There is just SOO much bad info in this thread. I am a turbo engine tuner from the import world and have tuned over 100 high HP turbo/sc and high compression NA motors. So I figured I would shed some of my knowledge of the subject. I've done tuning for Full-Race in the import world. Not yet for the Ford world but the basics still apply.

Now to explain how forced induction works. All it does is allow more fuel to be burned with less displacement. So with more air requires more fuel. That's the basics. Now with octane rating, lower octane fuels ignite sooner than higher octane fuels. With turbo motors you are basically increasing cylinder pressures which lowers knock threshold significantly due to increased heat. What happens with an improperly tuned motor is as the cylinder is on the compression stroke the fuel will ignite causing the explosion to push against the piston as it's rising causing that "pinging" noise you hear at times. This is EXTREMELY bad for your motor. With forced induction added to this in drastically increases cylinder pressures even more which increases heat even more. Now with drastic heat you get what is called pre-ignition which is what happens when the fuel starts to ignite before the plug fires. This is what causes catastrophic failure because you are basically getting the ignition stroke during the compression stroke pushing against the piston. This will bend or break rods, snap ring and ring lands and or blow the block out or head gaskets. Not good. knock sensors will listen for the beginnings of these sounds to stop the plug from firing to try and prevent this.


Now with fuel and octane rating the lower the octane the easier the fuel ignites. Higher octane fuel will not ignite as easily and or as quickly based on heat. As air is compressed it makes more and more heat. The more air you try to compress, obviously the more heat you make. So with a higher octane fuel it can withstand these compression heat sources and allow for the spark from the motor to be ignited sooner on the power stroke of the piston which is what increases power. Now, with an active knock control system, which is what Ford uses, it allows the use of multi octane fuels by retarding ignition based on real time knock sensing. The knock sensor is a microphone tuned to pick up the knock frequency that the specific motor will produce at compression stroke. There is knock that you can't even hear that Ford tunes into the knock system. Once the PCM senses any voltage from those sensors it's pulls ignition based on pre programmed parameters in the knock control tables in the tune. Some systems will have a self learning active knock tables based on the octane used most often and will retard ignition based on what it has learned over time.

Now with 87 octane, the fuel will ignite MUCH sooner in the compression stroke causing the motor to hold off ignition firing until much later to prevent knock and or pre ignition (catastrophic engine failure). This will usually significantly lower power and torque output when compared to a higher octane premium fuel. It allows the pistons to slow down ever so slightly before exploding the fuel causing power loss and the fact that the fuel is igniting sooner it actually slows the pistons speed down as it is moving up on the compression stroke. Now comparing this to a higher octane fuel it will allow the ignition system to fire sooner at the top of the pistons power stroke and because the higher octane fuel is more resistant to heat detonation the piston can keep more momentum and speed through the compression stroke through the firing stroke. This is why high HP cars require higher octane fuels. For example ethanol usually runs at about 100 octane to 110 octane and they burn much cooler allowing more boost and more ignition because this raises the knock threshold of the motor.

This is just the basics of this subject. Now someone mentioned more heat from ignition retard. That is very true. EGT (exhaust gas temps) rise significantly as it pulls ignition. That is usually bad for the turbos and exhaust valves. But that's a totally different subject. The difference between 87 and 93 octane is tremendous with a turbo motor. I used to make significantly more power just between 91 and 93 octane fuels when I would tune cars from CA compared to cars from MA where I'm from. With the turbo import cars regular 87 octane was not even possibly used at all. The direct injection is what allows that because it atomizes the fuel much better allowing more ignition control. but that is also another subject completely.

I just had to say something here because the amount of bad info being tossed around here was amazing. Some of you were on the right track but a lot of you are just terribly misinformed. So in conclusion, premium fuel is a much better choice for your turbo motor for safety, power and just about every other aspect.