T&T: Advice on Equalizing & Tackometer?

[Apa Ongpin]

Alternative tachometer solutions:

We are currently researching solutions for our older engine, whose maximum
rpm is only 900. Alternator-driven solutions were problematic. In order to
drive our alternator to some kind of realistic output, we had to pulley it
up something like 1:3, which means that any tach driving off this would read
the alternator rpm, or three times the engine speed. Sure, we could
constantly just divide the indicated rpms by 3, but that gets old. Also,
most tachs don't recalibrate easily by such a large factor.

The "bicycle solution" is a dynamo that puts out a predictable voltage per
rpm, in this case 1 volt AC per 100 rpm. Thus, 2 volts = 200 rpm, 3 volts =
300 rpm etc. This is a great solution, if you can find the parts, and we did
(a dynamo-driven  tach that reads to less than 1500 rpm, in our case, is
quite difficult to source in this day and age).

Fortunately, one tach we found works off DC as well, so it's easy to test
and calibrate. Stick a 1.5V battery on, it should read 150 rpm. A 9V battery
should read 900 RPM. We worried about "voltage drop" (it's a 35 ft wire run
from the engine room to the pilothouse, where the tach is installed), but
were told that the voltage won't drop, just the amperage.

I am told there are also optical sensor solutions for bicycles, but have not
personally laid my optics on one. Here's one for $28 (you have to provide
your own display):

http://secure.oatleyelectronics.com//product_info.php?products_id=230&osCsid
=d2745a6da3a340fe63be270e866d56bb

The motorcycle solution, also used in many cars, is a magnetic sensor, much
like those found in a gas engine spark distributor. Basically you mount a
stud on the flywheel or vibration damper, and the other is a 12V magnetic
pickup mounted directly above it. As the flywheel or damper spins, the stud
comes into proximity (close but doesn't touch) the magnetic pickup, which
senses the field and creates a pulse. The tach counts the pulses. This is
way more accurate than an alternator-type tach at low rpms.

In our case, for low rpms, we may need to use a motorcycle tach that reads
to 10,000 rpm and put ten studs, thus all we need to do is remove one zero.
You can pick up all the parts for this in any automotive junk shop.

We will probably end up with several of these and see which works best.

Rafael