Sams club in Santa Fe has really been dropping the gas prices $2.72 today for 87 and instead of the usual additional 50 cents more for premium it is only 30 cents more per gallon - of course it was $1.71 the day Brandon took the helm and did make it almost to $5

 

I've been running pure 93 and recently filled up (20 gallons) with pure 90. Seems about the same except it appears that my mpg improved. Not sure why but I'm going to try another tank of 90 when I fill up tomorrow.. Its a lot less expensive than the 93 which is now over $5 a gallon.

 

You should try some 87 and see what you get.

 

I did 3 tanks of 91 shell 0 ethanol and 87 I didn’t notice any difference and the mpg were almost the same. Here in Canada it’s 20-30 cents a litre more for 91

 

I live in a town about 100 km outside of Vancouver... Shell here used to sell 91 without ethanol until a year ago. Now, the only place I know in town that sells gas without ethanol is Chevron with their 94 octane.

 

Having driven my D1 across country multiple times and experimented with various gas available - running high octane pure gas yielded a little better gas mileage - no noticeable performance on flat land but at altitude and on grades yes better performance - but usually it was nearly $1 more per gallon when available I did not find it worth the spend

 

I’ll have to keep an eye on the ethanol content. I’m under the impression that 91 octane generally comes without ethanol here in BC.

 

Well, the 93 pure gas is now around $5.49 a gallon so its back to the normal 93 that's $3.53. The 90 pure gas near me is $3.99 a gallon but not very convenient. Maybe when/if gas prices come down, we can re-engage the 93 pure gas trial...
To be honest, I'm not sure that I can really notice much of a difference so less money spent on fuel the better.

 

Octane is not a measure of power in the fuel, just knock tolerance.

91 octane is recommended because of the compression ratio of the engine. Higher compression ratios mean that the fuel/air mixture is squeezed harder than a lower compression ratio engine which will make more power comparatively. The higher the compression ratio, the faster the flame propagates inside the cylinder which means the spark can be fired later in the compression stroke to achieve higher cylinder pressures (more power). Higher cylinder pressures mean higher temps which can lead to the fuel igniting on its own before the spark fires, or the flame propagates too quickly. That's called detonation (knock). Detonation is bad because if the explosion happens too soon, or the flame propagates too quickly, the crank / piston won't be in a position to transmit power downward during peak power and a lot of the power from the explosion is exerted on the piston while it's still moving upwards. Detonation can blow holes in pistons, bend connecting rods and damage cranks. Since octane is a quantitative measure of a fuel mixture's resistance to detonation (knocking), higher octane means that the fuel is more tolerant to higher pressures and temps.

The 4.6L has a compression ratio of 9.37:1 and the 4.0 has a ratio of 9.35:1. 91 octane is recommended for compression ratios at or above 9.3:1 to make full power and avoid detonation (knocking). We don't have 90 octane in the US so the recommended octane is 91 because 89 doesn't quite cut it (technically). An engine with a compression ratio under 9.3:1 will run just fine on 87. Disco engines are so close to the cutoff that 89 or 87 octane should be just fine especially when paired with knock sensors. It'd probably be best to use the higher octane during high-demand loads (like towing) to avoid the knock sensor telling the ECU to adjust timing and take away power.

Knock sensors make it safer to run lower octane fuels because they can detect detonation. Knock sensors work by sensing vibration in the engine which the ECU interprets to detect knock. If the ECU detects knock, it changes the timing of when the spark fires to ensure the power from the explosion is sent into the power stroke and not into the compression stroke. Knock sensors help a lot. For example, I swapped an LS-based L33 into my Disco. It has a compression ratio of 9.9:1. GM only recommended 89 octane or better for the engine because the ECU protects against knock.

Regarding ethanol; it has about 27% less energy than non-ethanol fuel. You'll lose some MPGs running ethanol gas. Since ethanol makes up a maximum of 10% of the volume of the fuel, only 10% of your tank (about 2.4 gallons) has about 30% less energy, the rest is regular gasoline. The difference in normal driving shouldn't be very noticeable and the cost difference should be close to a wash, if not a bit cheaper to run ethanol. Never run E85. It'll do bad things because the computer can't manage it. Vehicles that can run E85 have sensors in the fuel line that detect the level of ethanol and the ECU adjusts the tune appropriately. Without that, E85 can really damage an engine. A show called Engine Masters did a really neat episode on "flex fuel" recently (season 7, episode 21).

Long story short: 87 / 89 octane with or without ethanol should be just fine to run in your Disco under most conditions. Anything over 91 octane is wasted money because the engine doesn't need that level of knock tolerance.