I posted before about going to a howe radiator. It turns out that it's on back order...surprise, surprise. However, it gives me a chance to figure out a few other things. I found a part # for a double pass that has an overflow bung and a cap. The problem is that Howe only provides a 22-24psi cap. That's about what, twice the stock rating. My question is this: Since I have an S4 with a second cap up on the thermostat housing, will this stock cap bring the whole system to whatever it's rated PSI is? I would think the weakest link in the system would be the limiting factor.

 

Stock is 15# I think. I would agree with you and say that the system would pressurize to the lower of the two.

 

The stock radiator cap should be a 0.9bar (13psi) one. The cap on the filler neck isn't a pressure relief cap, it's just a plain cap, so it should hold 24psi okay.
Personally I don't see the need to run the cooling system that pressure. All it does is raise the boiling point slightly, but you're not supposed to be anywhere near that hot anyway. It will however put a lot of extra stress on the cooling system, and will quickly find any weak spots in the 15yo parts in your car...

 

I'm hoping that the filler neck on the howe radiator is similar to a chevy or something and put a regular pressure cap on it. Guess we'll find out as soon as it comes in from summit.