90% of torque (slightly made up number, but it is high) comes from displacement. That is where the saying "There is no replacement for displacement" comes from, and to an extent it is true. The alternative source is to force more air into a smaller cylinder at a higher pressure. So the only real replacement for displacement is forced induction.
Power is a function of the torque at a given RPM. Torque is the force that makes the vehicle move. Grossly over-simplifying vehicles tend to have 1 of three torque curves. One, where most of the torque at low RPM, two, have a flat torque curve (where most of the torque is available in the middle RPMS), and three, where the torque is available at high rpms. A torque curve skewed to the low RPMs will tend to have a higher peak torque, the flat curve has about the same torque over a wide range (although a lower peak number), and the high RPM engines tend to have the least peak torque.
Trucks tend to fall into the first category, with large displacement engines and they are tuned (with cams and such) to produce most of their torque at low RPMs so they can pull a load effectively. Some of the lighter duty trucks have a flatter torque curve. Most BMWs and the like are known for their flat torque curve, that is what makes them so nice to drive; the engine doesn't feel peaky (like you have to rev the crap out of it to get any power), although the power number might be slightly less. Honda's tend to have peaky engines with all their torque at high RPMS and you have to rev them to 7000 rpm to make any power. These tend to be smaller engines with lighter parts. The reason they are fast is they can accelerate longer in each gear (8k rpm redline). Their instantaneous acceleration will be much slower than a big truck, it is just that the truck has to shift by 15 mph where the little civic can keep revving.
The vehicles that accelerate the fastest tend to have slightly more horsepower than torque, but it depends a lot on the vehicle weight, gearing, etc. Larger engines and vehicles will be closer to 1:1, whereas smaller vehicles and smaller engines might be more like 3:2 hp to torque. Too little torque and the vehicle can't move, and too much torque and the vehicle won't be able to rev very high. Variable valve timing helps reduce this, having the ability to have some torque at low RPMs and have power at high RPMs by effectively having multiple cam profiles.
This is why the Acura 1.8 from the GSR performs so much better than the M42 from the same time. The BMW has only standard cams and no variable valve timing, whereas the Acura has VTEC. It can keep its flat torque curve, but still have the upper end revs that the BMW is lacking. Both vehicles have about the same torque and will accelerate about the same under 6500 rpm, but the Acura keeps going for another 1000 rpms, boosting its peak hp to 170 (or about 30 more than the bimmer). If you don't want to run your engines at that high of RPM (I think the GSR rev limits at around 8000), buy a larger displacement engine.
Forced induction forces more fuel and air into the cylinders, upping your torque at all RPMS where the supercharger works (turbos don't kick in until the exhuast gas gets the turbine spinning fast enough). Forced induction has a very similar effect as switching to a bigger engine.
Again, all these descriptions are over simplified.
To answer your question, if you are increasing horsepower you are increasing either your torque, or changing where it is distributed. For example, a free flowing exhaust may shift your torque curve to a slightly higher RPM, so you will see a decrease in low rpm torque for a trade off in peak power at higher rpm.
Forced induction (or adding displacement) are really your only means to gain significant torque AND power. Otherwise you are essentially tuning your torque curve to produce more torque and less power or visa versa, e.g. changing to a racing cam might make your car undrivable on the street but give a huge boost on the track. |