I'm asking this for a friend with an LT1 Z28 If so, how is it done. I know you don't just turn the optispark

 

Computer controlled only, I think. You might be able to change intial timing by reinstalling camshaft in a different position to get more advance or retarded timing.

 

What Siverbeast says...

There is no timing adjustment available to the optispark. And changing the cam timing would be simply dangerious... unless you want to get an adjustable timing gear set. I think Cloyes makes one.

The real downside? If you didn't like the changes, returning it to stock, or, the labour of tuning, would be a real bitch.

If you really want to mess with the timing curve, it will have to be done in the chip.