Non OEM primaries lose some benefits. Read attachment.

 

Good read and true. When I do something like a stepup kit with an aftermarket secondary rail and injectors and keep the stock primaries, I like to hook up the vacuum hose to the air bleed port to retain the function. It used to be the case when going with aftermarket primaries, it was difficult to get a good idle afr and drivability was affected to some degree. However with moder injectors like Injector Dynamics, its not the case so much.,