AEM, Skunk 2, Tein, ApexI, GReddy, etc. All spend UnGodly Amounts of money on research and development. To Make the parts they sell Fit and Perform the Best on and off the racetrack, as well as Comply with local and federal laws (in some cases/states) The Cheap "Knock Off" Parts Are made in mass quantitys and are usually poorly designed/produced disgraces.

And in the World of After Market Parts .. YOU always ALWAYS get what you pay for.