced with a solid wire so 12 volts goes to the HEI. The tach input wire going from the coil to the tach is also a solid wire.
So its 20% low? As in 1000rpm is 800rpm, 2000rpm is 1600rpm, etc? Not one defined number below, i.e. 300 rpm
And did it read normal with points? My idea would be to put in an in-line resistor to bring the signal from the tach input on the coil to the level that the points signal gave, if that makes sense. Someone please correct me in the morning if I'm wrong.
My original tachometer stopped working shortly after I bought my car in 1999. I replaced it with an Autometer 3 3/8" in dash tachometer. It was so dead on accurate, I left it in there for many years. A few years back, I sent my original tachometer to Scott (87GN_70GS) http://v8buick.com/index.php?members/87gn_70gs.1236/ He fixed my tach and calibrated it. When I installed it, I found it was fairly accurate at the ends, but it was off in the middle. With my GV, at 60 MPH, it should read about 2200 RPM. The tach was about 5-600 RPM high, it read up around 2700-2800. Back in went the Autometer. Maybe we expect too much from the factory tach? I run a Dave Ray distributor with an MSD Digital 6 box, so my signal comes off the box. Tom Telesco says D&M Restoration can install more modern guts in an older tachometer. It is on my list of things to do. http://dandmrestoration.com/tachometer-repair/
When I bought it it had been removed from a 1971 GS with points in operating condition although I never saw it operate. My engine has had an HEI ignition since I built it so no, this tach had never operated with points since I've owned it. I'm waiting for a tach buffer which is supposed to condition an HEI signal to work with vintage factory tachs. We'll see.
Yes, it reads about 20% lower that it should. i.e. reads 1500 when it should be 1800 (40 MPH), 1800 when it should be 2300 (50 MPH), 2200 when it should be 2700 (60 MPH), and 2700 when it should be 3200 (70 MPH). So, the error starts at about 17% low and increases to about 20%.
I'm still looking for feedback from anyone who has an HEI ignition feeding a bone stock factory hood or dash tach. Does the tach read accurately or did you have to modify it to make it work?
My friend runs HEI and a factory tach. No issues. He only replaced the resistance wire to the HEI. Sounds like your tach needs to be serviced. There was one or two listed above, but I used Bob's Speedometer. He wasn' cheap but hos service and workmanship was excellent. He is on the internet and Hemmings
My ignition simulator i use for calibration can replicate the duty cycle (dwell time) used by the HEI. I have seen very little difference in the reading between points and HEI duty cycle
Can you post a picture of the HEI ignition pulse or, describe it as to amplitude and pulse width (duty cycle)?
It varies. It is longer with low battery voltage, shorter with higher voltage. It becomes longer at higher rpm, shorter at lower rpm, and that is a function of charge time plus coil current limit; whenever the coil current limit is reached, it is reduced also. So it's a complex function. But you can vary the dwell, at a constant rpm, from approx 10% to approx 90% and indicated rpm will only drop a few percent or less with the original circuit
Sounds like an area under the curve problem (aka) dwell/PW issue. This is why newer tachs are driven by the leading edge instead of a fully formed wave. The really fancy tach circuits will distinguish frequency from the alternator like some 80's model early EFI vehicles. Anyway, I posted a short vid of a difference between an analog tach (the VDO shown) and a digital modern Sunpro tach. Yes the the Sunpro gauge face is analog but the driving circuit is not. In the attached video I am using a signal generator.
If you decide to send your tach anywhere send it to Scott, good prices and service/followup is excellent. Helped me out with the tach in my convertible, works great now.
