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Accuracy of Tach

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Old 07-16-2017, 11:49 PM   #1
Kim72
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Default Accuracy of Tach

Is there any way to check the accuracy of a tach BEFORE it goes back in the car? I need to replace the needle on mine. I did this years ago and once back in the car I discovered that the tach was 300 rpm low (checked with timing gun). I don't want to repeat this problem. PITA to get the tach in and out!! Any ideas?
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Old 07-17-2017, 04:09 AM   #2
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Quote:
Originally Posted by Kim72 View Post
Is there any way to check the accuracy of a tach BEFORE it goes back in the car? I need to replace the needle on mine. I did this years ago and once back in the car I discovered that the tach was 300 rpm low (checked with timing gun). I don't want to repeat this problem. PITA to get the tach in and out!! Any ideas?
Willcox sells a signal generator that is used for this.
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Old 07-17-2017, 05:42 AM   #3
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Being an old ET, I have some left over lab gear, and my Tektronics scope, and digital audio oscillator were employed years ago, drove the tach output connection with ignition on, and engine not running, and set the frequencies .....really simple, 600 rpm assume idle speed, that is 5 pulses per second/per cylinder X8 is obviously 40 pulses per second....so 40 hz from the oscillator, and proven with the tek scope set on line lock to the 60 hz power line freq......

my '72 has an HEI, and later shark electronic tach.....so I noted that up to ~2000 rpm, the tach was right on.....at ~2400 it was high by about 150 or so, by 4500 rpm the tach was reading over 5 grand, and at 6 grand it was meaningless.....

Tell you something else, years ago I got into a street race and thought I was going FAST, as in ~150 mph fast.....turns out I was RONG,

I was going by the tire sites, on tire diameter/size, rear ratio, and of course final drive ratio of trans....and of course the lieing tach.....

SO to prove it all out, I got a GPS ......at 40 dead on, at 60 dead on, at 80 it's only 78, at 100 indicated, it's only 94 REAL mph.....I ran outta room at that point, but it don't take a rocket scientist to see the curve on THAT......
AND something else, the gear count on the driven gear was supposed to be X teeth.....bullshit, it was reading ~10% high on speed, the above figgers are AFTER I got the correct number of teeth in the driven gear, as proven by GPS.....

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Old 07-17-2017, 12:04 PM   #4
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attach tach and cable to another car, or distributor machine, with tach drive.
for verification.
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Old 07-17-2017, 12:13 PM   #5
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If you knew the rotational speed of an electric screwdriver (they turn slowly), you could drive the tach with it and calculate what RPM the tach should read. Maybe Willcox has a table of "Input rpm vs. Tach scale reading" that you can use.

Anyone know the conversion factor for 'input rpm' to 'tach reading' for an early C3 tach?

Last edited by 7T1vette; 07-17-2017 at 12:14 PM.
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Old 07-17-2017, 04:34 PM   #6
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Quote:
Originally Posted by Kim72 View Post
Is there any way to check the accuracy of a tach BEFORE it goes back in the car? I need to replace the needle on mine. I did this years ago and once back in the car I discovered that the tach was 300 rpm low (checked with timing gun). I don't want to repeat this problem. PITA to get the tach in and out!! Any ideas?
If the car is a 72.... and distributor drive.

The tach is a 2 to 1 tach. So 1000 rev's at the tach head should read 2000 rpms on the face. If you have an electronic tach in the car then you'd probably be best suited to send in the tach to test the calibration or purchase the calibration unit we have.

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