Dear Nick Gammon.
My apologize, my brain is not good enough to understand your code.
Thats why I look for a dumb work around.
I got additional info about target engine
The engine have 132 tooth flywheel, and max RPM is 2500
and I will use this to convert from Frequency to Voltage
with Magnetic Pickup like above, I thing I can use (cmiiw) the math of :
Freq = Gear x Rpm
means that the max Freq will be : 132 x 2500 = 330,000 Hz
and the max Voltage will be 4925.373134328 V.
Using Voltage devider of factor 0.001 at the Vout point, I'll got max volt of 4.925373 V where the arduino analog pin can handle.
And I thing the max analogread value returnd will be (4.925373/5)*1023 => 1007.7313
That is the max value that will be reported to PC.
Thats what I can thing it so far.
Sincerely
-bino-