Pc3r has adjustable timing built in,but operates off of serial connection (slower processor) and can only be tuned in 500 rpm increments.
PC3usb-Much better unit-operates off of usb connection(faster processor),can be tuned in 250rpm increments,has direct injector controll(plugs directly to the injector harness not the ecu)and also has a expansion port where you can plug in all kinds of toys like a LCD screen,quick shifter,timing module,wide band,or a data logger.From a dyno operators standpoint,the usb is the way to go!!
