RyanR
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You're correct, but what I was talking about isn't about air/fuel ratio. It's about the intake air temperature (IAT) being cold enough that pre-ignition (detonation) is less likely. Think of it sort of like the Water/Meth injection effect, which is mostly used to cool intake air to reduce detonation under high loads/boost (compression). Your ECU and tune should keep your actual AFR almost exactly the same regardless of IAT, or season/temp. It does this by correcting fueling based on tables contained within the ecu mapping when under boost, and actual observed AFR under vacuum. I'm oversimplifying this for the sake of time, but if you want to learn more, you can do some reading about open/closed loop mode tuning for Subaru ECUs.
Higher IATs means that there is higher detonation potential, and lower IATs result in lower detonation potential. The level of compression, AFR, heat evacuation, and other factors all play into if detonation will occur. We run high octane fuel in turbo cars because under boost with higher (spring/summer/fall) IATs, there is more potential for detonation. Because ambient temps are much lower in the winter, detonation is less likely, and depending on the tune and IAT temps, less than 91 octane could be run. It is not recommended to run a lower-than-specified octane fuel. All I was saying is that I have seen customers put lower octane gas in turbo cars in wintertime with no ill effects. That DOES NOT mean that I am recommending it, or that everyone will have that same experience and/or luck. In fact, it is known that some winter blend gasoline is more prone to detonation than summer blend, even despite the colder intake temperatures.
Also, I should note that stock tunes knock just cruising around town on proper fuel. They're designed to run lean, especially when cruising, to keep MPGs up as much as possible. A little knock here and there is not necessarily going to destroy a motor, and happens to EVERY car out there, but the parts within the motor need to be able to sustain the shock.
Hopefully that was more helpful than confusing.
Higher IATs means that there is higher detonation potential, and lower IATs result in lower detonation potential. The level of compression, AFR, heat evacuation, and other factors all play into if detonation will occur. We run high octane fuel in turbo cars because under boost with higher (spring/summer/fall) IATs, there is more potential for detonation. Because ambient temps are much lower in the winter, detonation is less likely, and depending on the tune and IAT temps, less than 91 octane could be run. It is not recommended to run a lower-than-specified octane fuel. All I was saying is that I have seen customers put lower octane gas in turbo cars in wintertime with no ill effects. That DOES NOT mean that I am recommending it, or that everyone will have that same experience and/or luck. In fact, it is known that some winter blend gasoline is more prone to detonation than summer blend, even despite the colder intake temperatures.
Also, I should note that stock tunes knock just cruising around town on proper fuel. They're designed to run lean, especially when cruising, to keep MPGs up as much as possible. A little knock here and there is not necessarily going to destroy a motor, and happens to EVERY car out there, but the parts within the motor need to be able to sustain the shock.
Hopefully that was more helpful than confusing.
