Car is an ‘88 V12, engine confirmed in good mechanical shape. Distributor advance not seized and vacuum capsule new and working. TPS good and adjusted, throttle blades synchronized, and newer plugs and wires. Vacuum reducer works, as does vacuum dump.
Owners manual states to use 87 octane minimum fuel (before the days of E10 though), which is what I am using; the only affordable fuel in this area is top tier at Costco, which only carries 87 and premium (which is close to $1 more per gallon than the 87).
Timing set by ear, advancing until spark knock heard, then backing off slightly- this results with timing being 14 degrees BTC at 3000 rpm (4 degrees retarded from spec). Fuel economy has dropped by less than 1 mpg, averaging 13.5 mpg combined, and about 17 mpg all highway.
If the timing is set to 18 at 3000, I get continuous spark knock at freeway speeds with any additional load such as a slight incline, also a brief period at 1800 on acceleration.
Based on the above it makes no sense to me to spend $1 more per gallon to increase fuel economy less than1 mpg….
Opinions and comments?

 

Use the highest octane fuel you can buy!

 

Absolutely Greg!

I always put 98/99 RON in the Jags and the Range Rover. And it's nothing to do with the fuel economy.

Paul

 

Octane is not “if a little is good, a lot is better”. I know that I can use higher octane than the manufacturer recommends but to what benefit? Basically 25% more cost for less than 10% more mileage. Car runs fine with the timing retarded 4 degrees. I assume that the reason for the spark knock with the timing set at spec is due to E10 and different formulation than the gas of 35 years ago.

 

RGK20m3,

It may be different in the US, but in the UK, the Premium Fuels not only have a higher RON octane level, but have a number of additives, particularly detergents which can benefit the engine and PI system, and much lower levels of Ethanol. The Premium Fuels are 0-5% ethanol content, compared to the 10% ethanol in the lower octane fuels.

So for me, it's not just about the octane level as to why I use the Premium fuels. And whilst they also may benefit the fuel consumption, that's not really the reason I use them.

Cheers

Paul

 

The purpose of high octane fuel is to prevent preignition. If you can't set the timing correctly without knocking, you're using too low an octane rating.

 

Just about all fuel in Michigan,USA is E10 and “oxygenated”, whether regular or premium. Only fuel that is ethanol free is for off road use and carries a huge price premium. I get what others are saying “if it spark knocks, needs more octane “. But I think Jaguar knew what they were doing when they specified 87 octane….wondering if anyone has knowledge that E10 or oxygenated fuels may cause spark knock.
Or maybe there is something going on that I haven’t checked.

 

Well, the ROM says 91, not 87. Ethanol would actually help you, it has a much higher octane rating than gasoline, but it gets blended so that the final product has the advertised rating.

 

Additionally, the high octane fuels do not go off and gellify nearly as fast as the lower ones do, if at all.

 

We be different again.

We have 91 95 98 straight fuel, then the E10 and E85 (for specific cars).

I run our 98 in all mine, and at $2,54 /ltr, it is what it is.

The peace of mond that I can "stomp on it" if need be, and know the engine will do what it is meant to do.

Timing numbers, NO IDEA, I drive time them all, and be done with it.

The use of "P" spec spark plugs, meaning Projected Nose, has made a difference in MY opinion ONLY.

Running it retarded is OK, MAYBE, but if the situation arises when you need to "stand on it" it will cause issues, again, in MY opinion.

 

I’m sorry to say you are simply wrong. Ethanol is very very good. It has 105 octane plus its cooling effect. In addition it cleans deposits in your cylinders ( on the back side of the brake valve. ( The down side is more Ethanol is needed than gasoline)*. When I use E85( that 85% ethanol, 15% gasoline). In my flex fuel vehicle I lose between 1&1/2-2 mpg. But I only pay $2.39 a gallon so I come out way ahead, $20 per tankful. ( even after adjusting for the lower fuel mileage).
* it’s used in Race cars to add extra power.
You cannot use it in your Jaguar because it doesn’t have the sensors that adjust automatically for the fuel used.
Finally. Your manual is wrong. Your Jaguar has 11.5-1 compression. That means it needs the higher octane. If you switch to that in time you may save your engine. Otherwise it will blow up (technically what is most likely to happen is you will burn valves, or holes in pistons. You could also drop a valve seat which will ruin both the valve and the cylinder head plus's the piston maybe even the cylinder wall).
Then the only economical repair is a new engine.

 

My owners manual says "91RON" as well.

It further states that occasional light spark knock may be eliminated by using a higher than recommended octane

Cheers
DD

 

91 RON is equivalent to 87 AKI, which is what the US currently uses.
I wonder if the vacuum advance could be an issue; third replacement capsule in the 7 years that I’ve owned it. Perhaps to much advance range or advancing at wrong vacuum. I did check it before I installed it and the start/full vacuum readings were close, but actual link movement length was different by vacuum vs mechanically pushing it- and the stop adjustment didn’t seem to affect the vacuum movement range. The movement range (mechanically) varied about 1/16” between the units, one original, one reproduction, and the one on there now from British Vacuum Unit.
It would seem a variation there could affect part throttle spark knock, but it would be a pain to plot it…..

 

Octane, ethanol, compression ratio, and how they affect spark knock is a complex subject. Compression ratio, combustion chamber design, head material, camshaft, temperature , air density, and mixture all affect spark knock under any given operating condition. An aluminum cylinder head with a high swirl combustion chamber design can tolerate higher compression with low octane fuel, and will be more efficient with less spark advance. An iron head with an open chamber will require more spark advance for peak efficiency and also will require higher octane for a similar compression ratio. Ethanol being good or bad requires context; good in an engine designed to use it, good because of inherent resistance to detonation, but bad in engines not designed for it, as it is corrosive and provides slightly less energy ( which is why you see a decrease in fuel economy). Ethanol isn’t in today’s fuel to boost octane, it is used because it is a renewable and burns cleaner.
I should have said using higher octane than necessary is wasting money.
Back to my original topic, the amount of spark advance required at a specific load and rpm is affected by mixture, temperature, engine condition, and fuel; and what may have been determined in the eighties may not apply with today’s E10 fuel.

 

Modern engine designs incorporate computer controlled spark advance; detonation (knock) sensors provide a signal to retard timing. Also ethanol content is monitored on flex fuel engines to adjust fueling and ignition for peak efficiency. Engines designed to be tolerant to ethanol use stainless steel and different plastics, along with different hose. Ethanol in small amounts 10% or less were determined to not affect most vehicles in use when it was introduced. I believe that determination didn’t take into account vehicles that are driven infrequently, which may be why we see so many issues with our classics.

 

The octane rating, octane requirement, compression ratio, ignition timing subject is a complicated one. First two fuels with the same RON or MON don't necessarily have the same knock characteristics; the numbers are indicative and not absolute. The requirements of an engine will depend on its general condition and it's cleanliness. If you use a premium fuel (Ultimate, V-power, ...) from an oil major, the octane requirement of the engine might reduce. Another factor is mixture strength. Making the mixture richer than stoichiometric will generally reduce the tendency to knock. If you intend to keep your car long-term, use a premium from an oil major.

The original V12 in my Daimler knocked when cruising at about 80 mph on level or slightly uphill slopes. The knock stopped if I opened or closed the throttle a little. The knock margin must be small under that condition and the heads had been shaved at least once. I tried some of the additives, but they had little effect. After we swapped the engine, I've had no more problems.

Incidentally, both engines would just achieve the XJ12 target of 20 to 21 miles per imperial gallon with the car running at a constant 80 mph.

 

“Drive time” is essentially what I have done; Grant, it would be interesting to see where your timing is compared to the 18deg spec.

 

No you are wrong. America uses MON + RON divided by 2.
You should not have vacuum advance. ( if it’s an HE ). The HE distributor gives you vacuum retard ( hence the doggy acceleration ). Plus the source of Knock.

 

No, US spec HE distributors give vacuum advance with the Lucas distributor. I know, i own two of them, an 85 and an 87. Don't have a clue where you got the idea they have vacuum retard.

Jon

 

Some of the older models from the 70s had a vacuum retard system. Never have seen an HE so equipped, tho.

Cheers
DD