Wire/coil/plugs - Pros: In theory, a better coil and thicker/better insulated wires will deliver more electricity to the plugs, resulting in a more complete burn of the fuel/air mixture in your engine which should result in lower emissions, better fuel economy, and better hp. In practice, while there may be a difference, I would have to estimate it as being neglible - but that never keeps me from doing it anyway.
CONS: price.
UDP - Pros: UDPs work in two ways, first by being substantially lighter than the pulley you're replacing, thus reducing the rotational mass of the pulley and therefore the effort required by the engine to rotate the pulley, and second by changing the size of the pulley, thus reducing the torque required to rotate the pulley by changing the length of the lever arm (distance from center of pulley to outside edge where torque is applied - you physics gurus can help me out/correct me on this). This all means that the engine doesn't have to do as much work and hp should be freed, and your idle speed will likely drop.
CONS: It's called an underdrive pulley because it results in under-driving everything attached to the accessory belt. This means that your alternator will produce less electricity, your A/C won't blow as cold, etc... However, this is typically only noticable at idle, and often isn't noticable at all. Other cons - price, effort required to install, and minimal hp gains (maybe 2-3 at best).
Personally, I run a UDP that claims to underdrive accessories by around 20%. I've never noticed flickering lights even at idle with 2-12" 400W subs blasting. However, I do notice that the A/C doesn't blow cold air while I idle. Also, my idle dropped about 200 RPM after install.
CONS: price.
UDP - Pros: UDPs work in two ways, first by being substantially lighter than the pulley you're replacing, thus reducing the rotational mass of the pulley and therefore the effort required by the engine to rotate the pulley, and second by changing the size of the pulley, thus reducing the torque required to rotate the pulley by changing the length of the lever arm (distance from center of pulley to outside edge where torque is applied - you physics gurus can help me out/correct me on this). This all means that the engine doesn't have to do as much work and hp should be freed, and your idle speed will likely drop.
CONS: It's called an underdrive pulley because it results in under-driving everything attached to the accessory belt. This means that your alternator will produce less electricity, your A/C won't blow as cold, etc... However, this is typically only noticable at idle, and often isn't noticable at all. Other cons - price, effort required to install, and minimal hp gains (maybe 2-3 at best).
Personally, I run a UDP that claims to underdrive accessories by around 20%. I've never noticed flickering lights even at idle with 2-12" 400W subs blasting. However, I do notice that the A/C doesn't blow cold air while I idle. Also, my idle dropped about 200 RPM after install.
