This question isn't specific to Y-Blocks, although it might be if they are converted to Electronic Ignition. Can I hook up an old Tach/Dwell meter, to check an electronic ignition? I wanted to set the idle properly for an Emissions test. In this case it would be to a HEI distributor.

Paul

Never thought about it but I’m not sure an electronic ignition system has a “dwell” (or dwell angle, the degrees of rotation of the distributor cam during which the ignition contact breaker points in the distributor are closed thus saturating the coil) that can be measured??



But then again it will need time to build up the magnetic field in the coil. Interesting question...



Hope that doesn’t totally confuse the question??








Johnson Rod



Southern California

Paul -

The dwell / tach only wants a pulse signal from the ground side (-) of an inductive ignition coil to operate - probably might ought to avoid any capacitive discharge ignition units (MSD etc.) - but the HEI and Duraspark II types of "electronic ignition" are just substituting a grounding transistor for the points. 

If you switch the unit to tach, it counts the pulses and indictates one revolution for every four times the coil "fires" (for a V8). Most of the units have a switch where you can select the number of cylinders - 8/6/4 so the gadget knows what to use for the divisor. 

The dwell is "calculated" from the portion of time the switch is closed between each time the system cycles. On the early electronic ignitions the dwell was set to a fixed amount (by a calculator timing chip) but later versions (GM HEI) used a more powerful "brain" to alter dwell as the rev's increased (and thus give the iginition a little extra zip). Because our V8's fire every 45°, the ratio of time "on" to "off" compared to the time "on" to "on" can be displayed as degrees. For instance if the grounding transistor (which replaced the points) is closed and operating 50% of the time between "on to on" cycles - the dwell would be shown as 22.5° (half of the total angle).  

This number is a little too low for a traditional "hotrod" inductive ignition - it is/was pretty common practice to use dual points to jack up the dwell angle to something like 34° (a 75% duty cycle).

 
 Steve Metzger       Tucson, Arizona

Steve: So, I just need to arrange a connection to the negative side of the distributor, for the meter to give a RPM reading? And thanks for the explanation of how the electronic & capacitive discharge ignitions work.

Paul

Paul -

Correct - the trigger lead for the dwell tach should be on the negative (-) post of the coil.

 
 Steve Metzger       Tucson, Arizona