[TR] 4 or 7 PSI radiator cap

[Randall]

> Any ideas on what changed, why the difference might be in running  
> temps or other considerations?

Raising the pressure makes no difference to normal operating temperature, as
that is controlled by the thermostat.  In theory, it does give slightly
better cooling (meaning the thermostat stays more nearly closed under normal
operating conditions), but in practice I've never seen a difference.

The big difference is that higher pressure increases the boiling point of
the coolant, which makes the engine less likely to boil over in conditions
where cooling is inadequate.  A very common example is when stopping
suddenly after a high speed run (like pulling over in a rest stop); the
higher pressure cap helps prevent the little puddles of coolant.

For the TR6, I believe the cap changed when the engine was further detuned
for emissions, in 1973.  The emissions changes also increased the heat
generated, so the higher pressure cap helped prevent boilover.

Or it may have been an attempt to counter the popular notion that all
Triumphs overheat.  In 1972, the Stag cooling system got a complete
makeover, which included going from a 13 psi cap to 20 psi!

Randall