Also keep in mind the disty output is very unstable just because of
the slop in the gear drive/powerpulses as it's rotating. I saw the
same thing when we built our distributor replacement (replaced the
stock hall sender with a better square wave output hall sender) and we
saw this slop in the readings. Our pickup was given 72 pulses per rev
of the disty so I had lot's of granularity to see it and it needs some
filtering to be used for any real accuracy (like how we were using it
for fueling and spark control). But just rpm readings though you
should be good with some averaging.

Jim Akiba

On Wed, Dec 4, 2013 at 4:09 PM, Craig C Forney <craig@opus.com> wrote:
> David is right.  Multiply pulses per second times 30 to get engine RPM.  30
> pulses is 900 RPM.
>
> Target Vanagon idle RPM is 880 plus or minus 50, which is 29.33 pulses per
> second plus or minus 1.67 pulses.
>
> ICUs working properly typically perform at 850 to 910 RPM, with most
> performing at 860 to 900.
>
> Craig in Cupertino
>
>
> At 11:33 PM 12/3/2013, James wrote:
>>Distributor rotates at half crankshaft speed.... idle is 850 rpm....
>>half is 425 rpm... one Hall Effect signal per crankshaft rotation ( or
>>is that two per rotation...??).... nah, I'd better try to calculate
>>this tomorrow!
>
> One pulse per (piston @  TDC** on firing stroke) so two per crankshaft
> revolution.
>
> 850 rpm / 60  secs/minute = ~14 rev/second x 2 pulses/rev = ~28
> pulses/second.
>
>
> ** Really at the fixed timing advance point which is a few degrees before
> TDC on the 2.1l.