Best gas to use for xk

XK
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Hi Simon, on your octane question, here is one comparison data point. My 1950 Jaguar Mark V has been in California its entire life. The first 50 years were with an owner just up the street from San Jose and the last 20 years have been in the Los Angeles area under my ownership. For octane I always buy the lowest octane available at the pump, almost always that is regular unleaded with 87 octane. The car has been run 125,000 miles on the original engine only.

Higher octane is used for higher compression engines where ignition of the fuel under compression might happen before the spark plug is charged. Whether you might need higher than 87 octane depends on the compression ratio and internal condition of your engine. If that is not known, start with some either high or low octane fuel (say a quarter tank) and listen for pinging under load. If you don’t hear any, then when near empty gas tank, put the next grade in and repeat. I like the lowest octane available since I have never had a pinging problem on California gas and like to save the money.

Since my car has been on ethanol gas exclusively for more than 20 years (and MTBE fuel before that) and no fuel line problems have been observed, I have no concerns about the ethanol fuel (other than the enjoying smog reduction benefit it provides). My car had about 5,000 miles a year early in my ownership (after some years of storage in the 1980s and 90s) and now has about 1.000 miles per year. That means the fuel in my tank sits there for about 3 to 6 months between refills, all under weather conditions similar to San Jose.

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Looking at the entire production stream, it really only serves as welfare for large agribusiness.

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Production of ethanol is not free, it is normal commercial farming with all that comes with it. Monoculture, mostly corn, fertilizer, weed killers, processing. Subsidizing large farms with taxpayers’ money is not much better than giving it to developing countries. I doubt nature is that much better off, whether cars run the same or not…

Start with the cheapest fuel and only go up if you have to.

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Thank you so much Roger, this 54 xk120 has 21k original miles. Not sure if that makes a deference. Just wanted to see which gas is best for it to avoid any related problems as I’m not so handy with cars.

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Simon:

I would echo Rob’s observations. We have had only unleaded gasoline in my area (southern Ontario) since the early 90s and ethanol added for an equally long time. I run my XK (8:1 compression) on 87 octane which frequently has ethanol added (I don’t believe one can even buy non-ethanol fuel in this area) and have experienced no problems in the past 30 odd years. My car clocks up between 2,500-3,000 miles each year does not “ping” under load or exhibit any other issues. There are as many opinions on this issue I am sure as there are Forum members, sort of like the, “What’s the best looking XK?” debate. As was observed, start low and work up if you have to.

Chris.

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Sorry, didn’t mean to trouble you folks and have you spend time writing on fuel technicality. Just wanted to simply know what others use for their early Jags, mainly here in the US. Thanks again!

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Thank you Chris, been using 92, will go down to 87 and see what happens. The point is not saving $ on gas just thought the 92 might be too much for old cars. This is my 1st classic so learning as we go :slight_smile:

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92 burns slower than 87. There is not more energy in the fuel and you will not get more horsepower or torque or stronger explosions. The reason many think that it is more powerful is that more powerful engines need more octane. Why?

If your engine has a higher tune, more compression or advance, it will need fuel that is harder to ignite so it doesn’t do it by itself which is bad for the engine. This is why you go up in octane if you hear preignition.

Especially on old cars without knock sensors, octane ratings higher than necessary are just a waste of money better spent elsewhere.

And because old cars have charakter, you have to try out what fuel it needs, but seems like 87 can work which is the answer you wanted.

If anything, lower octane gas could be easier to start but you will never notice. E10 is another debate but replacing perishable components like rubber is not a bad idea and the new parts will be ethanol proof. Hopefully. Just don’t let it sit and nobody will complain.

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Since a number of points have been repeated by others, I’ll also repeat something I’ve already said (maybe restated) -

Use the lowest octane of fuel that doesn’t ping. That’s it.

You can use a higher octane if you want, no harm, it just burns money up. But then the hobby of old cars burns money up anyway.

Dave

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Back when I was driving my original Rover ('75-'80), I was having difficulty, even then, finding “hi-test,” and on the unleaded then, it pinged like mad.

Fast forward to the present, Margaret runs just fine on 87: they made it better.

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Barn Queen was originally built to race specs and timing set up for 98 octane. When I moved south the gas I could get here was of lower octane and contained ethanol. I have retarded the timing so that I can run the local high-test.

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I have been running 5 vintage British cars on 10% ethanol fuel for 12 plus years with no adverse impacts. They are all used year round. On a recent 200+ mile round trip in my 1965 3.8S (9:1 compression) I tested using the no ethanol 95 octane (US rating method) fuel on one leg of the trip, On the return I used 89 octane 10% ethanol fuel. The calculated fuel mileage was fractionally better on the return trip with the 10% ethanol fuel. Therefore I see no reason to fuel my cars with the no ethanol fuel at almost $1.00 more per gallon and pay for octane I do not need as the car does not ping or have reduced performance with the 10% ethanol fuel. Just one mans experience.

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some ethanol 10% fuel FACTS…you can look up, to verify:
ethanol fuel will absorb water many times more than no ethanol fuel, the water will separate and form a layer, this causes rust and corrosion. Ethanol fuel gains water over time, and also degrades over time. It is at first a high solvent and can dislodge old varnish that will then clog fuel lines, fuel filters and carbs. Then over time it loses its solvency and becomes gel like. Ethanol attacks certain older rubber compounds, older plastics and some metals. Maybe those are in your fuel delivery system, maybe not. The West Advisor


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Busting Ethanol Fuel Myths

Red Tow Boat US boat

Keeping water out of the fuel system may prevent a call to TowBoatUS for a tow.

By Tom Burden, Last updated 6/1/2020

What are ethanol and ethanol-blended fuels?

Ethanol is used as an “oxygenate” and is added to fuel to help reduce hydrocarbon emissions that cause air pollution. It is highly refined beverage (grain) alcohol, approximately 200-proof, that can be produced from natural products such as corn, sugar cane and wheat.

New technology allows ethanol to be made from cellulose-rich feedstocks including corn stalks, grain straw, paper, pulp, wood chips, municipal waste, switchgrass and other sources. Ethanol used for fuel has been denatured or rendered unsafe to drink by the addition of a hydrocarbon (usually gasoline).

The term “ethanol-blended fuel,” or E10, refers to fuel that contains 10 percent ethanol and 90 percent gasoline. Similary, E85 refers to fuel that contains 85 percent ethanol and 15 percent gasoline. E85 is intended only for engines specially designed to accept high-ethanol content fuel blends, such as the Flexible Fuel Vehicles (FFV) made by some car companies. Not all states require gas pumps to be labeled to indicate the presence of ethanol in the fuel, so you may be currently using E10 fuel and not be aware of it. There have been efforts in Washington D.C. to introduce gas with 15 percent ethanol (E15). More on E15 and the potential dangers it poses for marine engines later in this Advisor.

How is ethanol made?

In the U.S., ethanol is typically produced by removing the starch or sugar portion of corn and fermenting it. The fermented starch is then distilled into alcohol. Excess water is removed, resulting in very pure, 200-proof, ethyl alcohol (ethanol). In some parts of the world, ethanol is made from a variety of raw materials. For example, sugar cane is used to produce ethanol in Brazil, while sugar beets and wheat straw are commonly used in Europe.

Draining water from a fuel filter/ water separator

If you change over to E10 from non-ethanol gas, you should check for water in the water separating fuel filter. Carry spare filter elements, as ethanol is a solvent that loosens gunk in the fuel system.

Problems with the transition to E10 fuel

The most likely time for fuel problems occurs when you first begin using ethanol-blended fuel. When E10 is added to a fuel system that has been using non-ethanol gasoline, ethanol, as a new solvent, will tend to dissolve and loosen deposits that are present in the tank and fuel system. Phase separation may occur, resulting in an approximately 50/50 ethanol and water layer at the bottom of the fuel tank. Never use E 15. I use only no ethanol in classics, .and pay the few bucks…in a year of driving adds up to the cost of a few pizza.
Nick

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I don’t know if it’s true or not, but I’ve read that Ethanol is less energy-rich than petrol, and the fuel burns leaner, which requires richening up again. Some have reported 10% worse gas mileage…

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It is absolutely true: alcohol has fewer Btus/unit (energy density) than gasoline. It therefore tends to run leaner, taking more fuel to reach proper stoichiometry.

With ethanol, Greenie gets ~32 mpg: with non-ethanol, it gets ~35 mpg (based on observations over 200,000 miles, and numerous tanksful of fuel).

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That is true, and the difference is very small (I don’t use E10 unless it’s more than 3% cheaper)

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Yes there are a lot of opinions regarding ethanol-containing fuel and an octane rating suitable for an XK120. Here’s mine: I live in North Carolina and in the summer when the ambient temperature edges into the 90’s (F) my car is very hard to start when the engine is warm, coughs and wheezes until the rpm gets up and the fuel starts flowing, and threatens to stall if I have to stop for a traffic light. I switched to ethanol free and the problem disappeared. I’ve repeated the experiment a couple of times with the same results. I can run ethanol-containing fuel in the winter when, I guess, underhood temperatures are somewhat lower.

My car is a late 120 with, originally, an 8:1 engine. I rebuilt the engine several years ago and did skim the head, so I don’t know exactly what the CR is now, but I get compression pressures (cold, cranking speed) in the 150’s. I also tuned the carburetors with an oxygen sensor installed to air/fuel number of about 11- it varies a little bit with rpm and load.

So now I use ethanol-free gas which comes only in 92-93 (US) octane.

Bruce Wright #674699

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Bruce:

Just out of curiosity, does your car have the fibre insulators between the carb flanges and the inlet manifold?

Chris.

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While I realize there are many opinions about how ethanol 10 may perform…does the engine seem to run better or not etc…I make my decision based on what is known: to recap above: ethanol 10 is more solvent-at first, thus may loosen crud and clog filters, pumps, jets etc. then it ages faster and becomes “spoiled” and can gel: it collects water and the there can be phase separation–a layer of water in the tank; it will attach certain older rubbers and plastics-are you sure you have only new ones designed for ethanol fuel? and it can eat at certain metals especially the zinc and aluminium in carburettors and some inlet manifolds…these are facts, not opinion. . Are there any such metals in the XK fuel system? Aside from this, the idea of using it happily to help the environment has many flaws…I invite to read the report by Yale University Enviroment Studies Dept: The Case Against More Ethanol: It’s Simply Bad for Environment - Yale E360 but that is another side to it.
Nick

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I am with Yale on this one, and have been for a long time! As I’ve said before, the best thing that ethanol does is provide welfare for large agribusiness.

I did not notice any increase or decrease in performance in any of the cars that I tried it in, back-to-back.

Oddly enough, I did not see any increase in mileage on the Rover or the Jaguar, and only saw a fairly significant increase in my Hyundai.