We offer two types of fuel injectors:

The Siemens Deka injectors are a mass-production unit that is a more affordable option that has a flow variance of up to 5%. This is comparable to other mass-produced injector types on the market and is simply an artifact of mass-production. 

The Fuel Injector Development (FID) injectors are custom flow matched race injectors. They have a flow variance of 1% measured statically and 2% dynamically. These injectors are comparable to the popular units from Injector Dynamics but at a lower price point. 

Normally the higher output  the engine is, the more important it is that there is a minimal amount of flow variance injector to injector. A 5% variance on a moderately modified naturally aspirated engine while it could be a big problem on a high output boosted application, particularly if the engine's airflow also varies cylinder to cylinder and the lowest flowing injector is paired with the highest flowing cylinder. 

Sold as sets of 4 injectors. So 4 cyl listings are for 4 injectors.