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Because an engine needs enough fuel to match the peak amount of air it ingests and burns.Over estimating is fine but I still don't understand why you just select the higher out of torque and HP, the calculator is programmed to use HP so if you use torque it will give a different result?
The calculator assumes engines make more peak HP than torque, most engines do and peak numbers are above 5252 rpm. The stock F2T makes lots more torque than HP, if you fuel for 145hp with an engine that makes 190ft/lb torque it will lean out and the air fuel charge will pre-ignite.
For the last time, when you burn air and fuel in a combustion engine it generates torque at the wheels or crank, when you multiply torque by RPM and divide it by 5252 you get HP.
145hp @ 5000 rpm requires 152ft/lbf Torque at the wheels.
145hp @ 6000 rpm requires 126ft/lbf Torque at the wheels.
145hp @ 4000 rpm requires 190ft/lbf Torque at the wheels.
The underlined numbers represent the amount of air and how efficiently the engine is burning it. It will require different amounts of fuel for each of those three torque numbers, the calculator is designed to give you a rough idea of the minimum size injectors required.
The calculator works with HP for the Honda civic, If you size your injectors to the blue line on the chart (Peak HP) nothing goes above the that line on the dyno and the injectors are big enough to supply peak fuel requirements.
If you do the same thing with the Mazda F2T motor and size the injectors to peak HP:
Where does the engine get fuel from to burn all the air above the blue line? The engine will be lean to 5100rpm.
The engine doesn't know or care what the injector calculator says or how you filled it out, it ingest air and burns it and generates torque at the wheels, it requires fuel injectors that can deliver enough fuel to match the peak amount of air it ingests.
F2T Peak fuel requirement (blue line):
If you where going to use this F2T dyno to program a stand alone fuel injector map for your car would you write a fuel map that follow the HP band or the torque band?
The answer is your fuel map should follow the torque band bellow 5252rpm and the HP band above 5252rpm for all combustion engines.
Again most cars make more HP than torque and for those cars the calculator works.
If you do that to over estimate and be on the safe side then that's fair enough but it seems a bit arbitrary because I could choose to use pound-feet or Nm and they're both torque but will give different results.
If the injector calculator uses HP, (Torque (foot pound force) X RPM / 5252 = HP) you can't use Nm. If the calculator used kW than you couldn't use FT/LB. (Nm X RPM / 9.5488 = kW).
You need to convert Nm to ft/lb. One foot-pound = 1.356 Fig - newtons
FT/LB = Nm / 1.356
Nm = FT/LB x 1.356