GTorangeGT

I've been running the 87 octane tune from breenspeed and want to try the 91 octane. Do I have to run my engine full of 87 octane dry then fill up with 91 octane then do the tune? Or can i leave the engine with barely any 87 left ... like 1/2 gallon and fill up with 91 and do the tune?

2007VistaBlueGT

Run it till the fuel gauge goes red, fill it up then run it red again fill it up and install the tune. Always better to be safe then sorry.

thewarmachine

Just run it down as far as you can and fill up with 91. You're safe to run the 91 tune then.

TypeS

You'll probably be fine just going as low as you can then filling up with the new desired octane, but I've always been advised to run a 2-3 tanks of w/e octane through the car to make sure you wont hit a pocket of 87.

wetwille3

Just out of curiosity, what would happen if you filled up with 87 while running a 91?

East87

Bad news haha. I think it's better that you run a higher octane for a lower tune than a lower octane for a higher tune.

87 on a 91 tune = baaaad
91 on an 87 tune = probably doable

jbrosh

Will octane boost help? I'd like to know myself as I'm doing the 93 tune tomorrow and need to get rid of the 87 in the tank. Thanks.

jbrosh

Answered my own question with add'l searching. Looks to be a bad idea to use octane booster. I'll run the current tank to as close to empty as possible, fill with 93 and then install the 91 tune for the first few tanks of gas and then install the 93 tuen.