so im trying to get an arduino to read a 4 cycle 3 cylinder engine speed.
im using an lm2917. max speed 8k
so im assuming a 4cycle engine is 8000 rev per minute, 24000 ignition coil firings per minute, 200 hz at 8k rpm.
now the lm2917 math is
Vo = Vccc * Fin *c1 * r1 * k
so 924 = 14 * 200 * 0.000033 * 10000 * 1
where k is gain constant, about 1.0
im assuming Vo is volts out.
running this math i get 924.
so is this max voltage of 924V, or 92.4V or 9.24V or .924V?
some help please?
I've never used an LM2917 so I don't know about all of those calculations, but a 4-cycle engine fires once every other revolution. Each cycle is half a revolution. (Intake, Compression, Combustion, Exhaust.)
With 8000RPM and 3 cylinders, I get 12,000 firings per minute, but I agree with 200Hz. 
If you're using an Arduino, there are ways of directly counting pulses. If you're doing a lot of other processing, you might have to alternate between pulse-counting and other stuff, but it's probably best if you can avoid the analog stage.
P.S.
You can test your tachometer by modifying the Blink example t generate various known frequencies.
the idea here is simple, i really need an analog input.
Given the numbers you supply, 924V is the correct result.
But the various performance charts in the datasheet show values for C1 of 0.05uF or 0.01uF whereas the value you've used is 33uF.
Was that meant to be 33pF (.000000033)? If so, the result of your equation would be 0.924V.
If we rework that equation to calculate the capacitor value required to give an output of 5V, we get 178pF.
I think you need to sort out a reasonable value for C1.
Pete
no, 33uf was what i used to run the math.
thank you so much.
yeah, i will need to rework the values, t now i know where i stand and how to run that math.
Seems silly to use an analogue IC to convert from digital (pulses) to analogue (voltage).
And then to digital again.
Arduino should be able to convert pulses directly to an RPM reading.
Leo..
enter e.g. "car tachometer" in the search box on top of this page.