|
yeah, the resistor is the first step in this. resistors in the maf are used in two ways; in a fi application they are a scaler that pulls the maf output away from the upper limit of its operating range and into the middle. of course, the computer has to be told this via programming. the other way tuners use the resistor is to lean the mixture - the resistor reduces the voltage signal sent to the computer, computer thinks there is less air than there actually is and delivers less fuel. at low load operation this is noted by the o2 sensors which trim the maps to bring things back to 14:1. at high load operation there are no checks, so the engine will run leaner than designed. if the resistor is sized properly then you see high load operation at 14:1, get more power, use less fuel. all good, except that your engine is now running hotter, the computer starts to pull timing to try and fix it and, if it can't compensate adequately enough, things break.
when you buy a standard 'off the shelf' tune this is all that is done as well; the tuner just modifies the maps to run closer to stoichiometric during high load operation. more power, less fuel. in this case the tuner also has the ability to fiddle with timing to try and bootstrap the issues created by running lean during high load. the other thing a tuner can do is increase the rpm limit. if power increases with rpm then increasing the rpm limit gets more power. porsche set the rpm limits to protect engine internals, however, and increasing these limits has the opportunity to break things in new and innovative ways.
going back to the resistor, the best way to do it is to use a variable resistor (potentiometer) and set the resistance value based on feedback from a wide band o2 sensor so that you are getting the afr you want. the problem is, a good resistance for one situation may not be the best for another, so you should be constantly varying the resistance to achieve optimal results. this is what a piggyback computer does - you program the afrs you want into it, and it varies resistance accordingly.
a final point on high and low load. I've been associating high load with high rpm, but you can get into a high load situation at low rpm. the computer also monitors throttle position (TPS) so if you are in a situation with the throttle wide open (WOT) but rpms remain low (i.e., going up a hill, pulling a trailer) you are also in a high load situation. often you will see WOT used instead of high load to talk about the various maps and tuning. note that if you put a resistor on your maf so that the computer thinks less air is getting in than really is, the computer will look at the tps, see that it is WOT but think that there is not much air moving, and default to open loop operation sooner that it would have otherwise.
anyway, a bit of a hijack, but wanted to get you the whole picture on tuning so that this sc install is a success.
|