Ignition Theory.

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Ignition from a tuning point of view.

Understanding Ignition Timing as a tuner.

In our classes, we teach that in most cases if you pick a good calibration the ignition table should be great as far as peak power goes. The most critical component of chasing peak power numbers on a dyno is doing the runs correctly. Engine temp, oil temp, tire temp,… are some of the things that can change peak power numbers run. Which can easily be confused with an adjustment made to timing or some other table. More on this in a future tip.

Before adjusting timing you need to understand what you are doing.

Gasoline of a specific Octane burns at a specific rate.
The piston in an engine is always in motion.
It’s hard to conceive but true nonetheless but your piston travels some in the very very short time from the spark igniting your mixture until you have the pressure of full flame to push the piston down.
For that reason, we need to fire the spark before the piston reaches TDC (top dead center) of the compression stroke. How much sooner before is measured in degrees of crankshaft rotation. When you see 15 on an ignition table that means the plug will fire 15 degrees of crankshaft rotation before TDC on the compression stroke.

As RPM increases the piston speed increases which generally requires firing the spark sooner to get full flame travel with the piston in the sweet spot. So a typical ignition curve has more advance at higher RPMs to make up for the increased piston speed.

When you make your engine work harder (higher load); going uphill, carrying more weight, and hard acceleration,… it creates more heat in your engine, which can lead to detonation/pre-ignition (pinging) which can damage your engine.  Engine load is measured by the MAP (Manifold Air Pressure) the higher the MAP number the higher the load. To help cool the engine so it doesn’t ping we retard (smaller number) the timing advance.

Which leads to the simplest way to view timing, “temperature management”.

When you ignite a specific volume of air/fuel it makes a specific amount of heat. Changing (timing) when you ignite it changes what you do with the heat. As an example. Near the end of the power stroke, the exhaust valve starts to open. The later you ignite it (less timing) the more of the heat you send out the exhaust.

“Increasing the timing heats the engine and cools the exhaust”

“Decreasing the timing Cools the engine and heats the exhaust”

The most important thing to remember with chasing numbers using timing on the dyno is that advancing timing (heats engine) to make power can be a bad call.

On the Dyno, we control the head temp and if we are chasing a pretty HP graph heads should be near 200F.  When the customer rides his bike head temps can be near 400F.

Advancing the timing in the peak power areas increases the chance the bike will ping when ridden by the customer.  I have seen several pistons with holes in them over the years.

Decreasing timing helps with pinging

You can increase timing in the decel column (lowest kpa) to help with decel popping. Low load area no risk of ping

You can change timing in the rows below 1000RPM (Across the whole row) to help with start-up. I.E. decreasing timing can help with kickback. Most Harley V-Twins crank about 700 RPM

Increasing the timing in the wild-man area (High Load/Low RPM) can help the bike feel better leaving a stop, on a table that has really low numbers there (below 10).

Obviously, every bike and situation is different.

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