<VV> tach

[Bryan Blackwell]

I've seen a couple posts on the inner workings of the tach - I'm not  
enough of an electrical guy to recall any of it, but you might dig  
through the archives a bit.  Here's one post regarding calibration  
that may help from Frank "man of many nicknames" Burkhard:

Les, Jim and VVers,
     Jim jostled my memory on this tach calibration question.  The 6  
cyl.
Corvair engine produces 3 ignition coil pulses per revolution since  
all 6
cylinders fire every 2 revolutions of the engine.  If the engine is  
running
at 2400 rpm (or 40 revs per second) that makes 120 pulses per second  
sent
to
the tachometer.  This is exactly the same number of 12 volts pulses (or
peaks) of DC voltage produced by a full wave rectifier battery charger.
The
battery charger takes 120 volts AC at 60 cycles per second and  
converts it
into 12 (more like 15) volts DC with 120 pulses per second IF the  
charger
uses a full wave rectifier.  If it uses a half wave rectifier, it  
produces
60
pulses per second DC which would be equivalent to the Corvair engine at
1200
rpm.
The power company takes considerable pains to maintain the 60 cps quite
accurately since it governs the speed of electric clocks and other  
devices
that depend on the frequency to control motor speed of things like  
VCRs and
CD players, etc.
     See, Les, no begging required !!(:-)
     Frank "gauge lover" Burkhard

On Sep 25, 2008, at 4:09 PM, Arend Huisman wrote:

> i have installed a Spyder dash in my '64 monza; everything works  
> well except for the tach, which gives a reasonable good idle speed,  
> but doesn't go further up than 1500 rpm. I looks like it responds  
> to the vacuum directly. It is connected to the minus pole of the  
> distributor coil as discribed in the electrical scedule (Classic  
> Corvair, Bob Helt) Can someone help me out?