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I've always ignored the super unleaded pump purely on cost grounds but was wondering if there is any benefit to using it and if there is in what way does that benefit manifest itself?
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If you take Shell Optimax as an example, its higher RON rating will be good for some cars (e.g. my Golf VR6, on the filler-cap cover of which is expressed a preference for 98 RON) and superfluous for others (e.g. my MB E320 Coupé, which says 95 RON in the equivalent place). "Good" should mean smoother running, more power and lower fuel consumption. After many months of scepticism I am now convinced that the benefit of Optimax is showing in the Golf. There are also benefits in terms of detergents and other additives in Optimax, which should be good anyway for a car that doesn't need the higher RON. However, you can also get these from fuel additives (e.g. the VAG additive, part number G 001 700 03); you'll have to work out whether ordinary unleaded plus additive costs less than super unleaded or Optimax. I'd say the RON factor matters most, but the opinions of others may differ.
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Thanks Roger, that was very concise and understandable for the non-tech person
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My Rover Vitesse 2Litre Turbo definately runs much better on Optimax. The engine idles more smoothly and is less hesitant. Also, the oil is very clean in the engine. My Kawasaki ZX12R is also noticebly better on the stuff.
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Premium unleaded is 95 RON, Optimax is 98.5 RON. You will only gain benefit from using Optimax where your engine is fitted with a 'knock senosr'.
As the name implies, the knock sensor 'listens' for engine 'knock' (high fequency vibrations of the cylinder walls due to the mixture igniting explosively, rather than burning steadily). A higher octane number indicates a lower propensity of the petrol to detonate explosively, and hence the mixture can be ignited later in the compression cycle, giving greater thermal efficiency and more power (the engine can run with more 'advanced' timing).
Generally, where a knock sensor is fitted, the ECU advances the timing until knock is detected and then backs it off a little; the engine is then running at maximum efficiency without risk of knock-induced damage. By running with Optimax your engine will be running a little more advanced than with 95 RON petrol, and hence give a little more power.
If you do not have a knock sensor then you would have to manually advance the ignition timing - although this would have to be done with care since too much spark advance could damage the engine.
Of course there are claimed to be other beneficial additives in Optimax (eg detergents) which may benefit your engine, even if you do not advance the timing.
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I have been using Optimax for a year and I can confirm that I've seen a 10% increase in the MPG. Can't really say the performance is better, but most people I've talked to who use Optimax report *either* better MPG or better performance, not both. My car is a 99T Vectra 2.0 16v (Ecotec X20XEV engine) so presumably that one has said knock-sensor in it's ECU.
Conclusion: a 10% increase in MPG for a petrol that costs 5% more = a good deal!
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Rob:
For the benefit of the sceptics, myself included, would you please explain how you have measured the 10% increase in MPG?
Thanks
Andy
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All cars with catalytic convertors have knock sensors because expoding fuel going out of the exhaust doesn't do them any favours!
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No, no. There are plenty out there that don't - although I admit that most modern cars (made in last few years) would have.
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A knock sensor does not necessaily mean you benefit from Super unleaded, or Optimax.
The Rover T series for instance has a knock sensor. That will not advance the timing for optimax, but will retard it for lower quality fuel.
Engines that are not expected to be sold across different continents do not tend to have knock sensors.
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