- Runs on Regular and works up in the mountains

Why do people think that high octane fuel is more powerful?

The octane number, or rating, of gasoline, the correct name for this fuel, is based on a ratio of two flammable fluids, 2,2,4-trimethylpentane and heptane. 2,2,4-trimethylpentane has a rating of 100, heptane of 0. So a mixture of 90% of the first to 10% of the second has an equivalent octane number to gasoline with an octane number of 90.

So why is this important. Well gasoline engine, which are also known as spark ignition engines rely on precise timing of the burn cycle to give good performance. If the burn starts too early, say before the piston is at the top of the cylinder, then performance can be compromised. In a similar way if the burn starts too late, as the piston has already started down the cylinder, less power may be produced and a partially burnt charge exhausted out.

Now the odd thing is that a low octane fuel can self ignite easier than a high octane one. To get around this problem a number of things can be changed. The easy one is the ignition timing. This is when the sparking plug, sparks. This usually happens just before the piston reaches the top of the cylinder, or top dead centre, TDC, as some of us call it. The reason it sparks before the cylinder reaches the top is because, even at high revs, there is a slight time delay before full burn happens. This slight delay is optimised depending on the engine variables. So things like engine revs, load, and throttle position can all affect when it is time to light the blue touchpaper. Air pressure and temperature make a difference too, this is why we monitor it to optimise engine performance. More about this later.

Thankfully electronic ignition and fuel management make this very easy, unlike a carburettor, which is just rubbish really. It is why they looked so complicated. All those tubes, bypasses and flaps to make them work, and then they ice up.

The other way to control the pre-ignition caused by lower octane fuels is to use a lower compression ratio engine. Or more accurately a worse volumetric efficiency.

Volumetric efficiency is the difference, in mass, between what the cylinder can hold and what it actually holds in usage. An inefficiently engine may have a VE of 80%, while a really good racing engine can go over 100%, and that is before any super or turbo charging is done.

This is the reason that running at high altitude, say in Mexico City, Leadville, CO (odd, no need for lead in the fuel) and Bodmin, Cornwall (just because they are all Bodmin) can happily run on a lower octane fuel. There is less air getting in, so less fuel, therefore cooler engines, so less chance of auto ignition. Simple really. It is also the reason that they put a super charger on the Spitfires Merlin engine, they could fly higher while still delivering decent power.

Now oddly enough, high octane number fuels can actually have less energy in them. If you remember that energy is the ability to do work, and power is the rate that energy is delivered.

So another nonsense saying is that a thin person has lots of energy. Wrong, a fat person does. Just that fatty has lower power, usually.

The reason that high octane fuel is often lower in energy is because they add things to it to slow the burn rate down. Ethanol is a popular one in the USA, but that has about 65% of the specific energy content of neat gasoline. But you can run a much higher compression ratio, so they can be just as powerful, but use more fuel.

So there we have it, low octane, more energy, less power, high octane, less energy, higher power.

So a quantum leap in understanding, except a quanta is the smallest of things, so a quantum leap is the smallest of jumps.

It is all nonsense.

See, put a blower on it and it is all you need.

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