TECH QUICKIE: Timing Lights and Wasted Spark

 

Setting your crank sensor position, relative to TDC, in the ECU map is a crucial step in setting up an engine.

If you take out all the spark plugs except cylinder 1 and use the built-in crankshaft oscilloscope in the DTA software, you’ll usually get pretty close but it’s always best to double check,

To dial it in properly, we always recommend using a timing light, preferably of the “dial-back” variety.

If your light can handle wasted spark or has a “2-stroke” mode, you’re fine. If not, you’ll need to set the light to double of that which the ECU is commanding. E.g. ECU says 10 degrees in real time mapping, set the light to 20.

Why?

Timing lights that don’t support wasted spark expect to see 1 pulse per engine revolution. With wasted spark they see 2 pulses. Double the pulses means double the RPM to these lights. When you try and “dial-back”, the time it thinks the engine takes to rotate 1 degree ends up being half what it actually is. At 1000RPM takes around 166μs per degree of rotation. When the light thinks the RPM is doubled, a degree takes half the time, 83μs. However, the engine is running at 1000RPM, not 2000RPM so if the ECU is commanding 10 degrees of advance and the light is set to 10 degrees, the light flashes 833μs (5 degrees) before TDC instead of 1666μs (10 degrees) before TDC. So by doubling the degrees on the timing light, relative to what the ECU is commanding, you’re doubling the time it thinks it should flash before TDC, which is half the time it should actually be.


TECH QUICKIE: Timing Lights and Wasted Spark