ok, i think that last link did a much better job of putting it inunderstandable terms. now i figure fasteraccelaration is a result of the torque curve throughout the whole rpm range, but low end torque is more funfordaily driving.

 

Something to think about: a majority of classic hot rod modifications make power by simply moving the torque peak up the RPM band. Say you have an engine that makes its peak torque of 400 ft lbs at 4000 RPM and its peak HP of 400 HP at 5500 RPM (which a little calculation can show you means the engine was putting out 383 ft lbs at 5500 RPM).

Installing a high-duration cam will basically move the entire torque curve higher: let's say it moves it upby 10%, or 550 RPM: that 382 ft lbs is now operating at 6050 RPM which means the engine is producing 382*6000/5250=440 HP, a gain of 10% over stock. The engine is more powerful. It will do the 1/4 mile in less time.
But think about this, at every RPM below the new torque peak the engine will have less torque than before. The cam basically stretched the torque curve out in the RPM band by 10%: what torque the engine had at 4050 (10% below 4500 RPM) it now has at 4500, what torque it had at 1820, it now has at 2000. And it had less torque at 4050 than at 4500 and at 1820 than 2000 originally (remember, below the torque peak torque is climbing with RPM). So generally it has about a 10% drop throughout the band of RPM from idle to 4500 RPM. It's not as good a daily driver, and because torque at idle dropped, it may not even have sufficient power at idle to idle smoothly -- hence the poor idle.
Now, this is a bit of a simplified explanation -- looking at only the primary factors at work and neglecting a host of secondary factors that work on top of this in both good and bad ways;but this is the primary effect of mods such as:
- hotter cams
- higher lift rockers
- bigger valves and ported (wider) intake ports
- larger intake manifolds (say a FAST 90 mm on a stock engine)
They add HP at the top but most likely take away torque at the bottom of the RPM scale, and are the reasons "hot rodded" engine have their well deserved reputation for losing low end torque and driveability.
Bt contrast, mods such as:
- cold air induction
- headers
- added displacement via boring and stroking
Do not have this effect, they increase efficiency or capability across the entire RPM band, usually, so they add torque at all RPMs (and hence, HP at all RPM bands, too). So they are preferred, at least by me As to turbos and superchargers, their effect is too complicated to generalize on in this manner: depends on the type of SCr.

 

One of the biggest mistake the import crowd makes is the installation of a huge diameter exhaust system. This shifts the torque peak to a higher rpm and since Hondas already have weak low-end torque, the fun-factor is further reduced. The other mistake many of them (and some Corvette owners) makeis swapping out their 14-inch wheels for 18-inchers. The wheel weight increases by 2 times and the ride and acceleration suffers, big time. Some Corvette guys install 20-inchers, not checking to see how heavy they are.