What is the difference between normal and Premium unleaded petrol.

[Headtime]

I drive a (new but nonetheless) workhorse 1.6l estate. Would I see any benefit in using Premium unleaded, or is it really for performance cars.

Thanks.

[Beena72]

Yes, you would, but the difference is slight. The Premium burns better, and you would therefore see a slightly improved performance, and (subject to you driving the same as you do on standard) improved fuel consumtion cos the engine has to burn less to do the same work. Whether the difference is worth the extra couple of pence a litre I can't tell you, but it's interesting that most manufacturers advertised fuel consumption and performance figures are based on using the higher octance fuel (i.e premium/optimax etc.), although mention of that is generally reserved for the small print in the brochure.

[Impret-Sir]

Ummm, sorry Beena, but I dont think thats correct. Unless a car is set up to run on superUL it will not provide any performance advantage over standard fuel. The cars engine management chip will have to make adjustments to the engines timing, fuel/air mixture and other things to take account of the higher octain fuel, otherwise it will just produce the same amount of power as on normal. I think it is unlikely that a cooking 1.6 motor will have the capability to make these adjustments. My Impreza is "supposed" to be run on SUL, but can be run on normal, albeit with a slight loss of power, but my engines ECU can self adjust to suit. Headtime, the quoted extra power that running on SUL would give me is 3 to 5 BHP, hardly worth the extra cost IMO.

[Ward-Minter]

My S2000 has to run on Shell Optimax only. Something to do with the RON factor which is the degree the fuel with burn I assume (Mr Impet Sir may tell you what it really means)

I believe a more standard 1.6 (without being condescending) would run the same on any kind of unleaded.

The few times I have been unable to find a Shell garage and have opted for other Premium Fuels I have noted no difference whatsoever. Alas I believe damage can be caused if used too regularly.

[Beena72]

Agree to some extent, but I did say 'slightly', a more sophisticated engine will compensate and alter the air/fuel mixture and really take advantage of higher octane fuel, but even a basic engine (if any modern engines can be classed as basic) will run better on fuel that burns more efficiently, regardless of whether it alters mix to suit. A bit like driving on a downhill gradient as opposed to on the level. A minor improvement and possibly not worth the extra wonga.  

[KebabMeister]

Any fuel injected engine will have a knock sensor which will monitor the detonation of the fuel and adjust ignition timing accordingly.  If you change the type of fuel, i.e. 95RON to 98RON it will automatically compensate for it and inform the Engine Management System which will make the necessary adjustments (one part of the engine mapping system).

As Beena mentioned, car manufacturers will use the higher refined fuel (burns easier/more efficiently) since it will demonstrate the cars performance better (0-60mph etc).  However, when on economy runs there will be hardly any noticeable difference in fuel consumption.

[Headtime]

thanks all

[Impret-Sir]

If anyone is interested, petrol when it leaves the refinery its all very similar stuff, ie, with the same calorific value.(A lt of petrol contains about 7.5 million calories, compared to say, a lt of sweet orange juice, that contains about 500). What determines the amount of power that an engine can literally squeeze out of the fuel is defined as its "octane rating." This in indication of its propensity to be compressed without exploding. So, a fuels octane rating is an indication of the compression ratio at which it will damage an engine, and is determined like this:- a datum fuel, iso-octane, is fed into a small, single cylinder test engine on which the compression ratio can be varied. The point at which the compression ratio causes the fuel to detonate is said to be 100, and any fuel that detonates at that CR is said to be 100 octane fuel. The octane is then diluted with another fuel, n-heptane, and the CR at which this mixture detonates is equated to an octane raiting. A petrol that detonates as a 95% octane/ 5% heptane mix is a 95octane fuel. I fact there are two different test methods, using different engine speeds and ignition timings, hence we have the Motor Octane Number (MON) and the Research Octane Number (RON) ratings. Fuels tend to score higher under the RON test, so that is the one usually quoted. It is usefull to understand that a higher octane fuel will NOT without modifing the engine, produce more power. As engines have become better designed, in terms of combustion chambers and fuel delivery systems, detonation limits have been pushed back, allowing higher CRs with lower octane fuels. Octane ratings of over 100% have been achieved for certain competition engines by the addition of lead, although lead is now of course, thankfully, banned.

[KebabMeister]

Impret, what type of RON/MON were the WW2 engines that ran on 103 - 105 octane?  Or is the RON purely for petrol only and not hybred fuel?

[Impret-Sir]

Sorry KM, Im not sure I understand the question, what type of RON?? what do you mean?

[KebabMeister]

Sorry, I'm a bit confused myself.  You mentioned that 100 Octane was as high as it could, but I'm sure some WW2 fighter engines were to 103 - 105 Octane.  How is this?

[Impret-Sir]

No, I mention at the end that octanes of over 100 have been achieved by the use of additives, most commonly tetra-ethyl lead. I know that GP two stroke racing bikes used to run on stuff made by Elf with an octane of 119RON ! this was achieved by adding ten times the amount of lead that was present in the old 'four star' petrol. I would guess that the engines from WW2 that you refer to ran on a similar brew, but I dont really know, those 12 pot Merlins certainly blew out some smoke!

[KebabMeister]

The Merlins used really low octane to really high stuff depending on availability.  The same with the German's, they'd actually have a badge on the plane informing what octane rating it was running on.

[750dave]

I've ridden a medium/large motor cycle for the last 12 years and consistently get an extra 20-25extra miles from a full tank of supreme unleaded as opposed to the regular stuff.
My tank holds just over 15 litres and, depending on whether I'm doing motorway miles or commuting. I usually get about 130 miles before I need to switch over to the reserve tank. When using regular fuel this drops to about 110.
There is no noticeable effect on the engine power output or top speed.
My bike (a kawasaki vn 800) is fitted with an old fashioned carburettor set to the manufacturers recommended values. Maybe expensive non user serviceable fuel injection systems need further r&d work.
Simplest is often the best.