[darkpuppet]

Technically speaking, using 91 on a car engineered for 87 is just going to result in the air/fuel mixture burning slower and incompletely with the stock timing.

this can result in the build up of carbon and octane tolerance that will sooner 'chain' you to the 91 octane pump rather than give lasting performance.

When 91 can be beneficial is if your engine has built up carbon in the cylinders that provide sharp edges which results in hot spots that cause pre-ignition. This gets picked up by the knock sensor and timing gets pulled, killing performance.

Putting in 91 at this point in the car's life will help reduce the occurance of pre-ignitian, giving you back the timing that you lost with 87. But this is usually for very high mileage cars... and with proper maintenance, you shouldn't really ever have to go to 91.

So basically, putting in 91 prematurely is like teenage drinking, you're just going to have that much less liver when adulthood actually hits... i definitely wouldn't recommend it.

If your knock sensor pulling a lot of timing with 87, you may want to run some seafoam or something to help clean out the cylinders... and if you're going to run 91, you should really have a tune for it.

Ok.... also... depending on your mods, there are exceptions to this rule. -- IE, AEM and some other cold air intakes increas your car's horsepower by increasing the diameter of the MAF tube.

At part throttle, the car can compensate for the mild gain in diameter, but at full throttle, the car goes closed loop, and calculates the a/f assuming the old MAF transfer function. And at those loads, it'll exceed any loads you hit at part throttle, so you get no learning adjustments, and the mixture leans out and gives you horsepower increases.

Where does this put us?

Well, if you lean out the mixture under WOT using a CAI with larger-than-stock MAF tube, you could be experiencing pre-ignition and having timing pulled.. and putting in 91 will prevent that.

However, the car is going to lose power (comparatively speaking) to the points where 87 could be used without knocking (assuming a stock tune)... not to mention that your closed loop cruising is going to most likely experience lower fuel mileage.

My Experiences
I drove my focus 1500 km's in one day and made 4 gas stops, doing the 1st half with 87 and the second half with 91 With the tank filled (mostly) with 91 octane, I noticed a more than 100km drop per tank in mileage (approx 520km vs 650km) and the butt dyno picked up no horsepower gains.

I drove that length 1 week later in the other direction, filling up at the same gas stations, using 87 and 91 for the same legs I did going the other way, and the findings were about the same... 91 did nothing for the car -- mind you, it was < 1 year old at the time.

So there's my $0.02 ...