1. Ford caps (see amkproducts.com) during the early 60's were 13 or 14 lb caps. The pretty expansion tank will "bulge" at higher pressures...
2. Roughly 3 degrees raise in boiling temperature for each PSI.
3. AT SEA LEVEL (roughly) water boils at 212 F, 50/50
antifreeze/distilled water at 233 F.
Using advanced math, a 13 lb cap with near 50/50 mixture boils at ~270 F.
(ignored is the 5,000 ft elevation boiling temperature of water ~203 F)
(ignored is the 29,000 ft elevation boiling temperature of water ~162)(Mount Everest!!)
(totally ignored is the actual relationship to barometric pressure not altitude)
PRACTICAL EXPERIENCE:
A completely stock, all original foreign car was spec'ed as a 7 lb cap through 1972.
Factory changed little else, but switched to a 13 lb cap 1973+.
I managed to obtain an ORIGINAL 7 lb cap... for an event in the Southwest US, i.e. 95+ F outside.
Car ran great, no heating issues, TILL stopped quickly after a long interstate drive...
Sitting nearby about 15-20 minutes later, the car started having "gas"... and I don't mean the liquid kind!
It was obvious, burping, belching, shooting water/steam out the overflow pipe.
The performance lasted ~10 minutes.
Consistent performance on hot days until I changed to a 13 lb cap.
HEAT SOAK is real and will cause hard starting (mixture) and slow turning (starter), AND "gas", 5-30 minutes after a hot run.
NOTE: ideally ~40%
antifreeze to optimize the cooling process, lower freeze temperature, and higher boiling temperature.
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