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Is the Tacho Accurate...

7316 Views 4 Replies 2 Participants Last post by  Eclipze
[align=center]Is the Tacho Accurate?[/align]

Actually... it's more appropriate to ask, how accurate is the tacho?

I was logging the RPM digitally while riding the CBR250RR, and I was surprised that after bouncing off the rev limiter that the very accurate digital record showed more than 1,000rpm less than what I reckon I saw on the tacho gauge!

So I grabbed a CBR250R instrument that I've got lying around, and decided to test just how accurate it was...


I powered the gauge up with a constant 12.5V supply, and drove the tacho input with a square wave from a frequency generator, with similar amplitudes to what the bike gives out. To get accurate/repeatable results, I turned the input signal off, then on again to prevent the needle from getting 'stuck' by moving too slowly.

The blue graph is the ideal, red is the actual.


The worst was probably at the data point where the input was 16,935rpm and the display showed 17,800.

The needle covers the ground between two RPM values fairly quickly, basically within 0.7 to 0.8 seconds, however the last 500rpm is relatively slow.

Other tachos might have a different response, but I thought it was interesting... well... for perhaps one or two of you :p
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By looking at your gauge response graph and error distribution would I be correct in saying the the little CBR is hitting the rev limiter about 700rpm early?
Or does this effect the limiter at all.
The limiter isn't a function of the gauge. The CDI reads the crank angle sensor and drives the ignition system. The tacho output is just an output to the instrument. All limiting is done through the CDI.

When these 250cc bikes were being designed, there was an RPM war on between the Japanese manufacturers hahaa.... So they were embellishing on what RPM the gauge showed, vs what the engine was actually doing.
Okay so the ignition gets the angular velocity of the engine straight off the crank, does the tacho get its input from the CDI?
Sorry I'm just curious.
Yogi said:
Does the tacho get its input from the CDI?
Yup! :nod:
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