[u]NOTE FROM THE AUTHOR[/u]
This post is somewhat of a supplement to the Teg Tip relating to checking and adjusting ignition timing:
http://www.g2ic.com/forums/showthread.php?s=&threadid=28359
I did not want to get into details in that post regarding what octane to use, what timing you should set your G2 at, etc. Also, LL Cool B was asking me some real good questions when we were checking his timing. And personally, I didn’t have anything to back me up. So, I researched it a bit more and found some interesting info. Hopefully, this is something we can discuss and agree upon before we can even consider sticking this in the Archives. Rather than just simply cutting and pasting the info that I’ve found (which would fill up dozens of pages), I decided to crunch it down, explain things in lamen-terms as much as possible, and stick primarily to ignition timing.
Feel free to correct any info that I present here for you, however, I do ask that you try and back it up with some good evidence as much as possible. Hopefully, in the end we all have a better understanding on what octane we should use, and at what timing we should set our cars to.
[u]What is Ignition Timing and its importance?[/u]
Ignition timing is the synchronization of the spark plug to ignite the fuel so that large amounts of pressure can be applied to the piston when it reaches TDC (Top-Dead-Centre). It is this pressure that pushes the piston down to create power. A small pressure obviously relates to small power, and vice versa.
In virtually every gasoline engine, the spark plug is set to fire before the piston reaches TDC to allow the air/fuel mixture to properly burn and build high amounts of pressure. If the spark plug waits too long, the air/fuel mixture may not have time to burn properly to create a high pressure. Conversely, if the spark plug ignites the mixture too soon, the a/f mixture may build an irregular high pressure before the piston hits TDC (more on this later), thus inducing what we call “knock”.
[u]What is the physics behind a proper combustion?[/u]
Let’s take a normal B18A1 engine with stock settings (750RPM idle, 16deg. ignition timing, 87 octane gas).
The “Flame Front”
When the piston hits 16deg. BTDC (Before-Top-Dead-Centre), the spark plug ignites the nearby gasses to create what’s called a Flame Front. Since the spark plug is positioned at the top of the combustion chamber - in between the intake and exhaust valves - the flame front starts from the top, propagating itself downwards to meet with the piston to create high pressure.
Maximum Pressure
When the piston reaches TDC, maximum pressure is not yet attained. In fact, there are still more end-gasses left to be consumed by the flame front. This downward pressure - which started at 16deg. BTDC - reaches its peak roughly 15-20deg. ATDC (After-Top-Dead-Centre). Please note that I use 15-20deg. as an example, true numbers are sitting in Honda’s labs collecting dust, guarded by laser-guided machineguns:) Therefore, even though the piston is already descending, the combustion pressure is still applying a good amount of downward force creating more power.
[u]Octane and Knock[/u]
Now that you know how power is properly generated. Let’s take a look at what Octane really is.
What is Knock and why using a lower octane causes knock?
Knock happens when there is too much pressure built in the combustion chamber, and the end-gasses - that have not yet ignited by the flame front - ignites by itself. You now have an abnormal pressure rise before the piston reaches TDC. This abnormal pressure rise is trying to push the piston down as the piston is trying to reach TDC, rather than a smooth pressure rise pushing down on the piston ONCE it reaches TDC.
High Octane gasolines (91 and up) can handle more pressure before self-igniting, hence high-compression engines (B17A, B18C) require high octane gas.
What are the consequences of Knock?
Because of the abnormal pressure rise from self-ignition by unburnt end-gasses, engine temperatures will rise. The first to suffer damage are the pistons. Failure can happen in a matter of minutes depending on engine speed!
Are there any other variables that increase knock tendency?
Yes. They are:
Temperature - Increase in ambient temperature increases knock tendency, therefore increasing octane requirement.
Humidity - Increase in humidity decreases knock tendency, decreases octane requirement.
Altitude - Higher altitudes above sea level reduces knock tendency, reduces octane requirement. However, this is not proven in stone. There are many conflicting reports regarding altitude and knock tendency.
High speed driving - Increases knock tendency because of advanced timing (check below).
High loads - Increases knock tendency possibly due to richer a/f mixtures.
What is Octane?
Contrary to popular belief, the Octane rating that you normally see at gas station pumps is only telling you its Anti-knock index. The octane numbers 87, 89, 91, etc. does not correlate to more power, nor does it contain more potential energy. Rather, the number is a representation of the fuel’s resistance to self-ignition under compression (without the aid of the spark plug). Octane is rated using the RON method and MON method and combining the two - (R+M)/2.
What is RON and MON?
Research Octane Number (RON) measures the gas’ anti-knock index under typical driving conditions, without heavy loads. While Motor Octane Number (MON) measures the gas’ anti-knock index under severe, sustained high-speed, heavy load conditions.
Does Higher Octane burn faster than lower Octane?
Flame Front Speed does not have any correlation with the octane rating. Flame Front speed is affected by many factors (such as a/f mixture, spark plug location, turbulence, among many things). So, 91 and 87 octane “burn” just the same. It is “when” they burn that separates them.
[u]So, how does this all help me with Advanced Timing and My Teg?[/u]
What is the reason behind Advanced Timing?
If you did not know, Ignition timing increases as engine rpm speed increases. Therefore, if the B18A is set at 16deg. BTDC at 750RPM, it might be at 32deg. BTDC at 4000RPM (numbers are not true, just proving a point). The reason behind this is because the air/fuel mixture needs more time to burn as piston speed increases. The extra time causes the a/f mixture to create a much better combustion pressure, thus enabling to push the piston down with greater force when it reaches TDC (more HP).
I advanced my timing X degrees using 87 octane with no problems…
As I stated above, ignition timing increases based on engine speed. If, for example, you have advanced your timing to 20deg from 16deg at normal idle, you may not experience any knock. However, at 4000RPM, instead of the usual 32deg (example), it will be set to 36deg which may induce knock.
Also, do not forget that if you have a loud car (such as no intake resonator, loud exhaust, etc.), it may baffle the knocking sound when you are driving.
So, how much can I advance using 87 octane?
Provided that your engine is running in good condition, I suggest sticking with the stock setting of 16deg BTDC during summer climates, and no more than 18deg BTDC in winter climates.
What are the disadvantages of Advanced Timing?
Apart from an increase in knock tendency, the major concern I have with Advanced timing is increased cylinder pressure. Because you are allowing the a/f mixture to burn more fully and completely, maximum pressure is increased. Increased cylinder pressure can lead to increased engine running temperatures.
The first thing to suffer from Advanced Timing is the head gasket. Second are the spark plugs. Also, expect increased oil consumption due to high combustion chamber temperatures (oil may start to vaporize). These are the main things that I would worry about.
Then, how far can I advanced my Timing without sacrificing reliability?
IMHO, I would say no more than 20deg BTDC using at least 91 octane (higher if you live in hotter climates).
What HP gains should I expect?
Unfortunately, I have not seen any dyno graphs showing advanced timing gains for the B18A and B17A. It also depends on how far you have advanced your timing. I would venture roughly around 2-4HP peak gain with an average of 2HP gain throughout the powerband. These are modest estimations.
[u]THE END?[/u]
Well, I’ll leave it at that for now. Hopefully, I have enough info to start a good debate (if necessary). Of course, feel free to ask questions. I will answer them to the best of my ability. Please post your questions here rather then emailing me, so that I don’t get the same question over and over again 
If this goes well, I hope to share with you the importance of Oil Analysis and Interpreting its results.
Regards,
Oz
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