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Old 03-13-2018, 03:40 PM   #32
The Radium King
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Join Date: Apr 2010
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fra and rkat are both long term fuel trim indicators. fra is for closed loop trim, rkat is for idle trim. there are no trims for open loop operation. these are not short-term fuel trims. i explain trims in the thread referenced. i've attached an edited version below:


your engine has three modes of operation - idle, closed loop, open loop.

idle long term fuel trim is the rkat value measured by your durametric and can vary by +/- 4%.

closed loop long term fuel trim is the fra value measured by your durametric. 1 is ideal, but you can swing between .7 and 1.32 before you get a cel. what is a ltft? closed loop, or low load as it is often called, is part throttle, medium rpm operation. the car isn't working too hard. you are cruising along. maf measures air, looks up the appropriate amount of fuel to add in a chart, o2 checks products of combustion in the exhaust to make sure everything is burning properly and, if not, either adds or takes away ('trims') fuel (these trims are initially short-term fuel trims - stft - which become ltfts if they stay consistent over a period of time). this check by the o2 sensor is what makes it a 'closed loop' operation.

in closed loop, the amount of fuel added is done to maintain an air fuel ratio (afr) of 14.7 parts air to 1 part fuel (14.7:1 - stoichiometric). according to 'science' this is the most efficient, powerful ratio that creates the best combustion. as a result, the narrow band o2 sensors in your exhaust are designed to operate just around 14.7:1 (as opposed to a wide band o2 sensor like the one your tuner uses which has a much broader operating range).

fyi, the ecu knows how much air is getting in because of the maf - the maf measures the volume of air hitting it. the ecu knows the size of the intake tube, so multiplies the amount of air hitting the maf by the area of the tube to calculate the total volume air being ingested.

now, open loop operation. this is also called high load, or wot (wide open throttle). this is when the engine is hot, under load, banging against the rev limiter. the ecu switches to open loop based on a number of considerations; rpm (high) temp (hot) throttle position (ie, throttle is wide open but rpms are down - you are lugging the engine or going up a hill or something and ecu will go to open loop).

what is open loop? this is when the ecu moves to an afr of 12 to 12.5:1. it adds more fuel. the idea is that the extra fuel helps keep things cool, and prevents a hot, hard working engine from detonating. note that porsche was too cheap to use wide band o2 sensors in our cars (they do in the 996 turbos and the 987 cars; in their defense, this system is designed to address aging components and not yahoos doing crazy things with their intakes) so there is no feedback on whether you are hitting the target afr or not - the ecu just calculates how much fuel to add based on maf reading (no trims). this lack of checks is what makes it 'open loop' operation.

Last edited by The Radium King; 03-13-2018 at 03:48 PM.
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