Adjustable Ignition Timing

Discussion in 'Performance Mods' started by jaguar, Oct 3, 2014.

  1. jaguar

    jaguar Well-Known Member

    Adjustable CDI
    Here's why I think it is the only correct way to go:
    Everyone eventually modifies these weak unreliable vibrating engines to be stronger, more reliable, and less vibrating. Each different modification can require a different ignition timing at high RPM. A different cylinder head is usually high compression which requires retarded timing. A better carburetor atomizes the fuel better which makes it burn quicker which requires retarded timing. Longer exhaust port durations (due to raising the port) require more retarded timing. So basically all the common ways to modify these engines usually require more retarded timing, but how much? Each engine is different and only by testing can you determine how the high RPM timing should be. It is common for modified race bikes and karts to have adjustable timing CDIs because the engines are now so different than the stock engine for which the stock CDI was made.

    Why is timing so important?
    If timing is too advanced then it occurs earlier than it should before piston top dead center which causes more peak cylinder pressure (see first graph) which needs to be offset by the counter-balance holes in the crank flywheels which hasn't been done in the Grubee engine. A low compression poorly carbureted engine like this one needs advanced ignition but not as advanced as the stock CDI provides. What is strange is that there is little power difference between two engines that have different timings, so really power is not an issue although being too advanced keeps the engine from reving as high in the RPM range than it wants to. The overly advanced timing engine has two disadvantages from its timing though. It gets too hot and it vibrates more if the crank flywheel wasn't holed correctly to match. The more retarded timing engine has the advantage of a bit more top speed but the engine doesn't get as hot and doesn't vibrate as much. Engine heat decreases engine power though so if the two bikes were racing on a course that limits their top speed then eventually the overly advanced timing engine would fall behind as heat induced friction caused more power sucking resistance.

    This graph shows a 31% increase in peak cylinder pressure by advancing the ignition timing from 21 degrees BTDC to 31 BTDC.
    Picture 13.jpg

    This graph shows a 50% increase in peak cylinder pressure by increasing the compression ratio from 6:1 to 9:1 which is a typical increase when switching from the stock head to a high compression head.
    Picture 28.jpg