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MG MGB Technical - Octane rating for Premium UK petrol?

Just spent nine fantastic days in Cumbria and North Yorkshire with the MGB, together with 12 other Norwegian couples in MGAs and MGBS. During the stay, a number of us experienced some unusual engine pinking on steep hills, which made us wonder: What is the octane rating for "premium" petrol in the UK?
Tore

It varies Tore, I try to use Shell Optimax which I think is 98, sure they have a web site. My car will also run fine on BP Ultimate which they claim is 102. I have to say that even with octane booster it turned up it's nose when presented with Danish fuel, I have never taken it to Norway so I have no idea how that compares with your forecourt diet.
Stan Best

Premium unleaded is usually rated at 97 octane with standard unleaded rated at 95 octane. The original specification for the MGB was to run on 101 octane or "5 star" fuel as it was then.

Pinking is not good for the engine and it would have been prudent to have retarded the ignition slightly to compensate for the lower octane. Unfortunately this will also have a noticeable effect upon performance. Otherwise you could have used an octane booster, Shell Optimax or BP Ultimate if you could have found it (or afforded it!)
Mike BGT

Shell Optimax is rated at 98 RON. I think BP Ultimate (the normal Ultimate) is 97 RON and they also do an Ultimate 102 which (guess what) is 102RON. I've never seen the Ultimate 102 at a petrol station - has anyone else?
JH

From the BritishCarForums:

A quote from the above reference: http://www.ethanolrfa.org/objects/pdf/DAI960501classic_car.pdf
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"There are two test methods, the Research Method which yields a Research Octane Number (RON) and a Motor Method which yields a Motor Octane Number (MON). The number posted on the gasoline pump is an average of those two numbers, (R+M)/2.

Today, gasoline octanes range for 85 to 94 (R+M)/2 with the typical grades being regular unleaded at 87, midgrade at 89, and premium at 91 to 94. Prior to the eighties, gasoline octane was often posted based solely on the Research Octane Number which allowed postings as high as 100 octane. Premium gasolines sold today often have a research octane number of 100 or higher but must post the (R+M)/2 value. For instance, a 93 octane premium will likely have a motor octane of 85 and a research octane of 101 (101 + 85)/2 = 93.
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USA uses the (R+M)/2 rating system. As it says, prior to the 1980's USA used the "research" rating. Today, many European countries & some others still use the research rating method. Bigger numbers for the same equivalent octane rating. AV gas uses yet another different rating system.

Approximate equivalent ratings:
USA = 87 -- Euro = 91 -- RON = 91 -- MON = 82.5 -- R+M/2 = 87
USA = 89 -- Euro = 95 -- RON = 95 -- MON = 83 ---- R+M/2 = 89
USA = 93 -- Euro = 98 -- RON = 98 -- MON = 88 ---- R+M/2 = 93

USA producers are allowed some variation between their posted pump octane ratings & the actuals. Each batch of fuel may be slightly different & you may find that one brands actual octane ratings variy a bit from day to day. The "must have gotten a bad batch syndrome". In the end, it's a matter of what you are most satisfied with. For most modern cars with adaptive timing controls, few folks will notice much difference between 87 & 93 octane. Older cars excepted, of course.

Additionally, there are seasonal variations in the volatility (how easily the fuel is vaporized) from region to region. This can lead to a more volatile winter blend causing vapor lock on an unusually hot spring day.
Curtis Walker

This is interesting. With these octane ratings, we should not have had any problems. The pinking we experienced was not too serious, just very noticeable. Some of the petrol stations we used had brand names none of us had ever heard about, so perhaps the quality of that particular petrol was not too good.

In Norway, most big petrol stations offer 95 and 98 octane, both unleaded, small stations usually only 95.Of course most of us always run on 98. With ethanol fuel E85 on its way, I suppose we will soon have the simple choice of 95 octane, and have to adjust our engines to run on that. Never tried any of these octane boosters, though. Do they work?
Tore

For those in the UK Tesco do a 99RON fuel. Where I am it's on 80% of pumps. It makes a LOT of difference on the A series 101p/l at the moment.
Stuart Robson

This year pinking on my roadster has seemed worse than usual, so much so that I have been tinkering with different curves and knock-controlled ignition again. Wondered whether it was my car that had become more susceptible, but was then told by an authority on the subject that the petrol companies changed the formulation again early this year. Presumably however they test octane it still reaches the minimum stated value, but I bet they don't test it on a 1970s B-series engine! Maybe it used to be generally better than the stated minumum in case it fell below, but quality control has now improved so they can reduce it closer to the stated value and still not fall below it.

Not all Tesco stations, but this a (growing) list - http://www.greenergy.com/products/99_octane_locations.html - haven't had the opportunity to try it myself.

The MGOC mag recently said that both Optimax and Ultimate are 98 octane, but BP themselves only claim 97 for Ultimate. Ultimate 102 is about UKP2.50 per litre!

Bayford Thrust supply genuine leaded with at a claimed 99.7 octane at a number of independants - http://www.leadedpetrol.co.uk/list.htm - price significantly more than UKP1 per litre but nowhere near Ultimate 102.

Optimax is certainly noticeably better than other supers at 97 in my car, and very much better than 95. On a recent tour in the Yorkshire dales I made the mistake of not filling up at a Shell station shortly before I arrived in the area. Once in it there wasn't even any 97, let alone any Optimax, and I had to suffer mad pinking on the 'much steeper than at home' hills, on two occasions having to creep up in first not because of lack of power, just more pinking than I was prepared to tolerate.

The octane rating *should* always be at least the stated value, from independants as well as nationals. However it would take someone to complain to Weights and Measures and for them to test it before any adulteration would be found. And it has often been said by those in the industry that it all comes from the same base fuel anyway, it is only what is added at the distribution centres e.g. detergents that makes them different. I've heard that even different grades and types of fuels are transported around the country in the same pipelines, with a plastic ball inserted when the grade changes to keep them largely separated.

I tried the octane boosted version of Castrol Valvemaster but found it made no difference compared to the unboosted (and cheaper) version. Castrol reckons to have the additive with the best protection against valve seat recession, so that is what I have always used. Switching between additives is advised against as it can cause valve sticking if two or more additives react together.
Paul Hunt 2

Curtis, do you know which rating system they use here in NZ?
Simon Jansen

RON, as far as I know, Simon. Same as the UK, so I think our BP's 97 is the same as the UK's one. I've been using 97 Ultimate in my G to good effect (less run on than 95), and will be keepong an eye out for 98 Optimax to see how that goes (if Shell NZ has it!).
Curtis Walker

Ah, always wondered what we used and I would have guessed it was the same as the mother land! Someone was advertising higher than normal octane fuel but I think it was BP? My B isn't on the road yet and I barely use my other car so I don't need to buy petrol all that often.
Simon Jansen

This thread was discussed between 26/07/2006 and 27/07/2006

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