Which fuel should we use?

Echoing and amplifying some of what has already been said...

The computer primarily adjusts timing to keep the engine from knocking ... which is primarily a function of octane. On an older pre-computer motor, your timing is mechanically controlled, so you set it based on the type of fuel and conditions in which you were running. You can safely put any type of pump gas in your Armada and the engine will automatically adjust for the fuel, driving conditions, altitude, etc.

That said, because you can dial in a little more advance with a higher octane fuel, you should get better performance running the premium fuel...might be a consideration if you are doing towing. On my non-turbo Volvo, I run premium, because losing any of the 162hp really impacts performance. For the Armada, where you already have more than enough power, the price difference and the Armada's mid-teen's gas mileage should make regular unleaded your usual choice.

In regard to the Infiniti 91 octane recommendation, it's tuned to provide another 10hp and 5 ft-lbs of torque. It could be that these figures are only reached running the higher octane fuel, but again, the computer allows the motor to still run fine on 87 octane. But if you're paying the extra $$$ for the QX, maybe the additional coin at the pump doesn't make a difference to you.

Pardon the long post.

Matt

There are several excellent discussions of octane and our ECU's over on the Titantalk forum. They are technical and involve some pretty sophisticated engine analysis tools which have done real time timing advance measurements with different octane fuels under different load conditions.

I'll try to summarize and also supply some "back channel" input from an anonymous Endurance engine engineer at Nissan.

1. The engineer confirms what we have suspected. The ECU's and engines are identical on the production lines for the Armada and the Q56. We have the same stuff.

2. Our ECU's are of the new generation, smart "learning" ECU's that take data from the knock sensor and a lot of other inputs and will continue to advance timing until knock is detected, and retard it when driving conditions tell the computer to do so.

3. That means that it takes the computer a little time, perhaps hours, maybe days to adjust to different driving habits. Aggressive driving will teach the computer that you want full timing advance more quickly under WOT (wide open throttle) conditions. So changes in fuel can produce some unusual results. A change to premium might take a little time for any benefit to take place. Once the computer is used to running premium, a sudden change back to regular could result in a little loss of power until the computer readjusts.

4. This is what we believe happens: The premium fuel will, indeed, permit the timing to advance an additional 10 degrees or so, but only under full WOT situations. Once the computer recognizes the change and learns to permit the timing to advance more, it does, indeed produce about 10 more bhp and about 10 more pounds of torque. And as long as 93 octane or higher is fed to the engine, the computer will permit this advance at WOT on demand.

This means that the Armada and QX56 engines are identical and each will produce the power rating of the other when using the other's fuel, once the ECU adjusts.

5. The good news or bad news, depending on how you look at it is that 10 bhp and 10 pounds of torque is real, but cannot really be measured by the seat of the pants and adds very little to performance. It will show up on a chassis dyno or a good accelerometer. I can demonstrate that small gain on an accelerometer.

But, this is only under WOT conditions, and even then only when we push the engine up high into the power band. These are conditions most of us rarely use. But the fuel is much more expensive and is being burned all the time, not just when you are at WOT at or near redline.

In terms of performance, the net result will be less than one tenth of a second gain in the quarter mile. This might mean a ten foot difference over the 1320 feet distance, at best. It will not improve gas mileage, and might actually decrease it slightly, especially if you are getting into the right pedal more frequently because of the desire to want to "feel" this extra slight power, which you really can't feel.

6. With respect to the QX56 owners here, I suggest that some prestige car or truck owners are lured to the extra horsepower claim and the belief that if the car or truck is "premium" then the fuel should be also. I would suggest that they try their trucks with 87 octane and save the money. The QX56 is a beautiful truck, and nobody knows what is in the tank.

7. For us Armada owners, if you are going to tow a heavy trailer, the little extra oomph and timing advance may be a benefit, especially if you tow on steep grades. Or if you want to take your truck to the strip or engage in some other operation where maximum WOT performance is desired, you get a little gain. For the rest us, do like me and run 87 octane unless you have one of the above reasons. At twenty cents per gallon price difference (or greater) the difference adds up soon.

BTW, I get 11 mpg around town and 14 on the highway with the Big Tow package and the extra weight of the 4x4 front axle and transfer case. Running premium did not seem to change those numbers appreciably.

Just my admittedly wordy two cents worth.

Gas Mileage - not a strong point, but OK

I am at about 8200 miles on my 04 LE 2WD with Big Tow package. Have been getting 12 MPG (and less, depending on how many Accords I need to see in my rear view ).

Highway mileage is 17 constant, whether or not my pop-up tent trailer (2500 lb and low profile) is attached. Decline in acceleration and braking effectiveness with the trailer behind me are minor but noticeable. Went up about 3 miles of 7% grade last weekend and noticed the trans cooler indication going up some. Slacked off when I actually got my foot in it, pushing to a lower gear w/more RPMs.

I have never put anything but 87 in it (even a few fillups in Mexico over spring break) and I'm not wanting for power. My brother has an Xterra and doesn't even get up to 16 MPG on long trips. Really gets his goat: :censor:

Fred