[Engine] Petrol Octane Rating

[Softweigh]

Hi Guys

I just found a supplement to my normal M59 1.4 Petrol van which is applicable to Australian models and they say that you MUST only use unleaded fuel and it MUST be no less that 95 octane.   I have been using standard 91 octane but switched to 95 after reading the Australian manual supplement ...the van goes just as well (if not better) but to be honest I never found anything wrong when running on unleaded 91 octane.    The supplement states that using an octane rating below 95 could damage the engine.

What do you guys that still have the 1.4 i van use ?????

[cancunia]

Only thing that comes to mind is that lower octane petrol will cause more pre-ignition (pinking) under load. Retarding the ignition timing gets around that but you get reduced power as a result.

[oilyrag]

Most engines have knock sensors to detect 'pinking' and the timing is then automatically retarded to the point where pinking just about stops. At a guess there are limits to the breadth of timing available and a low octane fuel would take the engine into a much retarded mode which itself can have harmful effects, such as overheating, piston melting etc.  
91 Octane is pretty pretty low - our Donkey urine is 90  - so worth a few extra cents for something half-decent.

[cancunia]

My old MX5 had deliberately retarded timing from the factory to account for parts of the world with poor fuel. One of the quick ways to add a few HP was to advance the timing by a few degrees. Mine never overheated, apart from once going up a steep hill in Spain on a hot day, oh and once when the aux belt broke which then burst the radiator...

[Softweigh]

Thanks Guys

I have been running for a couple of weeks on 95 ..it is a bit more than 91 but the van seems to like it Our fuel runs on a 2 week cycle (pricewise) so this coming Tuesday it hits the bottom of the cycle ..I expect 95 to get down to AUS$2.00 per litre (91 will drop to AUS$1.89 probably ) That's around GBP1.12 roughly per litre I see that the average price in the UK is around 200 p per litre for super unleaded (is that 95 or 98) but then again you guys get really slugged with VAT on fuel?

[cancunia]

You could mix it and get 93

[Sol]

95 here is regular unleaded and 97-99 is super unleaded. I've never seen fuel below 95 RON for 35 years or more in Scotland and I expect, the rest of the UK.

Fuels with low octane numbers are a big issue for petrol turbos. I'm not sure it would trouble a 1.4i NA motor.

[oilyrag]

The 1.4 has a compression ratio of 10.5:1 which, (quite) a few years ago would only be found on distributor ignition, highly tuned engines, with normal road-going motors running closer to 8.5:1.
It's only the ECU and the vast array of sensors that allow such compression ratios - to gain performance, and in the case of the 1.4i, fairly mediocre fuel consumption. 
The ECU also expects a decent octane rated fuel, but maybe the ECU is re-mapped for poor fuel quality countries.

[Sol]

Wow that is high for a little NA engine.

[Softweigh]

Wow guys that is high .. in the "old day" only high performance motors had compression ratios over 8:1 .. I used to race a highly modified old Ford Anglia (the one with the reverse sloped back window) and my worked cylinder head was skimmed down to achieve a 10:1 compression ratio. Then again no ECU's in those days .. Our pumps here have 91, 95 and 98 at service stations Most people filling up seem to opt for 91 since that's the cheapest!! Something I have noticed is that my 1.4 seems to run a tiny bit cooler on 95 but fuel consumption seems to be much the the same if not slightly better