I was looking over the specs for a 98 Mazda 626 2L and noticed that it says the max power is at 5500 RPM.
Will the Max Power RPM also be the RPM at which the engine is most efficient assuming you're at a steady cruise?
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Power is a function of torque and RPM. Maximum torque is achieved because the engine is able to move the maximum amount of air and fuel in and out of the engine. The power continues to climb even when the torque starts falling because the RPM is climbing faster than the torque is falling. Eventually the torque begins to fall so fast because at the high speed the engine struggle to move the fuel and air in and out. Finally to torque drops of so drastically that the power also begins to fall.
No, the engine speed at which maximum fuel efficiency is obtained is different.
In fact, the faster you go in any given gear the worse the resultant fuel economy will be because it takes increasingly more power to overcome the vehicle's drag forces.
It worsens because the power required to overcome drag forces on a vehicle at a constant speed is proportional to the cube of the speed, so to drive at twice the speed, you need 8 times the power.
However, this doesn't mean that fuel economy is best at the lowest RPM, because the engine's fuel efficiency at that point is low (resulting in low torque).
Instead, there will be a sweet spot where the efficiency is best closer to the lower end of the RPM scale which represents a best compromise between the torque generated and vehicle speed (drag forces).
No.. Max torque is not the max force you will have available to accelerate your vehicle at any vehicle speed. that point happens at max HP. Efficiency means how much you got out for how much you put in.. For most high performance engines, that points to the high HP levels, again, not max torque. At the point of max HP you are burning the most amount of air and fuel possible, the most potential energy consumption is at that point and the greatest rate of doing work (HP) happens at peak HP, not at max torque.