What if I am waiting on my Xcal2 to come in soon? I figure I would "purge the tank" of the 87 stuff.

 

If your waiting for the tuner in the mail definitly purge your tank now. It wont harm anything with one tank.

 

here is what Mike Troyer has to say about octane:

For 05Silver - Not exactly - there's much more to octane than just a higher octane number having a higher resistance to one form of detonation (pre-ignition). While what you said in that vein is correct, more octane gives higher resistance to pre-ignition, you do NOT want to run premium gas on an 87 octane tune, as that will result in making less power and dropping the fuel mileage, as the fuel does not have enough time to burn properly.

See, the higher you go up in octane, the longer it takes to *initiate* the combustion event, in terms of real time and thus in terms of degrees of crankshaft rotation - thus when running higher octane, there MUST be more spark advance (additional timing) in order to allow more time to get that mixture lit and to burn as much of it as possible, enough to make proper power, etc. Now once it's lit, then the flame front propagates itself across the combustion chamber at about the same rate with either 87 or 93 octane, all else being equal and the ignition system being up to the task.

Running 93 octane on an 87 octane tune, you will not have enough time to burn enough of the air/fuel mixture, thus power will actually drop starting at about 4000 rpm at full-throttle in the 5.4 3V, and can do so even earlier on part-throttle - trust me, we've tested this, as a number of our customers have found this out on the dyno the hard way, calling us to complain why their engine stopped making any more power at 4000 rpm instead of peaking at at least 4800+ rpm - it was because they made the mistake of running premium gas on an 87 octane tune.

Always run as close as possible to the octane level the engine is actually tuned for - 1 point of octane HIGHER really isn't going to matter much, but much more than that, and it quickly hurts power. So you definitely want to run the same basic octane level that the engine is tuned for in order for the engine to run properly, and at it's best.

My earlier comment was simply predicated on the fact that when going from premium to 87, it's not quite as important to run the tank down as low as when changing from 87 to 93, as i that case you don't' want to invite detonation - so you want to run the tank down as low as is reasonably possible without running out. When going the opposite way, from premium gas back to 87, you could do that with as much as say, 1/4 tank of premium still in the tank, though most people aren't going to do that, as they want to get every bit of power & performance from even drop of premium they pumped in.

Just FYI.

 

The only thing to be careful of is the fact that since the 87 octane is not advertised as an upgrade it doesn't have to actually be 87 it could be 85. 93 has to be 93 if its advertised as an upgrade. So you may find 89 is better than 87 because its a blend of 87&93. My ranger and my girlfriends explorer both w/4.0 v6 ran better and gained 20-30 more miles a tank. I haven't seen this improvement with my 05 f150 but its something to keep in mind for the guys running an 87 tune. If it pings try useing 89 or go to a different station (exxon and mobil where rated high in an old study they had for snowmobiles).

 

I'm pretty sure this is a stupid question, but if 93 burns slower than 87, if I light a can of 93 on fire and a can of 87 on fire, will the 87 go up in flames faster?
I guess I'm in for a nice explosion either way.