Electromotive HPV1 tachometer inaccuracy problem

Curtis

Curtis

Well-known member
I have run these for years and never had this problem. Realized at dyno today that my tachometer and the optical eye set up for the dyno do not match. At 4000 rpm the tach and the dyno read the same. However at 8000rpm on my tachometer the dyno said the engine was turning 8800 rpms, a 10% difference. Spark seems good and controlled and it was the same with the rev limiter turned on or off. I have a spare unit to swap in but would like to understand, and fix this one, if possible. I am going to delve into it this week and figure out how the tach signal is sent but am hoping somebody else has been through this before and can help.

Anyone?
 
Bob, Thank you but I think there is a new problem as it worked fine for years as wired and this developed more recently. I am going to be looking for what might have changed.

Matt, I don't think anything I do surprises Sam anymore.
 
I'm an analytical chemist who fixes instrumentation for a living. And the first question that popped into my mind was this: Is the difference always 800 rpm? Or is it always 10% higher than it's supposed to be (500 rpm at 5000)? I know nothing of how the two units work but the answer to this question will probably help the brainiacs who do.

Dayle
 
Dayle,

It was accurate or pretty close from 0-4000 rpm and the difference got progressively larger as rpms were increased beyond there.
 
Doesn't Electromotive have an adjustment for advance?
Where do you have the advance change set to occur (4000)?
 
Curtis.. The advance on mine started increasing on it's own one time on the track and the tach didn't seem to be acting right at LOW rpm. Autometer tach was new so I sent the unit to Electromotive and the values of one of the pots had changed. They replaced the pot, put it on the analyzer, reset my advance curve and have had no problem since.

Bob
 
THANKS to everyone who offered ideas on and offline!

I am embarrassed to say it was not the Electromotive but rather the tachometer. Replacing the 12 year old ProComp tachometer with an even older Mallory fixed it. Should have checked this first :oops: but just couldn't imagine it being anything other than the ignition.
 
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