The NST Underdrive Crank Pulley is a 10% underdrive (slow down) pulley, and as you can see in the pic here, it is very close to the OEM pulley in size. This pulley is designed to slow the accessories by roughly 10%... not enough to "kill" any of the accessories, but just enough to free up a nice amount of hp and tq to send to the wheels. The excellent weight reduction also aids in providing better throttle response, quick acceleration, and better fuel economy. We did a bit of testing and decided that 10% is the sweet spot for this application. 20% or 30% underdrive may provide an extra 1/2 or 1 hp or tq at the wheels... but it may do so at the expense of some of the other accessories' functions, so we decided to stick to 10% for our pulley.
The NST Overdrive Alternator Pulley is a 10% overdrive (speed up) pulley, and as you can see in this pic, it is also very similar in size, to the OEM pulley. This pulley is designed to bring the alternator back to OEM speeds. No slower, no faster. There will be NO hp or tq loss when using this pulley. The crank pulley is the SENDER pulley and it is the pulley that decides how much power the accessories will receive. The size of each accessory pulley determines how this power is used. The alternator will not be overworked, underworked, robbed, killed, etc. when the NST pulley is used. It will work in the same efficiency range as it would with the OEM Nissan pulley.
NST Pulleys are NOT a "one size fits all" type of design. We take the needs of each application into consideration, we come up with a few prototypes/configurations, and ultimately we sell the KIT that works best based on the data we collect.
Both pulleys are compatible with the OEM belt and the OEM belt has shown to work very well during the past several months of testing. As long as you are using a new belt, or your current belt is in good condition, you should be good to go!
There are other options out there, and some of them are excellent options. We feel that NST offers the best all around kit, with the best all around options, at the best possible prices. Our customers feedback and reviews have consistently proved these to be the truth.
Thanks again everyone, good luck and have fun with all your projects!