I thought the theory of higher octance fuel was that is more resistance to pre-ignition, pinging/knocking - or it burnt more predictably and uniformly.

This means you can more accurately ignite fuel so it goes bang at exactly the right time (which I remember is around 15 ATDC).

So better octance fuel = less need for advance = better "go bang" timing control = more power.

Have I got it right?

the kidney shaped head has better swirl / squish properties than an open chamber head.

we're using a kidney shaped head on our turbo project - there's a lot more power and torque to be had and less chance of detonation with good fuel.

So the kidney shape sets up a better set of conditions to burn the fuel.

The swirl I guess makes it richer around the spark plug and leaner the further you get away from the plug....something along those lines?

I think thats how it works Ben, the shape of the kidney allows the flame front from the spark plug to spread evenly between the two lobes of the kidney
since it is smaller in area than an open chamber the flame burns quicker and mopre efficiently, although I could be wrong i just remember reading something like that here a while ago

Quicker burning fuel does not yeild more power. The slower the burn the better. The longer the fuel is burning the longer it is pushing on top of the piston thus creating more power. Its the difference between a tap and a drive.(golf lol) The one that you dont follow through with doesnt go as far. Nitro in a dag car burns burns around 10x slower than petrol.

In saying that if the engine is not set up for the higher octane no benefit is to be had. The differnece between nitro and petrol is hugh i know. The difference between 92 and 96/98 is mnimal.

It's not because it is a "kidney" shape -- it is the quench factor that causes a high swirl. That's good for power, but not so good for low emissions.

When you compress the fuel air mixture (the 'charge') it gets hot. The hotter it gets the more chance it has to detonate (or 'pre-ignite'). The 'octane' rating of a fuel is a measure of its ability to resist detonation due to heat.

Increasing the compression ratio on a motor results in more torque throughout the entire rev range. Its simple thermodynamic law and is the very basis why engines compress the mixture to begin with. If you increase the compression by 10%, expect roughly a 10% torque increase providing detonation and ignition timing is not affected. Bear in mind that if it was as easy as that, then all stock motors would have compression ratios of 50 to1. There is ALWAYS a compromise (ie downside).

Increasing compression increases how much heat is transfered to the block, head, pistons, ring, valves, oil, etc in the engine. You will need to consider the radiators ability to dissipate this heat. Road cars have periods of low speed. Traffic lights and McDonalds drive thru will cook a high compression motor with a stock radiator. High octance fuel is more expensive. It's harder on the starter motor and battery and cold weather starting will deteriorate. The engine will also idle rougher (not as smooth).

So running high octane fuel in a stock motor is a waste of money. But running low octane fuel in a high compression motor will lead to detonation and damage to your engine. The obvious question is how much octane do you need for a given compression ratio.

It depends very heavily on the cam you using, correct spark plug use (may not be ok to use factory type plugs) and the pistons and rings you use. Generally speaking, if you want to remain on premium unleaded fuel and have high quality pistons with chrome top rings, an upgraded radiator and don't much road driving you should limit your compression to about 7.5:1 DYNAMIC. For a road car, use something more sensible like 7:1

From here you can then use your cam figures to determine the maximum STATIC compression ratio you can run without running into detonation problems.

The kidney shaped combustion chambers are designed to provide 'squish'. If you had a large flat sheet of MDF (about 1m x 1m x 16mm) rested one edge on flat concrete then allowed the MDF to fall onto the ground you would notice there isn't much sound at all. During the last few mm of the fall, the air between the MDF sheet and the concrete act as a cushion. The air escapes this gap at very high speed. If the floor was dusty a big cloud of dust would emerge. That's squish.

In a combustion chamber, squish does one very important thing. By keeping the mixture turbulent it means that hot spots in the head do not get the chance to super-heat a part of the mixture and ignite it. So basically it allows the use of a higher compression ratio.

Race engine builders pay very close attention to squish. It's difficult to work with as connecting rod stretch and expansion due to heat affect this dimension. When race engines over-rev and destroy and engine, its often due to the piston slamming into the head because of rod stretch.

Hope this helps.

Quote:



lets clear things up:
Octane is a hydrocarbon. It is a major component of petrol. well actually the stuff in petrol is 2,2,4 methylpentane, this is a structural isomer of octane as it has the same molecular formula)

Octane is a longer molecule than heptane which has 7 carbon atoms as opposed to octane which has 8. The longer the molecule the higher the dispersion forces and therefore the less volatile it is, therefore the higher its autoignition temperature, therefore increasing the burn time, therefore creating a more controlled burn, therefore reducing the chance of detonation.

Detonation is the collision of flame fronts within the cylinder. ie if one part of the charge autoignites then when the spark plug fires up the rest, there will be a collision of flame fronts.
and thats what causes engine damage and that awful awful noise of your hard work going down the drain....

Octane is measured relative to a mixture of isooctane and heptane. a 95 octane fuel has 95% (by volume) isooctane and 5% heptane.

however some fuels have higher than 100 octane. how is this?

Because octane is not the most autoignition resistant compund know (its far from it...) and some fuels (such as ethanol) have a higher measure RON and MON than 100% octane. meaning that it will burn slower and more controlled than 100% octane.

RON (Research Octane Number) and MON (Motor Octane Number) are the ways of measuring the way the fuel burns. so, if a fuel has 90RON then it has 90% octane

MON however is a more thorough way of testing the burning characteristics of a fuel, and the MON rating will always be lower than the MON rating.
Note:
Octane semistructurale formula:
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH3
molecular formula C8H18
2,2,4 trimethylpentane semistructural formula:
CH3 C3H6 CH2 C2H4 CH3
making a molecular formula of C8H18 again

pentane semistructural formula:
CH3CH2CH2CH2CH2CH2CH3
making a molecular formula:
C7H16