![]() Ignition Timing for Fixed Advance (Locked Distributor or Magneto) On high percentages of nitro at a high level of enrichment, 55 degrees of timing was typical. Nitro fuels run a lot more fuel volume and, as a result, need more timing.Įxample: For a blown alcohol engine on low percentages of nitro at a low level of enrichment, 40 degrees of timing was typical. With a very powerful 44 primary ampere magneto, only 22 degrees of timing is common. With a higher power magneto near 4 primary amperes, 36 degrees of timing is common. For a blown alcohol engine with an old 2 primary ampere magneto, 38 degrees of timing was common. Less ignition timing is needed for higher power ignition systems. ![]() An increase in timing was needed for best power. For the same blown engine on alcohol at a richer mixture, 32 degrees of timing is common.Įxample: One methanol combustion engineering test measured a 22% reduction in methanol flame speed for a 19% increase in fuel enrichment. More enrichment of alcohol fuels usually needs more ignition timing.Įxample: For blown gasoline at around 2 atmospheres, 28 degrees of ignition timing is common for best power. The amount of increased timing for alcohol fuels is very dependent on the amount of enrichment. As a result, usually more ignition timing is needed. For rich alcohol mixtures, ignition delay is increased. Mixture enrichment is common for racing alcohol fuels. ![]() Less ignition timing was needed for methanol, however, spark advance in racing is usually greater for methanol than for gasoline. Methanol combustion flame speed was 42% faster than the combustion flame speed in gasoline. In one combustion engineering test, methanol flame speed was compared to gasoline flame speed at lean mixtures for each respective fuel. A knock sensor can be used to reduce timing when engine knock occurs.įlame speeds are greater in alcohol fuels than for gasoline fuels in lean, highway fuel mixtures. It may be varied for different engine temperatures, throttle positions, and engine loads. Timing in more recent ignition systems is computer controlled according to a closed loop ignition timing function. Removing or changing the vacuum signal to a different location in the intake affects drive-ability. In later engines, a vacuum signal was taken from the carburetor that had a different vacuum characteristic, more ideal for emissions. The flame speed slows down, so the spark advance was bumped up with a vacuum diaphragm to start the ignition earlier. At low power levels from a closed down throttle, fuel and air is reduced. At idle and low speed throttle positions, high manifold vacuum occurred from the closed throttle. In older engines such as from the ’40s and ’50s, the vacuum advance signal came from the intake manifold. This is typical in earlier mechanical fuel injection and lean burn carbureted engines since the late ’60s. ![]() More recent electronic ignitions modulate spark advance for different driving conditions. Vacuum advance if so equipped – variable up to 10 degrees, typically added with engine vacuum more advance usually at low engine speed.Įlectronic limiting of ignition timing is also added for smog control requirements. Initial advance – typically 10 to 15 degrees before TDC (top dead center)Ĭentrifugal advance if so equipped – typically up to 20 degrees before TDC, added with engine speed more advance with high engine speed Ignition Timing Control – Open Loopįor open loop systems commonly used in a carburetor or mechanical fuel injection: Ignition timing trendsĪs RPM increases, more ignition timing is neededįor higher octane fuels, more timing is needed due to slower flame speedįor large combustion chambers, more ignition timing is neededįor forced induction, less timing is needed because of faster flame speedįor emission controls, less timing is used to reduce smog compoundsįor richer fuel mixtures, more timing is needed due to slower flame speedįor alcohol fuels that are run richer than gasoline fuels, more timing is typicalįor nitro fuels that are richer than methanol fuels, even more timing is typical. This is done with a combination of initial advance, plus centrifugal or electronic controlled advance for engine speed, plus vacuum or electronic advance for engine loading and throttle effect. Best power is achieved when ignition timing is set to fire the spark ahead of time to reach that peak pressure at about 2 degrees after TDC. ![]()
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