In the past I have happily jumped on the bandwagon of concern about the "high" pressure our cooling systems hold: 150kPa or about 22 psi. However, after some thought, I am left wondering not only why I ever bought into it, but why so many others are concerned to the point of using lower pressure (75kPa or about 12 psi) caps on their cooling systems.

If you think about it, 150kPa/22psi is not that much. For example, your average house water system holds more than that. And you probably have soft hoses connecting your dishwasher, refrigerator, parts of your lawn sprinkler system, and possibly garden hoses that you leave pressurized all the time (which you shouldn't). The average house water system is holding around 300kPa/45 psi or even more. That pressure is in both sides, hot and cold, of your home water system. Mine can get up to 75 psi at full pressure. And the cheap hoses, tubing, and components that we use in our homes are nowhere near as strong as the genuine Volvo hoses and components are in our cars' cooling systems.

So, why give up the added protection against localized or nucleate boiling that the higher pressure gives you, especially in the turbo models? Simple maintenance of the cooling system, including hunting around for top quality heater valves, seems a low price to pay for the added protection against local overheating that higher system pressure gives.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)