Thursday, January 3, 2013

Fuel octane rating and vehicle performance

About a couple of years ago I had an argument with a motor diagnostic specialist about the use of hi octane fuels. I have always used 91 octane in all our hybrids. His opinion was that we will get 5% improvement in fuel economy if we changed to 95 or 98. I have always resisted this change as the Prius is not a particularly high compression engine and therefore I see no added advantage. In any case the extra cost would cancel out any improve performance.

Chris Read has this to add which to me settles the matter.

Nearly all modern cars (certainly Japanese cars) learn various characteristics, a key one of which is octane rating, any car that has learnt a particular grade will probably react very badly when a fuel at the other end of the scale is used.

If I had a pound for every time somebody had told me they only buy a particular brand or grade of fuel 'because the car suddenly performs badly if another type of fuel is used' then I would be a rich man.  They have all been right in the short term and wrong in the long term.  Every scientific test I have ever seen (last one was performed by the Consumer Association in the UK for the Which? magazine, you can't get any more independent than that) concludes that standard unleaded (91 RON in the UK) gives the best value for money and there is no discernible difference between fuel brands (Shell, BP, etc.).

I choose a RON rating that is within the suitable range for the vehicle and stick to it, avoid premium or performance fuels as the extra costs in producing and marketing such fuel make it uneconomic to use (once the car has learnt to use 'basic' fuel).  If you want to change fuels then disconnect the 12v battery for a few minutes just before changing fuels to force the car to re-learn quickly, this will stop the warning lights.

(If you can, watch the long and short term fuel trim on a scanner (most will do OBD2 or E-OBD), for a few minutes after the 12v is re-connected and the car started.  The long and short fuel trims learn at the same pace for a while, then the long term gets heavily weighted by previous data and changes very very slowly.  It's this slow change that makes the car run so badly for a while after fuel is changed and sometimes puts an error light on.  Without a 12v disconnect, after a few days of learning the long term fuel trim adjusts enough to put the error light off and the cars running improves as it matches the fuelling better, happens much quicker with a 12v disconnect.)



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