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Discussion Starter #1 (Edited)
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 ?



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



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 take to turn 1 crankshaft degree @ 8000 rpm ?


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?:



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?:



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?


cont'd->
 

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Discussion Starter #2 (Edited)
Now that we know how long one degree turn of the crankshaft takes, how far the spark flame must travel to completely ignite all of the air/fuel mix, and the speed of the flame propagated, let's start putting pieces of the puzzle together...


The key to making power is timing when the combustion event creates the most downward force upon the piston top. The expansion of combusted air and fuel pushes down upon the top of the piston for the power stroke. This downward force turns the crankshaft and propels your car forward.

All of the air-fuel mix inside the combustion chamber must be ignited for the biggest explosion to occur.


When is the best time to have the combustion event completely finished?:

Below is a graph looking at what happens to cylinder pressure as the crankshaft rotates.

On the X-axis is crankshaft degrees in which zero degrees is Top Dead Center (TDC) or the top most point of the piston's travel as it moves up and down the cylinder.

On the Y-axis is cylinder pressure as the piston is squeezing the air-fuel mix through to the time the combustion event is finished.

The graph shows 5 consecutive combustion events in the same cylinder at the same engine settings. The spark is ignited in this Mercedes diesel engine at 30 degrees BTDC on the far left (-30 on the X-axis).



Notice that there is variation in peak cylinder pressures between each of the 5 combustion events , even within the same engine and igntion timing, fuel injector timing, and cam timing. This can be caused by variations in how the air/fuel mix fills the cylinder (mixture quality) and the mixing in the cylinder during each intake stroke.

The dotted line at the very bottom represents the cylinder pressure if there was no ignition and lighting up the air/fuel mix. It shows you the level of the cylinder pressure from the piston squeezing air/fuel mix alone without combustion. This is what you measure when you perform a compression test. It is also called the "cranking pressure". Clearly, the combustion event spikes the cylinder pressure much higher than the cranking pressure.

cont'd->
 

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Discussion Starter #3
The spark is ignited in the figure shown at 30 degrees BTDC (-30 on the X-axis to the far left).

At about 10 degrees BTDC on the graph (-10 on the X-axis), the igniting of the air-fuel mix is finished and the air-fuel mix starts to increase cylinder pressure from the expanding force of the combustion event. Cylinder pressure rises, as the burning air-fuel mix expands and pushes down on the piston top.



The greatest force or highest cylinder pressure occurs at 20-25 degrees After TDC (ATDC) [+20-25 degrees on the x-axis]. This is the finish of the combustion event.

You want to "time" the start of the igniting process so that the combustion is finished (the highest peak cylinder pressure) when there is a good connecting-rod-to-crankshaft angle.


What do I mean by that?:



What is a "good" Connecting Rod to Crankshaft Angle for making more power? When should peak cylinder pressures happen?

Cartoon of a Piston ( [O] ) and Connecting Rod ( I ) On Side View:

1. ...........[O]............... 2. [O]
.................I..................... /

.........Rod @ TDC...........Rod @ 12-15 degrees ATDC


If the force of the explosion (or peak cylinder pressure) occurs at TDC, the connecting rod is straight up and down or "in line" vertically with the piston (1. in the above cartoon figure). A downward force or push by the explosion on top of the piston at this time does NOT help turn the crank. The force just travels down the rod vertically.

If the highest cylinder pressure occurs later ATDC (2. in the above cartoon figure) , the connecting rod to crankshaft angle is more ideal. The downward force or push of the explosion on the piston top helps turn the crankshaft due to the rod not being straight up and down or in line vertically with the piston. The rod is travelling clockwise and downward already at 12-15 degrees ATDC from inertia and an additional "push" from the peak cylinder pressure would accelerate it faster in this direction. As a result, the force turning the crank would be greater.

Most people say that the downward push should be happening no earlier than 12-15 degrees ATDC (+ 12-15 degrees on the X-axis) so that there is a mechancial advantage of turning the crank.


The time at which you START the ignition of the air-fuel mix influences when the combustion event finishes and where peak cylinder pressures occur.

You want the peak cylinder pressure or greatest downward push to occur at a time when the rod is at a good angle relative to the crankshaft.


This ensures that you get the most out of the downward force from the explosion to turn the crank.

cont'd ->
 

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Discussion Starter #4
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.

----------------------------------------------------------------------------
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.

And that's no "bull" :
 

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wow, you never fail to amaze me baxter. i mean blake. hopefully the people that have "ignition timing advanced" under their sig will stop and think for a sec. def good write up, thanks for posting it up for all of us.
 

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Good to see you putting things up to help these guys out blake! You know id rep but the 24hr rule owns me today :ninja
 

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If only the censorship wasn't in place, there would be tons of other useful articles for everyone to see. I'm not a fan of copy and paste so I'll just leave it be.
 

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+1 for a great writeup.
 

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thats the thing that i still have a hard time with is timing especialy when dohc is mentiond the belt can slip one tooth and you get bent vavles or worse. let alone how persice and accuarte things have to be, but that's how you get alot of power form small displacement.

i rememberd when i got into building engines playing around with sbc 350's
it like i know every piece ,how to accurate time and adjust valves, and tuning ,but it's all stick and rock simple with that old skool stuff .

but the more play this honda the more fun it is ,so thanx for the info .
 

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Personally I think a tooth slipping is rare considering there are tons of cars out there that have timing belts on even for a SOHC engine. DOHC's are no different.
 
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