KevinK2

twokrx7: the AST normally has the pressure control cap, with the visible spring. The filler neck gets the springless cap, which would function as a 'zero psi cap'.

Zero pressure is less likely to cause leaks, and if a marginal connection exists it will be less likely to leak.

There still will be pressure exit the pump, and the cap establishes the zero point. With thicker evans vs normal coolant, some pressure drop across the rad at high rpm could occur, esp with stock rad. This tends to put the lower rad hose at a slight vacumm, and I don't think oem hose was reinforced to avoid collapse.

IF this were an issue, using an AST with an orifice in the side port (stock has .086" at bottom port) would set the zero point at the pump suction, eliminating hose collapse as an issue. I think replacement asts have no orifice, create a significant bypass to the rad (not good), but would tend to reduce any suction at the pump inlet (that would occur with no ast) by 50%.

The 7 psi cap helps prevent vacuum at the pump suction, but since liquids are highly non-compressible, you are not assured 7 psi will exist at all times, regardless of coolant temp, which points out a huge advantage of the NPG+ system ... no pressure needed.

A standard pressure control cap with spring, as normally found at the ast, can be easily converted to 'zero psi'. Just carefully remove a few 'pie slices' from the smaller, lower rubber gasket (under the small central disc).