First question... if it is a 5.0 set, up on your engine, do you have the correct water pump rotation?

When illiminating all the other accessories with a 5.0 wide belt set-up, if I remember correctly, the water pump rotates in a different direction then the normal 5.0 set-up, because the belt is routed around the pump differently then a standard 5.0.

If it is spining backwards, that can be your issue because the pump must work a lot harder to spin as the rotor in the pump is designed to spin in only it's normal direction. The harder force, means the belt may slip under the heavy load.

Easy way to tell, are you using the 5.0 style fan, or the one that goes with that year engine? Then which pump are you using, the one for your engine or the one for a 5.0?

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66 Stang 385+ HP 306 .494 /.520 225 durr @.05, 200cc Windsor Sr's, Edle RPM, C4, 3.00 posi, 575 Annular Mighty Demon, 22 MPG,