Tach accuracy....

mrvette

Phantom of the Opera
Joined
Mar 24, 2008
Messages
15,207
Location
NE Florida
anyone ever wonder if your tachs are accurate....know how to test it??

something I got curious about so decided to prove the point....

I happen to have a digital audio oscillator goes from damn nearly DC to very fast on the output....

nothing for a Tektronics scope though...got one of them too....

so I decided one day to find out if my tach was accurate or not...since it IS an old OEM POS from a 75-77 vette.....

how to know?? easy...figger 600 rpm idle....that ten trips up and down per second for a piston....damnit, that fast....

5 sparks per second being a 4 cycle engine...duh.....

times 8 cylinders means the sparks are flying at 40 hz....not too fast by electronic standards.....

so to take a old horizontal output transistor from a color TV set, and just clip it to the tach output terminal on my HEI....fire the base lead (transistor input) with the oscillator.... and read the tach...with the ignition on but engine still.....

I can run that puppy from 40 hz which is 600 rpm as above...to 4000 hz which is 6000 rpm, and of course anything in between....

proving the time base is easy with the scope that way I KNOW it's dead nutz on accurate....

I found my tach high at anything above 3000 rpm same 300 rpm all acrocc to 6 grand....slightly high....at 1500 rpm it's dead nutz on....

at idle it's slightly lo....maybe 50 rpm...

just so you know....

I know, I"M bored....
 
I guess I'm bored too. Just checked my timing today. Tack is about 50RPM low at idle compared to the tach reading on the timing light.
 
I read once where a guy used a modified 60 htz cell phone/battery charger. Should read 800RPM.
 
I read once where a guy used a modified 60 htz cell phone/battery charger. Should read 800RPM.


heh...I have a couple old crank phones with the magneto there, fun to turn it and hear the bells....love to take a islamic an connect to their nads, and ring the balls a couple times....

stormy weather.....


:bounce::bounce::bounce:

call it electro shock therapy......

:bump:
 
Your dial back

why you say that?? those things digital with a 3 meg clock or something???

I never did trust a pot with a dial on it for degree in say a setback timing light....can't be all THAT accurate, but I sure can be by marking the balancer and reading directly....
 
As a matter of fact my dial back timming light is Digital :devil: And i would have to agree with TT,that the dial back RPM is more accurate than the 30 year old tach in the car.
 
As a matter of fact my dial back timming light is Digital :devil: And i would have to agree with TT,that the dial back RPM is more accurate than the 30 year old tach in the car.

Only dial back light I seen is one a buddy bought some 30+ years ago....I never bothered....just wasn't impressed....

but ti's all up to the computer in the car NOW...so hell with it....

:smash:
 
Your dial back

why you say that?? those things digital with a 3 meg clock or something???

I never did trust a pot with a dial on it for degree in say a setback timing light....can't be all THAT accurate, but I sure can be by marking the balancer and reading directly....

I have found the Sears unit to be very accurate. Checking it against the tape.:D
 
Your dial back

why you say that?? those things digital with a 3 meg clock or something???

I never did trust a pot with a dial on it for degree in say a setback timing light....can't be all THAT accurate, but I sure can be by marking the balancer and reading directly....

I have found the Sears unit to be very accurate. Checking it against the tape.:D

Yes, on the sears light.:bounce:
 
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