When someone mentions tuning the car for low end torque, do they mean changing something mechanical (gear ratios/final drive) or something that has to do with the ECU ? if so what is done exactly to tune for low, high or mid end torque? I don't think changing the fuel ratio or something similar gives such effect, so what exactly can be done to achieve that?
Can two identical engines with identical power trains out put different torque and power figures based on tuning alone?
If the powertrain is identical, there's not much the ECU can do.
The word tuning is thrown around a lot and means different things to different people.
ECU tuning is one thing, and basically you can set the ECU to run the engine more aggressively (for power and torque) or more conservatively (for fuel economy). This usually involves ignition timing and fuel injector pulses.
However, two engines with the same engine displacement, same number of cylinders and same number of valves can have different power outputs and different torques based on how they are designed. Changing this design involves major cost and surgery, but engines are already tuned from the factory based on the design decisions that were made. Basically:
To change bore and stroke, you need machining, new piston heads, maybe new piston rods too. To change piston rods, you will change the compression ratio and may need new piston heads. Intake runners can be changed, sometimes there's the entire intake plenum of a similar engine that you can bolt on, but usually, a new part needs to be machined. Exhaust piping is a more common bolt modification because it if physically simpler. An exhaust header would do this.
Depending what you do, it will shift the power band and torque band to higher or lower RPMs.
The only thing you can tune while keeping an engine mechanically 'identical' as you word it, is playing with ignition timing and AFR. If you have variable valve timing and you know what you're doing, you can gain some power or efficiency by adjusting that. For these things you still need a custom ECU or some other hardware though, but requires the least physical changes in your car.
Keep in mind that you always make sacrifices. You don't get power or efficiency for free. More power often means shorter lifetime and more fuel consumption. More efficiency often means less power and less driveability. And for almost everyone there's the MOT that you need to pass, so the room to play is limited.
normally it would just be changing gear ratios/final drive just like you said, you could also do different tires/tire sizes as well, the person could go as far as changing the transmission if they really wanted.
so yes, typically it would be something mechanical. (gear ratios I would say)
