Is premium fuel for the XK carburetted engine worth it?

Here in the UK unleaded petrol is offered at standard and “premium” grades - the price difference being about 8% - 24%. As far as I can make out the claimed advantages of “premium” grade fuels are that the premium grade provides:- 98 octane instead of 95 octane rating; friction reducing additives; cleaning additives; more MPG. It seems that both standard and premium grade petrol contains a proportion of ethanol.

I have been trying out the premium grade fuel for a couple of months in my 1971 Daimler Sovereign 4.2 litre and am struggling to experience any benefit - particularly given the average 16% additional cost. With these older pre - computer controlled engine systems and relatively crude carburettor fuel systems I am doubtful that the additional fuel cost is worth it. If I want to clean my inlet valves and fuel system I can always use a bottle of fuel additive from time to time.

What do others think?


Running 98 octane fuel will allow you to advance the ignition timing and gain a little more power, but if you’re just using 98 octane in a 95 octane setup there will be no noticeable benefit. Whether the extra power is worth the extra cost is probably unlikely! Are the other additives going to give noticeable benefit? I think there may be a clue in the careful wording the oil companies use when describing the potential benefits.

I think that higher grade can be useful when it starts to ping on hot summer days. Apart from this it is more of a waste I’d say.


Personally I feel I can improve/maximise performance by adjusting the ignition advance and plug gaps, getting the valve timing spot on, setting up the SUs correctly and even using a lighter/heavier grade oil in the dashpots. There are so many manually adjustable variables I feel the benefit (if any) of the more expensive fuel diminishes into insignificance on these engines.


1 Like

When I had the engine rebuilt 2 years ago we discussed which compression ratio to go to since I was getting new pistons. I asked if going to 8-1 would allow the use of regular, 87 octane, gasoline. I was told no it won’t, so I went with 9-1.

If I go to Gas Buddy and figure the cost of a trip from here to Orlando and back, 2026 miles using 16 city and 20 highway as the mileage I’d save about $60 by using regular instead of premium. Big deal.

Well, that seems bassackward…???

I run 95 in my 4.2 1975 S11 our petrol is rated the same as in the UK sans ethanol. I had to knock back the timing slightly as it was pinging on hard acceleration but apart from that its fine.

Same with 9:1 SIII XJ and 4.2 E.
I argued with somebody much more competent than I (Crespin or Dwyer?) on fuel consumption long ago and if the timing is set just right and fixed it won’t make a difference. If it can be advanced further it will make a difference, but not justifying the price for normal driving.
If the timing is adjusted it will save fuel, and if it is pinging or unnecessarily retarded the engine loses power.

Something might hold up about all the additives, but the XK ran on fuel that was much worse than todays (which satisfies high-horsepower, incredibly efficient motors with very fine tolerances).

If you do set it up for better fuel, it will save you money. But for everyday driving, the cheapest fuel (from the best station) that doesn’t ping is the best.
My manual says if you don’t use four-star leaded fuel, the timing has to be adjusted, the engine driven very careful otherwise catastrophic damage blah blah.
Without lambda feedback mixture might lean out slightly but in any case, compare the fuels yourself and look for objective differences.

I’ve never heard an XK engine ping, heard others but not an XK engine. How loud or noticeable is it?

Oh, they’ll knock! It sounds like any other engine’s detonation.

Pinging or knocking if I don’t have the terminology, it makes a distinct noise that can’t really be heard with windows open… similar to the cracking noise knock-sensing cars make when you floor them? It goes away if you ease off a little.
If you hear it you know it.

Thanks guys. Oh, I’ve heard it in other cars, Just not in a Jaguar. I’ve always pictured an air popcorn popper filled full of steel balls when I hear it. I’ve got the timing spot on at 10 degrees BTDC. I might move that to 12 and see how the car performs.

And here we have it - the definitive answer from the lovely Vicki!


Yes, but on a modern car.
Old fixed-advance designs are different… I would say! Just my opinion and probably worthless.

No opinion is ever worthless!

I’m no expert, but I would think the positive effect (if any) on a pre - computer carburettor engine would be even less.

The claims for the “premium fuel” remind me of the fuel saving devices that proliferated in the 1970’s oil crisis. I remember two in particular - an “atomiser” which looked uncannily like a tea strainer and fitted between the carburettor and inlet manifold; and a propeller that fitted over the exhaust tail pipe - designed to reduce back pressure by “extracting” exhaust gases. To my eternal same I actually bought one!


The key word here is YOU. Unless, of course YOU previously added the sensors and ECU to do it for you as you drive.

In general, you can get more performance through advanced timing up to a point prior to detonation. This is one function of an ECU (to deal with whatever fuel is being provided at the moment). ECUs programming is often concerned with issues other than performance as well (such as emissions).

One, reasonable, approach is to add a distributor that supports multiple, customized, advance curves. If you want to feel the difference in order to see if you feel premium is better, then fill up with premium, select the curve you have developed for that and take a vigorous drive. Then select the curve you have developed for a lower octane fuel and repeat the drive. If you don’t feel (or care about) the difference then you likely want to save your money.

Note the word ‘developed’ which likely involves a data logger and detonation sensor. Having the ‘right’ curve for your car would be a good thing no matter what fuel you choose to use.

1 Like

Much as I enjoy Vickie, that would have been a more meaningful test drive if it had been a blind test for the actual test track segment.


It would be, except for one scenario:

If the car is set up for premium and you use lower grades, it should lose power and consume more.
On very hot days premium is definitely useful to prevent detonation.
If it works on regular you wouldn’t need premium. Modern cars self-adjust and will use less, but will it be cheaper? No XK ever came with adjustments save for the cat-equipped SIIIs and they only adjusted the mixture. Carbed cars might be so imprecise that if not dialled in perfectly they‘d waste enough power elsewhere.

I always had a hair too much advance but at least my pistons look like new and so do the chambers :slightly_smiling_face:

Jaguars were specified to run on Premium, probably a shade higher than what is available today. Damage in the engine can occur from Pinking, pre-ignition, knocking, or whatever you call it. Main cause is the ignition happening at some other time than what is optimal for the engine. Whether this be from the ignition set incorrectly or hot spots in the combustion chamber. Not many people drive their E-Types 10s or thousand miles each year and engines are expensive to rebuild. It is also a Jaguar by the way. How cheap can you be to worry about the small difference in the cost of Premium vs Regular with the potential drawbacks of not running premium. If you drive a lot of miles, get a Prius. If you want to drive your XKE as it was intended, follow manufacturers recommendations as far as fuel. If your car will run on regular, you have something wrong with it and are not getting full potential out of it.


On a Jaguar that cost $2500, isn’t in concourse condition and has good part availability I dare save more than $100 a year in unnecessary fuel cost. Besides it did exceed the factory time 0-60 and shows absolutely no damage from the grade of fuel used.
On a hot engine I may not have the full potential but in cruising it does well. Downshifts and slowing down are a possibility that easily negates the effect of lower fuel grades.
With 8:1 I bet regular isn’t a problem at all anymore, I have 9:1 and do not care for premium.

I cannot see anything wrong with that. Sorry.
Agreed that $100 are not much on a good car with timing set for premium.

1 Like