I just reread that part. This engine should not be operated on anything lower than 87 R/M octane fuel (R/M is the octane calculation for cars in the U.S.).
If you have access to 85 oct then you're probably at altitude (the username also gives it away
). Naturally aspirated engines made to run on 87oct can get away with it because the air at high altitudes is less than, which effectively lowers the compression ratio which lowers the octane requirement necessary to prevent knock. This is why NA engines are so much weaker in Denver vs say Bonneville, they're making less power.
Turbo engines are not the same, in particular ours. Turbos naturally equalize power and maintain the same engine output as altitude increases (which made them very favorable for piston aircraft). On our cars the ECM is "torque based" meaning it sees the gas pedal as a "torque request" with pedal position and RPM = a certain desired torque, regardless of air density, temperature, etc. To maintain that torque output across huge ranges of altitudes and ambient conditions the ECM will modify engine parameters. With boost pressure being its biggest tool.
For example let's say you're in Texas @ 100ft above sea level and It's 15F outside, it might only take 8psi of boost pressure to make It's max torque (149 ft lbs).
Now you drive to the top of the Rocky Mountains and It's 1000ft above sea level and 100F, now It's taking 16 psi of boost pressure to reach max torque.
The ECM is monitoring and making these changes for every Gas pedal position, RPM, and a host of other factors.
Now the reality of it is that unlike an NA engine that loses power and experiences lower dynamic compression ratios (running comp ratio) as altitude or temperature increases our car maintains power and dynamic compression ratio regardless of temperate, because extra boost pressure is making up the difference.
This means it needs the same octane fuel, otherwise it will run into knock and timing will constantly have to be pulled back and the fuel mixture run rich to combat it. The result is lower performance, worse fuel economy, and potential engine damage (although the ECM is rather quick at combating knock and keeping it extremely minor. None the less fuel economy and power will suffer).
In short run atleast 87 octane fuel in this engine.
