Correct RPM? - Team Camaro Tech
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post #1 of 6 (permalink) Old Nov 28th, 06, 11:12 AM Thread Starter
 
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Correct RPM?

My factory tach appears to be off about 1000 RPM's. I have got pointless electronic ignition. At 60mph tach shows 3800 RPM's tire size and gears ratio I should be about 2500-2800.
Any thoughts on this, do I need a resitor in my tach lead from distributor?

69 Z/28 Hugger Orange X33 Four Speed
Tuck Jones
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post #2 of 6 (permalink) Old Nov 28th, 06, 02:56 PM
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Re: Correct RPM?

Tachometers are fickle things - especially the OEM units. You might try a tach filter (search the archives) to see if that helps. Seems like Jim M. posted a home-made tach filter in the Tech Reference or in the sticky at the top of the page.

Dave
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post #3 of 6 (permalink) Old Nov 29th, 06, 04:11 AM Thread Starter
 
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Thumbs up Re: Correct RPM?

Thanks Dave.
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post #4 of 6 (permalink) Old Nov 29th, 06, 10:18 AM
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Dave
 
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Re: Correct RPM?

I thought the tach filters were for bouncing tachs not calibration type issues? My tach is off by 350 rpm and I've been waiting to send it out for calibration. Has anyone solved the issue with a filter?
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post #5 of 6 (permalink) Old Nov 29th, 06, 01:09 PM
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Re: Correct RPM?

The tach circuit is a frequency to voltage converter. The signal on the tach lead from the coil will be a 12 volt square wave (or 9 volt) with large voltage spikes riding on it that reach 50 volts or more (up to a few hundred volts). The tach circuit attempts to strip off those pulses and convert them to a voltage. Trouble is HEI and other non-stock ignition systems present a different signal that can fool the circuit. Bouncy tachs occurr when the voltage is high enough to break down the diodes and/or transistor(s) in the circuit. But calibration can also be affected. The filter is a pretty simple thing to construct from junk parts and worth a try.

Dave
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68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #6 of 6 (permalink) Old Nov 29th, 06, 02:21 PM
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Re: Correct RPM?

My filter is in the electrical basics sticky.
It helps bouncy needles, don't know about it helping accuracy, it could.

I believe I also recall a post about adjusting a tach. Think it was by farm boy, and may have referenced a 68 repro tach, I know my tach has a tiny adjustment pot on it's printed circuit board.





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