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So I picked up a clean 2.3L 2007 Ford Focus ST the other day, I love the car only problem is that this is my first time dealing with a car that’s had any after market work done. The main issue i’m having is what i can best identify as detonation, the car is tuned on 91 octane but the compression ratio is said to be 11.6:1, i did some research and most everything i’ve found says that i need to be running 95 octane, the highest octane in my town is 91. Friends have recommended i mix e85 with 91 to create higher octane, but i’ve seen everywhere that the ethanol will damage my fuel components. is there a work around for this until i’m able to fix the tune/CR? The car also has a very rough idle and will tend to almost die when coming to a complete stop, it’s showing a P0135 code which is O2 sensor, bank 1 sensor 1. I have a new sensor coming in but will the rough idle stay until i fix the detonation? Anything helps! Much thanks from a newbie. This is my current performance mod list:
Cool flo intake
Trubendz exhaust
OBX Headers
FSWERKS tuned on 91
11.6:1 pistons(not sure if they are upgraded pistons)
Eagle Rods
Stage 2 Clutch
 

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2013 Focus SE gray 5 spd hatch
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You're not clear as to where the 11.6:1 compression ratio is coming from. Did you test your compression, told that by the gut you bought it from, or read that somewhere? Higher compression necessitates high octane. 11.6:1 would require at least 93+ octane. You could buy octane booster from an auto parts store and add it to the tank every fill up or just drive outside town to get 93 and see if that fixes it. 91 is only mid grade here in NJ.
The P0135 means that you have a short in the O2 sensor itself or the wiring to it. Check your wire and connector to make sure that there's no damage or corrosion.
 
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