Best gas to use for xk

Quick question please, what is the best gas to use for a 1954 xk120 ? Octane number.

In MY opinion, AND if you can find itā€¦ you want to use a NON-Ethanol laced,
high octane (93+) gasoline. This assumes an 8-1 Comp Ratioā€¦ A 7-1CR would
require less octane. 'Too Much" octane will NOT harm an engineā€¦ just your wallet.
Charles #677556

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Put whatever fuel doesnā€™t ping or knock. Start with a quarter tank, and if it pings then go up to the next grade.

I just drove a 500+ mile round trip, ethanol outbound and non-ethanol on the return leg, 24 hour timespan. I could detect no difference whatsoever.



Iā€™ve just discovered that in the UK, some (all?) Esso service stations sell 99 octane ethanol-free fuel. Itā€™s called Synergy Supreme + 99. On the pump it legally has to say E5 (5% ethanol) but it actually means ā€œup to 5%ā€ and in fact is 0% ethanol. Worth seeking out? Iā€™ve noticed that the fuel at the carbs in my XK120 is bright yellow (I normally use Shell V-Power E5 98 octane) and I can only assume that the yellow colour is caused by some reaction between the ethanol and brass components in the fuel systemā€¦

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Check the fuel as it goes in, it could have had that yellow tinge all the time. 8-1 requires high octane? (Donā€™t forget the ROZ/MOZ difference, 99 UK is more like 93 US)


David, I use the super unleaded mainly because the regular unleaded here is now E10, while the super (98 or 99 octane) is E5 or (as Iā€™ve discovered) even ethanol-free. Iā€™m not so concerned about the octane ratiing. My car is probably a bit higher than 8:1 now, as itā€™s had a .040" overbore, and the headā€™s had a light skim as well. Still, 98 (or 92 in US) octane should be plenty high enough. Maybe the fuel is dyed yellow, as you suggestā€¦?

people will disagree as to if ethanol fuel is an issue on rubber hoses and metal in the carbsā€¦butā€¦it is an issue, some will say all the old rubber lines have been replaced by nowā€¦not necessarily tho by the new type resistant to ethanol:ā€¦anywayā€¦that argument will go on foreeverā€¦If it is available anywhere even close to you, do use no ethanol in 91, 92 , or 93 octane if it is available where you areā€¦just do it. If you canā€™tā€¦then use 91 minimumā€¦to 92-93 and ADD an ethanol treatment additive each fill up as directions on the labelā€¦Startron or Lucas, or Stabil brands all have em. When storing for winterā€¦always use a Stabilā€¦if ethanol fuel is in the tank use the one for ethanol: ethanol fuel goes bad much fasterā€¦as in 2 months: ā€¦if non ethanol is in the tank use the normal Stabilā€¦ If it has to be ethanol try to use brand nameā€¦Shell, Exxon, Chevron, Mobilā€¦as their additive package has fuel system cleaner, anti-corrosion, already in it. Avoid grocery store, zip trip gas. The no ethanol is usually an off major brand supplier even if at a brand name station. Since I donā€™t know what additive package is in itā€¦I add some Chevron Techron to it. Autozone or Napa have what you need. Nick

Thank you Nick! will try that as soon as I get a chance and report back. Much appreciated.

The following is a quote from a Government minister in Parliament in November 1950 answering a question from an MP as to why pool petrol could not be raised to 80 octane

'Before the war we had three grades of petrolā€”commercial 68 octane, No. 1 grade 75 octane and top grade 80 octane. I am advised that very few vehicles used 80 octane. Nearly all of them used the other grades. To raise our petrol even to 75 octane would mean a loss of output.ā€™

So the best petrol an XK owner could obtain then was no better than 70 octane?

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Yes, thatā€™s why the 120 was offered with 7:1 compression ratio in some markets.
Mine is 8:1 and Iā€™ve always used Shell or BP 87 octane.

So to keep it simple as in the US most gas stations have 3 options (87, 89 and 91) which should I use for my 1954 XK120, it has little over 21k miles. Thank you all so much!

The best gas is 91, but try a a few gallons of 89 and accelerate uphill in top - does it knock? If not, try the same with 87. if it does knock just drive gently and fill it up with higher octane - every engine is different in its ignition settings, exact compression, cam timing and much more.

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This international forum needs to be careful quoting octane ratings as well as things like added ethanol, and indeed leaded or unleaded, and indeed other aspects of fuel that affect exhaust emissions capability.
Trouble is the ā€˜simple quotes and numbersā€™ do differ from Country to Country, so its best if asking questions about fuel, advise your country, and then seek local (expert) advice. So up front, this thread started from Simon re his 1954 XK120, but country not declared, but his pic shows a LHD XK120 so probably USA/Canada.
Octane rating NUMBERS differ from USA to the rest of the world, and percentage ethanol or not, also differs country to country, and indeed any legal requirement to declare ethanol content at the pump or notā€¦
In Australia, the base level fuel is 91 Octane (RON) and this is only fuel that (so far) has any added Ethanol. In Australia the mid rated 95-6 (RON) fuel and the premium 98 (RON) fuel are not allowed to include ethanol, much to the dismay of those with a vested interest in producing/selling ethanol.
So if you pull up to a Service Station (as we call them) you get a choice of 91(RON)E5 with >5% ethanol, 95 (RON) with no ethanol, or 98 (RON) with no ethanol, but all our fuels are still considered ā€˜high sulphurā€™ content, thus cars built to the latest Euro Emissions standards do not actually comply, because of the high sulphur content. But whatever the spin doctors try to say, ethanol does cause problems for any car not designed from the start to run on ethanol mixes, with potentially all sorts of problems with synthetic hoses/gaskets/seals and any use of non-ferrous materials used within the entire fuel system. Modern cars are designed to address these materials problems, but cars built prior to the mandatory introduction of ethanol blend fuels are potentially at greater or less risk depending on the luck of how they were built/designed and actual materials used. Anything with SU carburetters will have a problem in the long term, if not readily apparent in the short term, and indeed Burlen in UK, who makes all the reproduction SU parts, are now modifying seals and floats etc. to be ethanol compatible.
In Australia, if you have an 8:1 cr XK engine, whether an XK, E-type or saloon, you need a minimum 95/96 RON fuel, or for a few extra cents/litre with other benefits I use 98 RON fuel, and in Australia neither of these have any ethanol in them. If you have a 7:1cr XK (as all XK120s and many XK140 sold new in Australia/NZ had) then you can get away with entry-level 91RON fuel, but given that has >5% ethanol added, my standard advice is donā€™t use 91 E5 fuel, but instead for a few cents/litre use 96RON without ethanol. If you have 9:1 cr as in many E-types (Australian new E-types were mostly 8:1cr) then you must use the best available 98RON fuel (no ethanol) but it is marginal, given the 9:1cr E-type was designed to run on 100RON fuel (the then UK five-star fuel), but with some marginā€¦ You can slightly retard the ignition, or just be careful not to excessively load your engine as apparent by ā€˜pinkingā€™ which if persisted, can damage your engine. But for normal and touring driving, and occasional spirited driving 98RON is OK, but I really warn people when rebuilding engines that if they take anything of the head (try NOT to!) then that increases the compression ratio, so seriously consider fitting new 8:1cr pistons and not 9:1 . The loss in maximum power and torque is unnoticeable.

So thatā€™s the ā€˜localā€™ Australian situation, which will be different in USA and in UK and in Europe and who knows about NZ or SA etc; but note my comments about Ethanol and indeed in the State of NSW (only) the local NSW government removed the requirement for E5 to be declared at the pump for 91RON E5 fuel, because of the justified concerns/wariness in using Ethanol Fuels.
And although there are many negatives using added ethanol in petrol and diesel fuels, I am not aware of ANY positives at all, despite the ā€˜spinā€™ from vested interestsā€¦

SOā€¦USA, You see my comment above as to fuel: I am in WA state where no ethanol is easily found: I use ONLY the highest octane I can find in ALL my carsā€¦91 to 93ā€¦some use non ethanol, still 91-93,ONLY this non ethanol in the XK120, Camaro, Thunderbird and Corvette, My newer ones use the ethanol 10% commonly foundā€¦for daily drivingā€¦they are made for it, but when I park one for the winterā€¦ONLY non ethanol. I pay a few cents more per gallonā€¦even over a full yearā€¦adds up to not much. I change the 15 quarts of oil tooā€¦often. I wax the car with Wolfgang, cover it when not in use, new tires every 6 years tho not worn out. Just part of having very nice classic cars. I would not put lower octane in any car I ownā€¦to save a few farthings or rupees or whatever they are. I do not know if lower octane also has a lesser additive package: I suspect it does. My cars run very well, without engine issues. Thatā€™s meā€¦thatā€™s what I do. Others can do as they may. I wonā€™t say it is wrongā€¦but I will ask why.

91 ā€¦no ethanol.
IF you must use ethanolā€¦91 and add ethanol treatment from first choice: Startronā€¦or Lucas.
If with ethanol, then For long term of same fuel in tank, ie winterā€¦absolutely treat the fuel.


Thank you Nick, I use 91 gas which is the highest we have here in San Jose California. I understand the ethanol treatment for long term, but I dont get the ā€œno ethanolā€ as I donā€™t see such option. Iā€™m assuming the gas we get has no ethanol? Sorry for all the questions, this whole classic car thing is very new to me :smiley:

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You just may have gas without ethanol in San Joseā€¦if you will type into your web searchā€¦no ethanol gas in San Joseā€¦a tab listing that has a list of stations with address and phone numbers should come up. Call them or go by and seeā€¦If you can get it there greatā€¦and use it. Otherwise most gas station pump gas that does not say clearly ā€œno ethanolā€ā€¦will have the ethanol in itā€¦if you must use thatā€¦then also use an ethanol treatment additive in each tank per the instructions on the bottleā€¦Startron, or Lucasā€¦just ask at an Autozone or Napaā€¦for it. Buy severalā€¦sometimes if you only add say 6 or 8 gallons you will not need the full bottle that normally treats 20 gal. so seal it up for next time. I adviseā€¦do not fill your fuel tank all the wayā€¦to about 3/4. Another ideaā€¦get a broom stickā€¦wipe cleanā€¦slide it into fuel filler til it stopsā€¦note gas level when you remove it and compare to what your fuel gauge indicatesā€¦after a few timesā€¦and adding fuel you can mark your broom stick with how many gallons marksā€¦to double checkā€¦as small airplane pilots doā€¦I find this useful to compare to the somewhat inaccurate fuel sender/gauge. It helps me know how much I haveā€¦and how much I add at a gas stopā€¦and to not overfill. The short broomstick (cut off a little) fits in the trunkā€¦(can be used as a hood prop as well at the hood front where the catch isā€¦the side rod puts a lot of bend stress on the hoodā€¦I removed the side rodā€¦only use the front with a broom stick to hold the hood (bonnet) up.

Whatā€™s all the fuss good for? If driving with ethanol containing fuel, one risk many claim is that it eats the rubber in the fuel system (pumps, carbs), the other is the fuel tank collects more water. So fill up with anything and store it with the expensive fuelā€¦ or pay extraā€¦ the engine itself doesnā€™t mind but ethanol means a slightly leaner mixture. Whether 5% more or less makes a difference you decide.

Iā€™ve been using 10% ethanol gas for over 10 years. I use fuel stabilizer of the gas is not going to be used up within one month. Entire fuel system was restored in the early 1980ā€™s.

Perhaps 10 years is too early for ill effects to make themselves evident?


About 20 years for me, always 87 octane, good brands, no pinging, and I havenā€™t noticed anything bad about ethanol in my 120. Two years ago in my '38 SS, I put new cork seals in the fuel changeover valve, weā€™ll see if it eats the cork.