Electrical magic?

archauto

archauto

Junior Member
I bought a old Motorola tachometer and found it was for a 6 cylinder truck according to Prestolite who has that Motorola as an obsolete item.

Thus the reading would be 3/4 of the actual rpms of the engine. Is there a way electrically to bump the reading by 1.33 times for it to read accurately?:rolleyes:
 
I think the reading of a 6 cyl tach would be 1.33 times the actual rpm if it was connected to an 8 cyl since it relies on 3 voltage pulses per rev for a 6 cyl. For an 8 cyl, it's actually getting 1 extra voltage kick per rev. So a resistor might drop a 4000 rpm reading to 3000 rpm. Not knowing the sensitivity of your tach, it's hard to guess how many ohms resistance you'd need but a ballpark figure might be about 5000 ohms for a 1950s era tach. If you had a high impedance rheostat or pot, you could probably dial it in --- that is rev the engine to 2000 rpm at 0 ohms, then gradually increase resistance till the tach reads 1500 rpm.

Jack E/NJ
 
If you have any friends with magical electronic skills you could have them build you a circuit like I did for an old Sun Tach; see my web page.
 
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