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Message: <blockquote data-quote="wc9922" data-source="post: 322224" data-attributes="member: 6000">How ECU adapt to fuel quality ?By checking the AFR target values for each rpm bin that is under closed loop control and adjusting the fuel trim constant for the effective fuel map area so it scales up or down the fuel trim, like a fudge factor. Open loop ecus cannot do this.The VE will be the same for the same (rpm,load) bin but the computed required fuel per injection cycle (ms)  will be less. The injector pulse width (ms) will be smaller. Don't mistake this for duty cycle.ECU can adjust your timing map according to you fuel by detect knock value recorded by knock sensor.ECU will retard timing when knock sensor gives out knock signal. But under normal operations, it will try and advance the ignition up and reduce the fuel trim for the map cells that fall under close loop operation. The limit will be when knock is detected. Usually it backs off by a safe margin of 5 degrees or so. but depens on the tuner. A BMW will not be the same as a Subaru. Secondly, the ECU will also back off the timing when the intake air temperature and/or coolant tempreature hits certain limit. This high temps indicate to the ECU the engine is running under severe operating conditions. One example, The M series engine ECU's will not allow the engine to rev above a certain rpm before the engine warms up to preserve the engine. The rev limiter is set by retarding the timing to very little advance so u cant rev past it. That's how rev limiters are set in ECU's, cut out the timing, too risky to cut out the fuel injectors.   So the ecu can adjust timing not just based on knock sensor only. Most ECU (or DME) recheck/re-read the data everytime you start the car.What specific data are you referring to? [Octane rating has nothing to do with fuel quality. As you said, Octane rating is merely a value for knock resistance. So you argree with me? Fuel quality encompass more means the mixture %'s , the energy value (heat energy released during combustion)  of the mix is important too.Higher octance rating doesn't mean better fuel economy too.So you saying it's only a measure of the anti-knock characteristics? Partly true. If I have acess to a fuel that has a higher knock trash hold, won't I as a tuner take advantage of this positive attributes and bump up the timing  and reduce the fuelling VE values of the ignition/fuel map in the cruise load areas so the car produces  great lean torque on light throttle positions and be able to run AFR's between 15 ~16.5 without fearing the exhaust valves getting burned or the piston melting. That's what every tuner is trying to get, great power and great economy which is a possible combination by tuning. E85 has as high as Octane Rating (R+M)/2 of 105, but you get 30% less mileage compare to regular gas.Is E85 refering to Ethanol ? Ifyou are referring to Ethanol, it's  a biofuel, almost a pure component fuel compared to gasoline which is a mixture of up to 13-15 organic compounds. By the way, gasoline has a higher heating energy value than ethanol per mol.So during combustion, you need more ethanol by mass to produce the same amount of energy or heat compared to gasoline. This is a fact. That is why the fuel economy sucks on ethanol grade fuel.Thanks for refreshing my thermodynamics.</blockquote>

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