Regular or High Octane gasolene

[Burnzyb]

Pica-Pica said:

My E36 I6 2.5 Nat Asp benefitted from 98 RON over the standard 95 RON. Much smoother, better fuel consumption and more power. This was a 1998 built car, standard apart from the fuel. About 2 to 3 mpg improvement and smoother. Once I had done several comparisons over a year or two, I stuck with 98+ RON. The car’s knock sensor adjusted to the higher octane fuel.

I don’t see the point in my quote, I said turbo cars benifit a lot more than N/A cars which is true, any engine will benifit from higher octane fuel in one way or another, but turbo cars make a bigger percentage of gain if the standard map is for 95 Ron.

[E65Ross]

Pica-Pica said:

Garvin said:

For a vehicle with a modern electronic engine control unit then higher octane fuel will give more performance. However from my experience over a number of cars:

- The improvement in performance won't be 'earth shattering' for NA engines. In fact, it's usually hardly noticeable.
- The improvement in performance can be quite noticeable for turbo engines.
- The cost reduction in improved fuel efficiency is likely to be marginal, if anything at all, when compared to the extra cost (particularly here in the UK) of higher octane fuel.

In other words - turbo engine, go for it; NA engine, don't bother.

Not in my experience.

Nor mine, car dependant. A few years ago my old man had an E92 M3, it was actually a fair chunk more economical on the 99RON stuff which negated the difference in cost, plus it felt a bit better for it too, more so seemingly at higher rpm. Cars which are not quite so highly strung....seems pointless.

[HappyMidget]

For a start, a car does not advance timing till knock is detected, it removes timing when knock is detected. Cars with knock sensors generally have two spark maps, one high octane and one low octane. The high octane map is developed using a specific fuel octane rating to be the most advanced timing possible across the map with no knock present at all loads and RPM. The low octane map is then set a certain amount of degrees retarded from the high map to allow the ecu to choose a lower timing if knock is detected. This is how cars can and do make more power on higher octane fuel when they have been properly tuned to take advantage of the higher octane rating.

[kooky1976]

My CLS55 runs like st on 95. Lovely on 99. Well worth the 5p/ltr.

[Mr E]

Only one of my cars has a knock sensor.

kambites said:

Makes bugger all difference to my Elise but the Octavia definitely feels marginally happier on 99.

Elise has no knock sensor. I put super-u in mine just in case (it does minimal mileage so cost is irrelevant).

[Pica-Pica]

HappyMidget said:

For a start, a car does not advance timing till knock is detected, it removes timing when knock is detected. Cars with knock sensors generally have two spark maps, one high octane and one low octane. The high octane map is developed using a specific fuel octane rating to be the most advanced timing possible across the map with no knock present at all loads and RPM. The low octane map is then set a certain amount of degrees retarded from the high map to allow the ecu to choose a lower timing if knock is detected. This is how cars can and do make more power on higher octane fuel when they have been properly tuned to take advantage of the higher octane rating.

So if my previous E36, could run from 91 to 98 RON, then while 95 is the usual RON in UK, and 98/99 the higher octane petrol, would that mean three maps were present on this engine (2.5 I6)?

[HappyMidget]

Pica-Pica said:

So if my previous E36, could run from 91 to 98 RON, then while 95 is the usual RON in UK, and 98/99 the higher octane petrol, would that mean three maps were present on this engine (2.5 I6)?

No, only two, it scales between the upper and lower maps. The low map is basically a safety map.