*Writeup done by Tuan of www.automotivetech.org*

**What is meant by "advancing" the ignition timing?**

If you do not know how to adjust your ignition timing on your Honda, please check out this article.

However, before you go ahead and advance your spark timing, do you really understand what is happening inside the combustion chamber when you advance ?

A stock Bseries Honda ignition timing range is 14-18 degrees Before Top Dead Center (BTDC).

Let's say we start with 14 degrees BTDC timing and advance to 16 degrees BTDC. What is happening in the combustion chamber (cylinder)?

Many beginners incorrectly believe that the combustion event occurs instantaneously or "all at once" (say over 1-2 crankshaft degrees). If this were true, the shock to the rotating assembly (piston , rods, rod bearings and crank) would disintegrate it, after several combustion cycles. The events do occur very very quickly in the order of milliseconds (faster than a blink of an eye) but they do

**not**occur instantaneously. There is an order of events that occurs.

Do you know how many crankshaft degrees it takes to start and finish a combustion?

How long (in milliseconds) is a 1 degree turn of the crankshaft?

You have to visualize, in your mind's eye, that a spark jumps across the electrode and a flame is started in the shape of a "kernel" (like a corn kernel) around the spark plug electrode. This flame must then travel from the centrally located spark plug outwards at a distance equivalent to half the cylinder's bore and downwards towards the piston top. Remember, during all this time, the piston is rising towards the spark plug at the top of the combustion chamber and squeezing the air/fuel mix . The piston top is the "floor" of the combustion chamber and like an elevator, it is coming up towards the spark plug at the "roof" of the combustion chamber.

In the combustion chamber, the air-fuel mix sits as a series of layers with different air/fuel ratios. The richest air/fuel ratio layer is closest to the spark plug and the leanest air/fuel ratio layer is at the very bottom of the chamber or the piston top.

The air/fuel mix layers are sequentially lit and the igniting process or combustion event is cascading outwards from the spark plug electrode, like dominoes falling in a row. As the air/fuel mix is lit , the mix combusts or explodes which creates an expanding force outwards. This explosion occurs over several milliseconds or crankshaft degrees. This expanding force of the combustion event also raises the pressure inside the cylinder.

**When you "advance" your ignition timing, you are starting the lighting of the air/fuel mix earlier during the compression stroke.**

let's take a break, to let that sink in a bit:

**A Concrete Example**Let's say we are at full throttle in an Integra Type R and the rpms are at 8000 rpm (peak hp) , nearing the redline (8400 rpm). We need to first work out some numbers to show what advancing the start of the ignition process does.

Sorry about the math but it's meant to illustrate to you what happens when you advance spark timing.

**The main question is:**

How much time or how many crankshaft degrees does the spark have to completely ignite all of the air-fuel mix, when we are are at wide open throttle (WOT) at 8000 rpm ?

How much time or how many crankshaft degrees does the spark have to completely ignite all of the air-fuel mix, when we are are at wide open throttle (WOT) at 8000 rpm ?

To get the answer, we need 4 estimated numbers.:

**1. How long it takes for 1 crankshaft degree rotation when the engine is at 8000 rpm.**

Answer: 7.5 milliseconds

Answer: 7.5 milliseconds

**The Math (if you are interested):**

The calculation involves 2 Steps.

1 a) Calculate first how many revolutions of the crankshaft there are per second instead of per minute.

8000 revolutions/min x 1 min/60 sec.

= 133 revolutions / sec.

1 b) Then using rev/sec, calculate how long it takes to complete 1 crankshaft revolution @8000 rpm?

Using straight ratios:

133 revolutions / sec. = 1 revolution / ? sec.

? seconds = 1 rev. / 133 rev/sec.

= 0.0075 sec.

= 7.5 millisec.

**2. How much time does it ta**

Since there are 360 degrees in 1 revolution and it takes 7.5 msec for 1 revolution,

7.5 msec. / 360 degrees = 0.021 msec. / degree

3.

The above is a graph of flame speed in meters per second (m/sec.) at various air fuel ratios.

Turbulent flame speeds can range from

Hopefully, we have tuned our engine on the dyno to get optimal air/fuel ratios already to get the most power before advancing our ignition timing. In most cases, people have used 12.7-13.8:1 air/fuel ratio as a reference range for tuning. Looking at that graph above, the flame speed is around

The shortest distance for the flame to travel is vertically down to the piston top.

The piston to head clearance of a Honda Integra is 0.89 mm (0.035 in.). This is also the distance from the centrally located spark plug to the top edge of the piston top when it is TDC.

Whew! That was a mind-bender to the people who hate math and aren't engineers. We'll use these later on below to work out when is the best time to start the ignition event. File these 4 numbers away in your head for now and we'll use them later on below.

Next, we have to find:

cont'd->

**ke to turn 1 crankshaft degree @ 8000 rpm ?**

Answer: 0.021 millisec. per degree

The Math (if you are interested):Answer: 0.021 millisec. per degree

The Math (if you are interested):

Since there are 360 degrees in 1 revolution and it takes 7.5 msec for 1 revolution,

7.5 msec. / 360 degrees = 0.021 msec. / degree

3.

**What is the average turbulent spark flame speed in gasoline at a "performance" air fuel ratio**(assuming common octane rating fuel and the usual cylinder pressure ranges and temp)?**Answer: Spark Flame Speed = 1.35-2.2 meters / sec.**

How did you get that?:How did you get that?:

The above is a graph of flame speed in meters per second (m/sec.) at various air fuel ratios.

Turbulent flame speeds can range from

**1.35 to 2.2 m/sec**. in a well-mixed combustion chamber using a stratified charge at various cylinder pressures.Hopefully, we have tuned our engine on the dyno to get optimal air/fuel ratios already to get the most power before advancing our ignition timing. In most cases, people have used 12.7-13.8:1 air/fuel ratio as a reference range for tuning. Looking at that graph above, the flame speed is around

**2-2.2 m/sec. at 12:1-13.5:1 air/fuel ratio .****We'll use the quoted range for turbulent flame speed at the usual cylinder pressures and temp. of 1.35-2.2 m/sec.**

4. How far does the flame have to travel?

Answer: 0.89 mm

How did you get that?:4. How far does the flame have to travel?

Answer: 0.89 mm

How did you get that?:

The shortest distance for the flame to travel is vertically down to the piston top.

The piston to head clearance of a Honda Integra is 0.89 mm (0.035 in.). This is also the distance from the centrally located spark plug to the top edge of the piston top when it is TDC.

Whew! That was a mind-bender to the people who hate math and aren't engineers. We'll use these later on below to work out when is the best time to start the ignition event. File these 4 numbers away in your head for now and we'll use them later on below.

Next, we have to find:

When is the best time (in crankshaft degrees) to have the combustion event push down on top of the piston?When is the best time (in crankshaft degrees) to have the combustion event push down on top of the piston?

cont'd->