Detonation, Octane, Flash-point and CR questions

[Gunter]

As I understand it, detonation can be caused by lower octane, pre-ignition and higher CR.

Example: If the timing is set to max advance (33 deg at 4-5000 RPM) vac disconnected, how does different octane affect the timing?

Since higher octane prevents pinging, does higher octane slightly retard the combustion by having a LOWER flash-point?
Meaning: 91 octane ignites slower than 87?

In other words: With higher octane the combustion takes place a little later to prevent pinging and by pinging we mean combustion taking place before TDC has been achieved?

I don't have detonation but would like to be really clear on this issue.
To address detonation one would retard the ignition a few degrees and go with 91-92 octane?

Why, exactly does higher octane help with pinging/detonation?

Thanks.

[Zef]

http://oldeloohuis.com/octane3.html

[Two Tone]

From what I can remember from Chemistry class:

Lower Octane fuel is less stable and more reactive, which makes it more likely to pre-ignite during the compression stroke.

Higher Octane fuel is composed of a higher ratio of stable molecules (Octane) and is therefore less likely to ignite prematurely.

So, ironically, for a given mass of fuel, lower octane fuel contains more chemical energy than higher octane fuel. But the higher octane fuel contains more useable energy than the lower octane fuel in an engine in which the lower octane fuel is pre-igniting, ie. a high compression engine or one that is too hot, etc.

That's the way I understand it anyway.

[911pcars]

"So, ironically, for a given mass of fuel, lower octane fuel contains more chemical energy than higher octane fuel. But the higher octane fuel contains more useable energy than the lower octane fuel in an engine in which the lower octane fuel is pre-igniting, ie. a high compression engine or one that is too hot, etc."

I'm not sure I follow your explanation, but the main difference between low and high octane fuel is the resistance to self-detonate under pressure.

You also use the terms "pre-ignition" and "detonation" interchangeably. While related, pre-ignition is the condition where another ignition source, other than the spark plug, initiates combustion. Alternate sources could include an incorrect heat range spark plug, red-hot carbon deposits, an overheated thin edge of valve head/piston crown/head gasket, and/or any engine condition that overheats these parts that initiate combustion. (e.g. lean A/F mixture, retarded ign. timing, inadequate cooling system).

Detonation is a term used to describe combustion due to excessive combustion or compression pressure. Some causes include high compression ratio, low-octane fuel, poor combustion chamber design, incorrect spark advance.

Sherwood

[psalt]

Why, exactly does higher octane help with pinging/detonation?

Do you mean chemically ? The word "Octane" is the measurement of a fuel's resistance to detonation, so by definition, higher octane has higher anti knock quality. Octane numbers up to 100 are roughly the % content of trimethyl pentane, which is extremely knock resistant. Knock is a difficult subject to discuss because there are many different terms with weak definitions used interchangeably in many text. Basically, every engine has an octane requirement for max power, based on combustion chamber shape, piston design, temperature, speed and other factors. The octane requirement varies greatly with temperature, 16F of coolant temp is 1 RON , and 20F of intake air temp is 1 RON. An ideal ignition system is designed to ignite the fuel mixture so it burns and reaches peak pressure around 15 degree ATDC to get max work from the fuel. The burn time is relatively constant (2ms), so as rpm increases, you need to fire the ignition earlier (advance) to reach peak pressure at the desired point. At higher rpm, the burn time takes more degrees of crank rotation. Knock occurs when the unburned fuel mixture spontaneously ignites before the flame front reaches it. The term octane refers to a fuel's resistance to spontaneously ignite. Engines with wide VIA's and large combustion chambers (like hemis) need more time to burn all the mixture and are more knock prone. Add domed pistons which make a poor combustion chamber at TDC and things get worse. Add air cooling and it gets even worse. Most 911 engines are detuned with compression and timing by the factory to operate with a margin of safety for a "chosen" octane fuel. Many of them will make more power with more timing if you have the octane. If you study the history of hemi engines, you will find that the highest specific outputs were made with very high +12:1 compression, twin plugs, and exotic alcohol based fuels with theoretical octanes over 110. Water cooled, DOHC 4 valve, pent roof engines with compact combustion chambers and flat pistons have a much lower octane requirement.