A question about fuel and ignition timing

Effect of modern fuel on ignition timing

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old man emu
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A question about fuel and ignition timing

#1

Post by old man emu »

I was watching a video from a museum in North Carolina and as an aside the host mentioned that early H-D engines were designed to run on low octane fuel, and that nowadays the fuel we use has octane ratings in the 90's, therefore burns slower than the old stuff. This is further affected by the L-shaped combustion chamber which has the source of ignition at a distance from the crown of the piston where the pressure of the expanding gases exerts its force to produce power.

I have to clarify something about octane numbers on petrol pumps. The petrol in Australia is described by its RON (Research Octane Number). So the lowest I can get here is ethanol-free 91octane. That is equivalent to petrol sold in the USA as 87 octane.

In most countries in Europe (also in Australia, Pakistan and New Zealand) the "headline" octane rating shown on the petrol pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the simple mean or average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel.

I'm not going to enter into the false argument that higher octane fuels produce more power. For a particular engine, say a 5:1 45 incher, the amount of power produced is ultimately dependant on the compression ratio. Since you can't change that at the pump, the engine will produce the same amount of power regardless of the octane rating as long as it is above the rating the engine was designed to use.

Now, the thing I'm looking at is the timing of the ignition. The workshop manual for the 45 sets out the timing for an engine running 1940's type fuel. That setting will produce a flame front from 1940's fuel that will cross the space from the spark plug and intercept the crown of the piston just as the piston is in a position to be able to move down. Thereby producing the maximum amount of power from the fuel - all other things being in good order.

The flame speed is the measured rate of expansion of the flame front in a combustion reaction. The flame is generally propagated spherically and the radial flame propagation velocity is defined as the flame speed. In an internal combustion engine, the flame speed of a fuel is a property which determines its ability to undergo controlled combustion without detonation. That's why you don't put low octane fuel in a high compression engine. The fuel will explode, not burn, and that's called detonation.

So, to my question. If modern fuels have a slower flame front than the fuel the engine was originally designed for, should the timing be advanced to a point earlier than set out in the 1940's manual?
svkiwi
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Re: A question about fuel and ignition timing

#2

Post by svkiwi »

if you advance your timing, then you'll really find out what detonation is!!
1950Panhead
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Re: A question about fuel and ignition timing

#3

Post by 1950Panhead »

So, to my question. If modern fuels have a slower flame front than the fuel the engine was originally designed for, should the timing be advanced to a point earlier than set out in the 1940's manual?
Possibly or possibly not.
You want the fuel to burn and expand which pushes the piston down and off you go.
Full advance is 34 degree, that 34 degree is so the process can start and most of the expansion is after tdc.
If you have to much expansion before tdc it damages the engine and you lose power.
If you install quit mufflers you can test your theory and listen to the engine.
You are likely to find 34 degree is the optimal setting.
old man emu
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Re: A question about fuel and ignition timing

#4

Post by old man emu »

Panhead,
What octane fuel are you running in a 45?
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