Water pump flywheel?
 

Can anyone tell me why I need this? Seems like just added weight what is it doing for me?

Sounds like the Frisbee. You can remove it.

http://forums.corvetteforum.com/c4-t...e-removal.html

Indeed it's the "frisbee" that was installed to dampen out any vibration when the new (in '88) A/C compressor kicked on/off.

Toss it and use lock washers under the bolt heads; should be worth ~10 chp gain.

I took it off. My first trip to work without it I noticed the engine will buck at low rpm right before the downshift from 4th to 3rd. Engine was not fully warmed up yet never had this before? Didn't seem to do it after engine was up to temp.

Doesn't seem likely that the frisbee prevented bucking during a downshift but I suppose there's small possibility it was helping to mask a problem you might have with injectors or tuning/tune-up. Or...it was total coincidence. If it continues, simply reinstall and report if the problem disappears.

FWIW: Extra weight you add to a motor will create an interia that helps keep the motor running. That's why a heavier FW is easier to drive with a stick than a lightweight alum one. Considering they put that weight on the WPump vs the A/C, I'm more inclined to think that extra inertia keeps the engine RUNNING smooth when the A/C kicks in.

Any chance your A/C was running and the compressor happened to be kicking on JUST as you were downshifting?

Could have been a fluke as I didn't notice it toady. Ac was not running at the time. I will keep testing.