You’re building a high performance car and you’re thinking about installing nitrous injection to give it a higher top end and better acceleration. Nitrous is nitrous oxide, a chemical that is used to increase the amount of oxygen available to be burned in the engine, by cooling the incoming air, which greatly increases engine horsepower output. Determining what size shot to use depends upon how much extra power you want and what you’ve done to modify your engine to strengthen it for nitrous. Here is a calculator to help you with some of the more technical issues regarding jet size determination.
For more info, see How Nitrous Oxide Increases a Vehicle’s Performance