If you do a little looking around in the E-85 section you'll find that you can run up to a 30% mix. The closest to this to make it easy is 10 gallons of 93 and 4 gallons of E-85. This will give you a mix of 28% and with gas being E-10 already that's about as much as I'd add. Make sure you have alky on the car as well and you should get some decent power out of it.