The real effect of higher vs. lower octane fuel

[Sharky]

here we only have 2 types of gasoline. 86 octane and 91 octane referred to as Regular and Premium respectively. Now, from what I understand as the air-fuel mix is compressed it increases it's temperature and therefore the temperature could be so high as to light up the mix before the spak plug. Higher octane would give a higher resistance to temperature and therefore avoid this problem. This should be, from what I understand, the reason why cars with higher compression ratios need higher octane fuel. Likewise cars at higher altitudes (like the city where I live which is at 2600 meters above sea level or 8500ft) would need lower octane as there's less air, therefore less pressure as it's compressed and less temperature (PV=nRT).

Anyway, my car has a compression ratio of 9.1:1. Theoretically it would need Premium at sea level but I was told that at this altitude I could use regular and I have done so always. Likewise with my dad's car which has a ratio of 9:1. However, I was told that cars with compression ratios above 9.5:1 need premium even at this altitude. But a friend at work who had a car with a compression ratio of 10:1 used regular even at sea level and he said he didn't have any problems with the car.

But my question is, what are the risks of running a car with a lower octane fuel than the one thet is recommended?

[johnh875]

If the car does not ping (fuel ignite prematurely as described above) it should be ok. It sounds like a light rattle under accelleration.

[bartman71]

the higher the octane the more ignition timing you can run and more power is produced

[Mackmot]

In the near future there will be no need to worry, direct fuel injection mean zero pre det with whatever fuel at whatever atmos conditions.

[Sharky]

This ping is like a metallic rattle right? I wasn't sure how it's called in english. I sometimes hear that sound in my car, particularly when going up a steep hill. But the use of premium do9esn't always reduce it.

But what could be the damage caused to the engine caused by the use of lower octane fuel?

[Bluebottle]

Originally Posted by Sharky

This ping is like a metallic rattle right? I wasn't sure how it's called in english.

In the UK it's known as pinking.

[bartman71]

if it is pinking you are risking serious engine damage
eg holed piston in a bad case

[C32]

Sharky,

I can get 95 octane here.

[TimD]

I used to have a Rover 3500 V8 which in manufactured form required 105 octane. I had to retard the timing massively to get it to run cleanly on four-star. The thought of what the pinking was doing to that lovely aluminium engine was too much to bear.

[Sharky]

I see. It's a real shame that the government hasn't allowed for the sale of higher octane fuel here although i would think that at the current prices nobody would buy it anyway.

105 octane. Damn!!!

What about all those octane boosters on the market? Are they any good? I wouldn't think so but I've never tried them