i have a sunpro tach that is adjustable from 4 to 6 to 8 cylinders. what would happen if you are running it on a v8 with it in another setting? i have reason to believe that my tach is off and this is the only solution that i have come up with.
got a RPM question
- Thread starter Cobain03
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dodgestang
Active Member
what if i said it was reading too low? i even have a dyno grap showing that the driver took the car up to a requested rpm (4200) while the graph shows around 5200.
Great68
Founding Member
Dodgestang is correct... Personal experience: When I first got my 66, the PO had installed a tach that was set to the 6cyl setting instead of the correct 8cyl one. With the 3.00 rear, I calculated that I actually had a 4.11 comparing the tach readings vs the speedometer. When I was actually going 3000, the tach would read 4000 (and with the unfamiliar engine sounds I had no reason to doubt it). Being young and not so bright, I went ahead and bought a 3.55 track-loc as a replacement, thinking I was backing off on gearing... Although an expensive "mistake" I really enjoyed the new gears! 
Before I drove it, I counted the teeth on the old differential and realized my error. After a five minute corrective procedure, the tach has read accurately ever since. Not all tachs are accurate, though. For example, the mid-eighties factory tachs were notorious for not telling the truth.
Daniel
Before I drove it, I counted the teeth on the old differential and realized my error. After a five minute corrective procedure, the tach has read accurately ever since. Not all tachs are accurate, though. For example, the mid-eighties factory tachs were notorious for not telling the truth.Daniel
65ShelbyClone
Founding Member
Do you know how old your Sun tach is? Old electric tachometers are more likely to be off than the newer air core stuff like even the current $35 Super Tach II.
If it's a newer one, it's possible to adjust them by playing with resistor values inside, but it's not really an easy thing to do if all you want to do is fix a $35 tach.
On a side note, I took a newer Super Tach II and converted it to 1, 2, and 4cylinder operation. VDO gets $90 and Autometer charges $110 for tachs to do the same thing.
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