a few definitions:

Torque is the amount of twisting force that an engine can produce while horspower (power force "over" time) is the amount of force that an engine can produce at a certain RPM Hp = torque x (RPM/5252)

That said...torque, in a lot of ways, is related to the actual force that each piston pushes down on the crankshaft..This twisting force (torque) then is directly related to the size of the "explosions" that each cylinder makes...the bigger the piston and the more pistons there are, the more force you have twisting the crankshaft

Now...as the RPM increases..you also need to be able to suck and push enough air and fuel in to supply the demands of the big pistons at speed, so HP can be thought of as being related to the amount of flow in and out (big webers, big cams, big ports in the head, big exhaust)

Now..the smaller size intakes and exhausts will be more helpful for torque at low rpm than high rpm because the pulses of exhaust flow out of the combustion chamber will pull the mixture IN from the intake (think of the exhaust as a series of high and low pressure areas, waves, as each cylinder fires...high pressure areas are attracted to low pressure areas..so the closer these pulses are (as they would be stacked up in a small exhaust pipe)...the more the attraction between high/low areas exists..and thus more scavenging at low RPM.) whew.

try:

www.howstuffworks.com

Per
http://www.grassrootsmotorsports.com