I have a 91Z 5.7 and under the hood it say's the plug gap should be 0.35, really my plug's were gapped at 0.45. Would that hurt performance much, and could it cause spark knock??

 

plugs can cause knock but not normally because of the gaps.
IMO tighter gaps work much better for performance.

 

If your running the stock setup(ie wires, coil, etc) the "stock" gap will probably be your best bet. If your running anything aftermarket for the ignition, then you'll want to go with a larger gap..

 

depends on the plug and what company its from. Range from .35 to .45 is good. It would only cause knock and ping if the plugs were not right for the car, i.e. you got the wrong plugs. What kinda plugs did you get and what model?

 

They are AC delco, but I will have to get back to you on the part #.