I currently have a 93 tune uploaded, but would like to switch to the 87. I dont have to do anything special to switch like go back to stock, just select 87 and upload, right?

 

Right. Also rule of thumb is you can use higher octane w/ lower tune but never vice versa

 

Are you selecting an 87 tune or putting back to stock tune?
Either way, you can run 93 octane gas.
The reason I ask is because I have an 87 tune and a stock tune loaded on mine.
I can select either.

 

I had the 93 tune in with 93 octane gas. I had an empty tank and wanted to switch back to the 87 tune. So I uploaded the 87 tune and filled it with 87 octane gas.

 

To add specifics, the only difference in octane tune files is the amount of spark advance. Typically, it's almost one degree of timing for one point octane. This means that a typical 87 octane tune will have a maximum spark advance in the borderline knock table of, say, 22 degrees. A 91 octane tune will have 26 degrees. An engine can tolerate more advance with higher octane so if you run 87 octane on a 91 octane tune, you will increase your chance of detonation BUT please keep in mind that this is ONLY AT WOT (open loops). If you're just cruising around town and hot-******* (just a little), you're in closed loop and the PCM adjusts fuel and timing based on rpm & load so it is at or near stoich. You will never reach the high load and rpm values on the BKT table at WOT that command these timing figures I mentioned above. VYI, in closed loop, you will be running anywhere in the neighborhood of 35 to 42 degrees without knock regardless of which tune file you're running.

 

I would still never run a 93 tune with 87 gas.

 

I would still never run a 93 tune with 87 gas.

As long as you don't go WOT open loop, determined by the one-dimensional Fuel Open Loop TP matrix, you're perfectly fine. Might I suggest learning about writing your own tune files as I've done so you can fully understand what's going on.

As far as going WOT open loop, the knock sensor circuit has the ability to advance AND retard timing when knock is / is not detected. This is an insurance policy BUT I'm not convinced it functions properly -- I read of too many occasions where guys complain about hearing knock.....

Willie

 

It's a little comforting to know that if you screwed up at the gas station and did this(and didn't have your tuner w/you) that you could drive home slowly and change it w/ out screwing up the motor. I can't tell you how many time's when I freak out a few days later thinking'Oh my god did I put the wrong gas in"

 

To tell you the truth, I dont even notice a difference between the 93 and 87 tunes. I have a terrible butt dyno.

 

To tell you thruth, I couldn't really either. I run mid grade now .

 

Getting a 91 tune was a waste for me because its not even offered here, only 87, 89, and 93.

 

Not a waste.
Just think of it as a conservative 93 tune, there really isn't that much difference (a degree or two).
A 91 tune should never knock when you run 93, some aggressive 93 tunes might (in hot weather especially)
Besides, there is more to a tune than just octane.
That may be why you don't see so much difference between the 2 tunes.
Go back to stock and you will notice a difference.
Maybe Willie can explain better.