Hey so this might be a bit technical, and although I'm a newbie at engine maintenance, power and torque are something I understand pretty well. As your RPMs increase in your engine, your torque and horsepower increase almost linearly. Eventually, though, they start falling off because of friction and other opposing forces that occur at high RPM. The point is that there's a peak. Typically the torque curve falls off faster than the horsepower curve (see picture). Engineers will design transmissions so that the engine operates somewhere within this optimum power band.
Although horsepower gives you the "kick" into the back of your seat, torque is what you're actually driving. Torque is your engine's ability to rotate your wheels. In order to maintain maximum gas mileage, you always want to operate your engine at peak torque. That's why diesel engines get so many more MPGs that gasoline engines - they have WAY more torque! Horsepower is your engines ability to create kinetic or "movement" energy. Although your horsepower might be peaking, your engine isn't operating at it's optimum efficiency. Medic, that's why your vacuum will peak too. The intake vacuum is a function of the efficiency of the engine.
If you're a race car driver, you want to operate your engine so the RPMs are at peak horsepower. If you're a penny pincher, you want to operate you engine at peak torque. You should be able to find a torqe-horsepower curve for your model and engine. These curves will change as your engine ages. You could go to your nearest university and get some nice engineer (like me) to run a test for you if you want an accurate curve. Simply operate your engine at the RPMs as close to possible to your peak torque. That's why they have those fancy double clutch engines and expansion gears so the engine spends more time at optimum RPM.
If you want to know EXACTLY what speeds to shift your transmission to maximize gas mileage (maximize torque), there could be a way to optimize the RPM at which you change gears by integrating the torque over the time spent there at a certain speed. I know, I just talked about calculus on a car forum. If you want me to run the optimization for you, let me know. All I'll need is your torque vs. RPM curve.
Friday, June 22nd, 2012 AT 4:49 AM