What is the difference between a blow-off valve and a compressor bypass valve? And do I need either of them on a 01' Stillen S/C w/3.33 pulley?

 

Blow off valves are normally open air blow offs. Which means when your let the gas go the excess charged air is blown off into the atmospehere.

Bypass Valves actually take the excess charged air and re-circulate that back into the intake pipe of the SC or Turbo. Most car manufacturers and turbo/sc kit makers do this to make it smog legal.

And you need ANY one of the two if you have any type of SC or Turbo. Im sure your Stillen came with the stock bosch bypass valve unless you bought it used or had it replaced.

Dixit

 

I'm not sure what the difference is in functionality. They both open when manifold vacuum is high (when the throttle is closed) and stay shut when manifold pressure develops. Same animal with different taxonomy?

(Edit: Dixit answered this.)

You can run without either (I have for a short time) if you block off the bov exit port on the crossover pipe. You will get some interesting noises, though, due to compressor surge when you close the throttle at high rpm.

Compressor surge is reported to be bad for compressors, but I have never heard exactly what damage is caused.

 

Here is a small snipit about why compressor surge is bad. Makes sense.

Dixit