On 6/21/2007, someone (who shall remain unnamed) wrote:
>>"I highly recommend that people have their filler neck replaced at the time they recore the rad so that they can run more than a 7 lb. cap. Using a 15 lb. cap allows you a wider margin."

Oh? How hot do you intend to run the cooling system? 50/50 mix of glycol/water boils at 223dF at sea level. Boiling temperature increases by3.25dF for each psi of applied pressure, so the 7psi pressure cap takes the boiling point up to about 245dF.

My MGA temperature gauge goes up to 230dF. It is verified correct at 212dF with the sensor in boiling water. Last weekend I managed to drive the coolant temperature up to near the 230dF mark with extended puttering around a few small towns during a club rally (with 90dF+ ambient temperature). When I shut the engine off the indicated temperature would go higher, in fact peaking out at 85 PSI in the oil pressure scale on the combination gauge. That would be at least 240dF, and still it didn't boil or dribble.

The only thing to be gained by increasing the pressure rating of the radiator cap above 7psi is the capability to drive the cooling system temperature even higher than 245dF without boiling (235dF with plain water). Such higher operating temperature is far beyond the end of the gauge scale reading, so you would have no indication of the real temperature until it might start to boil and spit. If you have any intention of going into that operating range you will need a different temperature gauge.

As noted by several others, as long as it doesn't boil the engine is safe (if not entirely happy). The greater problem these days with our carbureted vintage cars is boiling of fuel in the carburetor, which is aggravated by the presence of alcohol in the fuel. This is not a problem during cruising at road speed when air intake and fuel flow is sufficient to cool the carbs. But puttering around near idle with slow ground speed on a hot day allows greater heating of the carbs. Under those conditions my MGA has problems with vapor bubbles in the fuel causing a lean running condition (at anything over 220dF). In this case I have to pull the choke out to keep it running.

As such, I cannot imagine wanting to run the cooling system hotter than 240dF, so no reason to use a pressure cap higher than 7psi.

And then the same unnamed person wrote:
>>"I like a 15 lb. cap. I use a 25 lb. cap on the ...."

Wow. 15 psi brings the boiling point up to 272dF (with 50/50 glycol/water). 25 psi is more like 304dF. I know some new cars with fuel injection and fuel recirculation systems may run into that range, but for our vintage carbureted MGs the higher pressure cap only puts more pressure stress on the radiator structure (apt to burst a solder seam). Due to thermal expansion of the coolant, as soon as the system starts to warm up it starts to expel fluid past the pressure cap, and system pressure rises immediately to the relief pressure rating of the cap (even when coolant temperature is well below the boiling point at atmospheric pressure).

I use the factory prescribed 7 psi pressure cap on my car. I do occasionally peg the temperature gauge at 230dF, and it will never boil or lose fluid. The lesson here is that use of a higher rated pressure cap only serves to apply higher pressure stress on the radiator and hoses and heater core. It would have no benefit at all in cooling the MGA.