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so, here's your math lesson.
area of the oem 3" maf holder = pi x radius squared = 7 sq in
area of the new 3.25" maf holder = 8.3 sq in
difference = 18.6% (and you can check my math; can't confirm i was sober when i did it)
what does this mean?
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.
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).
ok, so 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.
increase the diameter of the tube by 18.6% and you have 18.6% more air getting in than the ecu thinks there is. as a result it adds less fuel than needed, o2 sensors figure this out and ltfts are increased to add more fuel. so, you should be able to put your durametric on it and see a significant increase in your fra values (perhaps an 18.6% increase ?!?!?).
big deal, as long as you are within the .7 to 1.32 margin then you won't get a cel and life is good. except ... 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.
the problem? porsche was too cheap to use wide band o2 sensors in our cars (they do in the turbos, and the 987.2 cars; in their defense, this sytem 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). now then, take a 12.5:1 afr and increase the amount of air by 18.6% and what afr do you get - 14.8:1? pretty lean for a hard working engine. your fuel economy will be great, and your car will feel like it is on steriods (remember, 14.7:1 makes most power) but not so good for the engine.
my advice? get your tune asap, although i expect once your car balances our in 50 kms or so and your short term trims get moved into ltfts that you will get a cel for your idle rkats being screwed and perhaps transition to limp mode (where ecu meters air based on throttle position instead of maf reading).
or i could be wrong; you get what you pay for on the internet, hey.
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