can anyone explain what these mods do to the car?
Simple explanations - others here can get more detailed on request.
(1) Underdrive pulleys
Raise the ratio rate of your motor to your alternator. This helps to alleviate
the drag caused by the AC, Radio, and whatever else you have hooked up to the
electrical system
(2) B&B Header System (race kit)
A "race-kit" system is a very low resistance exhaust header setup (the exhaust
tubes coming out of the sides of the motor.) Chances are you have a basic
upgrade which allows for better flowing air and less backbuild pressure.
(3) Hi-Flow cats (metal substrate)
Hi-flo cats complement the headers - it allows even more airflow out of the
system and helps cool the side sills by producing less heat.
(4) B&B catback system
The cat back system is simply the exhaust from the catalytic converters to the
rear of the car. Again more airflow = better performance and this also helps
change the exhaust note. Also looks better then the stock tailpipes.
(5) Tune PCM & Dyno
Tune PCM sounds like someone altered the stock computer (PCM) to gain some
engine management advantages. This usually isn't done unless either the car has
some more significant modifications (like roller rockers) or the PO was just
trying to "chip" the car some added HP. An improperly modified PCM can be
disastrous in the long run so that would be my single only worry.
The dyno is simply were they put the car on a machine to see how much
horsepower/torque it produces.
Sounds to me like you either got a once heavily modified car where the PO took
all the mods not permanently bolted into the car with him, or you got a car
where the PO didn't really understand the dynamics of building viper HP. Some
of these mods are not the typically route a performance viper is taken.
We're having our fall cruise the weekend after next - stop in and say hi and
the gang will go over the car for you and see what we can see.