I just bought a new, digital timing light to replace my old original light. When I push the button, the timing moves but I’m not sure I know exactly what’s really going on here. There’s probably an easy answer but it escapes me! Thanks
Jeff Smith: What you are using is what is commonly referred to as a dial-back timing light. A normal timing light flashes the instant the Number One spark plug fires. This works great as long as you are only setting initial timing and only if you can line up the marks from the timing tab on the engine to the TDC mark on the balancer. A typical timing tab offers numbers up to 14 to 16 degrees Before Top Dead Center (BTDC). But if you need to set your initial timing at a figure greater than what is offered on the timing tab, you have to guess, which is never good. The simplest way to compensate is to use an MSD timing tape. These are adhesive tapes that adhere to the balancer and offer accurate timing numbers so that you can use a standard timing light to check maximum advance. There are several tapes on one sheet based on the diameter of the balancer.
But if you work on multiple cars/engines, it is time-consuming and cumbersome to attach a timing tape to each engine. A dial-back light is far more useful. The original dial-back lights employed a simple dial on the back of the light. In this case, you rev the engine to maximum advance and use the dial to line up the TDC mark with 0 (zero) timing on the tab. The dial on the light then indicated the total amount of advance.
The modern version is a digital dial-back light. Instead of a dial, these lights generally use a digital display screen with not just the amount of timing dialed back but also all kinds of other information. Instead of a traditional knob, these digital lights generally use up or down arrows to adjust the timing mark. You use these lights exactly the same way as the traditional dial-back lights, but the display and the options can be somewhat confusing. With the engine revved to full advance, the technique is the same — adjust the crank TDC mark until it lines up with the zero (TDC) mark on the indicator tab and the number on the back of the light will indicate the amount of total timing.
For example, the Innova digital timing lights that Summit Racing sells offer a display that will show system voltage (it uses power from the battery to run the light, so volts is accessed), dwell (if your engine is equipped with points), rpm, and the amount of timing the light has been delayed. The light that we referenced (PN INO-5568) is pretty sophisticated, but Innova does offer others that are less expensive.
What’s interesting is that the modern distributorless ignition systems (DIS) employed on all late model engines don’t require a timing light and don’t even offer a TDC mark on the balancer. It’s a new digital world out there.