How does this all relate to advancing you're ignition timing?


Why did we do all that math at the start of the post?:



If the slowest turbulent flame speed is 1.35 m/sec or 1,350 mm/sec. and the distance from the plug to the piston top at TDC is 0.89 mm:

1. How long do you have @8000 rpm WOT for the ignition process and combustion process to be completed in an Integra ?

Answer : approximately a 0.6 millisecond window to light the air fuel mix and finish the combustion event.

The Math (if you are interested):



0.89 mm / 1,350 mm/sec. = 0.0006 sec. = 0.6 milliseconds




2. How much is that in terms crankshaft degrees?


Answer: 29 crankshaft degrees.


The Math (if you are interested) :

if you have a 0.6 msec window from the start of igniting the air-fuel mix to the finish of the combustion event and 1 crankshaft degree takes 0.021 msec.


0.6 msec / 0.021 msec. / degree = 29 degrees

[Aside: If we do the same calculation for a flame speed of 2.2 m/sec, the window for the complete igniting of the air fuel mix and the expanding explosion is only 19 degrees. ]



3. What's happening when you advance the ignition timing? :


When we advance the ignition timing, we start the igniting process and combustion event earlier. The cylinder pressure builds up and finishes earlier. Peak cylinder pressure (highest downward push on the piston top) occurs earlier.



4. What's our goal or how do we make more power?


We want to minimize the time that the expanding combustion event occurs BTDC while the piston is coming up and squeezing the mix. If the air-fuel mix is expanding while you are still squeezing the mix with the piston BTDC on the compression stroke, you are making the piston work harder.

You also want the maximum cylinder pressure (i.e. the finish of the combustion or expanding event) to be located at least 12-25 degrees ATDC to get a good rod angle to help turn the crank. If you nail the timing for the highest downward force at the right rod to crank angle, you make more power.

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Back To Our Example of the Integra Type R @8000 rpm

A. 14 degrees BTDC Ignition Timing

We calculated that the time window from start to finish is 29 degrees. If we ignite the mix at 14 degrees BTDC (-14 on the X-axis of the graph above), the whole event should finish at 15 degrees ATDC (+15 on the X-axis). This is still within a good rod to crank angle to generate power.


B. 16 degrees BTDC Ignition Timing

When we advance the timing to 16 degrees BTDC (-16 on the X-axis), we move the peak cylinder pressure location earlier (i.e. the finish of the combustion event).

The start of the combustion event or expanding force is earlier. More combustion time is spent BTDC making the piston work harder by having it squeeze the mix as the force is expanding. A side effect of this is the peak cylinder pressures also increase to a much higher level.

In this example when, we advance to 16 degrees BTDC, the peak pressures will happen at 13 degrees ATDC (+13 on the X-axis) which is still at a good rod to crankshaft angle.

C. What if we keep on advancing the start of the ignition event to say 20 degrees BTDC?

Sooner or later when you advance too far, you will have most of the expanding force occur while the piston is still squeezing the mix BTDC and the peak cylinder pressure will occur too early at a nonadvantageous rod to crankshaft angle. The peak cylinder pressures now would occur at 9 degrees ATDC, if the timing was advanced to 20 degrees BTDC. Now, the rod to crank angle is too vertical and most of your combustion window occurs BTDC, when the piston is still on it's way up and squeezing the mix. You stop gaining power and have very, very high detonation-friendly cylinder pressures BTDC.

High cylinder pressures wear out components faster and can initiate auto-ignition (i.e. start combustion at the wrong time without a spark) which is also called " detonation" .

There's a trade-off or compromise:

On the one hand, you "advance" or move the start of the ignition process earlier so that the peak cylinder pressure is higher but still located earlier at a good rod to crankshaft angle to make more power. Higher cylinder pressures generate more downward force onto the piston top.

However, when you advance the start of the igniting process too far, the peak cylinder pressures are located too early at a rod to crankshaft angle that is almost vertical or inline with the piston which does not help turn the crank. The higher peak cylinder pressures create a condition that can generate detonation.

You are trying to locate the "sweet spot" (best rod to crank angle) for when the peak cylinder pressures should happen when you adjust ignition timing.

If you advance too far then you risk detonation from too high of a cylinder pressure or decrease the life of the engine over time.


Bottomline Take Home Message:

From this you should have surmised that the best way to go is to find the least advance you can get away with.

It puts less of a strain on the engine bottom end components, makes your pistons work less harder when they are squeezing the mix, and still places your peak cylinder pressures within a good rod to crank angle range from 12-25 degrees ATDC.

Good stuff....