Here is Nitrous Express's 75-page instruction manual and wiring harness.
Do Consider Completeness
Some companies are in the habit of not including things such as an instruction manual, wiring harness, and relay, as well as some hardware, such as bolts for the bottle bracket, and so on in the interest of cutting the overall cost to make the kit cheaper. However, as a consumer, you don't want to be spending more money out-of-pocket. Some also provide vague instructions, which ultimately cause the user to go to a shop for the install instead of doing it his or herself. Nitrous Express provides a 75-page in-depth instruction manual, as do we. Ultimately, you want to make sure that you have everything you need to do the install.
Do Consider How Much Power You Want To Make
The type of system (wet, dry, direct-port) comes down to how much you want to spray. For your average person who doesn't need more than 100-125 hp, a dry kit is simple and easy. Some sort of nozzle goes into the airbox or air stream before the MAF so that the density is picked up and fuel is added (by the injectors). A single-nozzle wet kit is good in the 35-200hp ranges, and also mixes fuel with the nitrous as it sprays (hence the name "wet"). Meanwhile, for 250-plus, you want to go with a direct-port setup, which I recommend using a dedicated fuel system for. While you may need to upgrade your fuel system and motor, achieving a larger shot of nitrous should be as simple as increasing the jetting. However, some kits only come with .063 solenoids, which max out at 200 hp. NX, on the other hand, uses solenoids with a flow rating of .093 that will support 300 hp. This is something you want to verify if you plan to upgrade later. The bottle valve may be another limiting factor because orifice size will affect how much it will support just like solenoids. You will find that some brands even promote the use of larger valves in the sales pitch of its products.
Do ask around about the company's reputation for customer service
Trust me, this counts because, if it comes time to take care of that warranty claim, you want them to be there. With the Internet today, companies cannot afford not to have good customer service, as it won't take long before everyone knows about it. And as a consumer, it's really important that you get tech help so you can complete the install and do it correctly.
Do Make Quality The Most Important Factor When Purchasing A System
Take a close look at the materials the components are made from. The quality and material of the fittings is important. Most companies use fittings from Aeroquip, which are made of stainless steel. In comparison, brass isn't as sexy, and it is neither as hard nor as resistant to rust as stainless. Some companies use nickel-plated brass, which doesn't corrode, and still saves money over stainless. The solenoids should use virgin Teflon on the piston and Viton for the fuel with a stainless housing. There are other things that will add value to the kit, but may also add to the price. For example, NX uses a bulkier hand wheel (on the bottle) to give extra support. Some bottles have a port on the side of the valve for a nitrous pressure gauge, and this will allow you to read the pressure without opening the bottle. The bottle valve also needs to be heavy-duty because it has been known to bend.