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What to Look for in an SC14
Im no expert, I only looked at a few before choosing one and buying it. But I picked up a thing or two along the way:
For the Nissan Silvia, you need the SC14 supercharger, and NOT THE SC12!!! The SC12 will only create boost if it is spinning in the right direction—the Wrong direction for the CA18. The SC14 will go either way, just the outlet will be your inlet.
*** Correction: I was corrected by the guys on The SC12 WILL go either way and make boost - the problem with it is that it was built for the 1.6L MR2 engine, and wont make enough boost for a huge difference on the SR20, and the CA18 could easily handle more than this guy could make. Maybe a good option for the CA16 cars however, or pulsars looking for some bolt-in boost. The SC12 is smaller, so easier to fit in a small car like that.

SC14 showing pulley and clutch wiring
Those two pipes are the inlet and the outlet. Which is which depends on the way your engine spins
A Simple Test
When looking at them, place your palm flat over one of the pipes to block it off, and turn the pulley by hand. A) The pulley should move easily by hand (it will feel ‘lumpy’ to turn, but it shouldn't be hard to turn) and B) You will feel either suction or force on your hand, depending on which way you turned the pulley. Now, turn it the other way—If you felt suction the first time, now you should feel force, and vise versa.

Oil Lubricated
If it doesn't turn by hand, or feels rough, try checking that it has oil in it, they are oil lubricated with a special supercharger oil - 90 grade synthetic oil is almost identical, or normal oils can be used (the SC14 is quite robust) but I wont be held responsible for damage, not that it should occur, but just in case.

Other Points
Other points are that I don't think the SC12 has the magnetic clutch that makes the SC14 so convenient, and the SC12 (I believe) came from the MR2, so from those that came supercharged it would be a bit harder to locate an SC12 anyhow.
The best part of the magnetic clutch is that it makes negligible drag on the engine when not engaged, so the only time you need to spend some power to make more power, is when you are already making that power anyway!