As far as I'm aware, there is three different types of HID kits:

Type 1: Single arc, like a H1 bulb, no good for the standard sierra lights, they have H4's which are twin filament. You can get them in H4 though.

Type 2: HID Bi-xenon, these have their low beam as HID and a normal bulb for high beam. These are the most common type of H4 HID lights

Type 3: HID Hi/Low, These use magnets to move the angle of the bulb, so you get high and low beams. Alot dearer than type 2 HID kits

Aswell as the different type of kits, HID's come in differents colours! ranging a yellow to a voilet. the best colour being the blue.
HID are measured in 'k's' the blue range is 6000k to 8000k approx.

I'm not an expert on these lights, this is what I have learn because I have just brought a set of HID Hi/Low's off eBay for £169

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