Rev counter precision

[Philippe-J.]

Guys,

I tested my rev counter sender on the bench, rotating it with my column drill at different speeds:

I noted that the reading is 8% overestimated, except at 5300 RPM where it's perfect.
Question: what is the expected precision for the engine rev reading? Should it be better than 8%? Is it worth sophisticating my test, e.g. with a rev counter instrument to check the drill speed? And to have my instrument re-calibrated?

Thanks for your ideas.

[mgcjag]

If you dont know your drill speed then you can't tell how accurate your rev counter is...led rev counter testers are cheep...and will measure your drill revs accurately....Steve...ps you then determine if the problem is the generator by measuring ng the voltage or the rev counter

[DWW]

I remember there is a phone app to measure rpm from the days I flew rc helicopters

[Philippe-J.]

Thank you for your advise.
I do imagine they are solutions to check my set up. My question is whether it is worth doing so, as my test shows a discrepancy of 8%, with two unknows (more or less): the drill rev and the tachometer. As it is unlikely that their respective inaccuracies are huge and compensate, I can assume that my tachometer is accurate to better than 8%. Is this actually much worse than the expected precision of a nominal tachometer reading in an E Type?

[mgcjag]

There is no accuracy spec in the servic manual....My reply was how do you get 8% when you don't know your drill speed?....is the inaccuracy unaceptable to you..if yes then test it all and have it recalibrated..Steve..ps have you checked the voltage output at the generator across the rev range.....

[abowie]

The tachometer when brand new with a brand new generator should have been pretty accurate. I’d expect it to be a couple of percent perhaps.

Your 60 year old generator and AC voltmeter however.. less so

I would have thought that 8% error was pretty good. However whether it’s adequate for your engine or not is another question.

Personally I’d buy a $20 digital tachometer from EBay and set it up so you can compare what your tachometer says with reality. Then you can decide whether it is good enough.

You can have the tachometer upgraded to run directly off the coil signal. This removes the generator from the equation.

[Philippe-J.]

That's two interesting replies, thanks.
- to check the voltage output from the generator. I remember indications of this voltage as a function of the RPM. I just have to find them again, I don't remember where that was
- compared to Andrew's 2% estimate for a new stuff, I find that 8% is not bad but not excellent. I guess it's enough to drive, but worth checking with a 15E rev counter and Steve's idea.
At least, I now know that the magnets in the generator are not tired, nor the counter stuck, which I feared, I must confess

[mgcjag]

Details in service manual electric/instrument section.. ac generator 1volt/100revs

[Philippe-J.]

Thank you Steve, I found it, that's the info I needed (even though it is given "as a rough guide" . So back to my initial question: how much is "rough"? )

[Tommd]

That is 1vac per 100 cam rpm’s.

[Philippe-J.]

In my case, "rough" is a "very rough" 1 Vac per cam rpm's. So I ordered a rev counter to make it clear. Then I'll know.

[Philippe-J.]

I eventually got my remote rev counter (for E 12, including postage ) and the measurements are much clearer. They exhibit the true rpm of my drill, that, compared to the rev instrument reading, is +/- 2 % away from it at low speed. Only the 5500 RPM is under estimated by 4 % by the instrument.
I think I can live with it, can't I?

[RICHOT]

When I had my car rolling road tested many years ago, I was advised that the rev counter was 8-10% optimistic, so I just live with it.

[Philippe-J.]

Thanks for your confirmation!