One could assume they were trying to deal with over heating issues and increased the Coolant System Pressure. To me this is the only thing that makes sense. But it also doesn't make sense to increase pressures on a system that is prone to rupture coolant hoses/fittings. OK course when everything is new the system can probably handle it fine, it is years later when out of warranty do the aging coolant hoses/fittings become a problem...for US. LOL

Just a thought,

Dave

 

Right, I was thinking the same initially Dave. A 29psi cap makes sense for the track. That's why I was asking if there was a bulletin on it. I suppose with a 50/50 mix of coolant it may help. I'm confident there's good logic behind it.

 

Well, it's been 8 years since this posting and I just stumbled across this issue while doing some mid winter's nap maintenance on my 2012 XK X150 Portfolio.

After doing the following analysis, I've chosen to go with the 140 kPa cap;

The boiling point of a liquid increases as the pressure increases, and for coolant systems, the boiling point is raised by both the system pressure and the coolant mixture (typically a 50/50 mix of water and ethylene glycol, which raises the boiling point by about 12°F at atmospheric pressure).

Water’s base boiling point at sea level is 212°F, and pressure increases the boiling point by approximately 0.27°F per kPa. For a 200 kPa cap, that equates to approximately 29 psi absolute pressure (including atmospheric pressure), the boiling point increase from pressure is (29 - 14.7) × 0.27 = 14.3°F, making the total boiling point 212°F + 12°F + 14.3°F = 238.3°F.

For the 140 kPa cap, which is about 21 psi absolute pressure, the boiling point increase from pressure is (21 - 14.7) × 0.27 = 6.8°F, resulting in a total boiling point of 212°F + 12°F + 6.8°F = 230.8°F.

My XK's X150's normal operating temperature range of 190–220°F is below both boiling points and the Engine Temperature limit of 230°F, so either cap should work under normal conditions. The 200 kPa cap does provide more margin before the coolant starts to boil if the system runs hotter (near 230°F) but operating at a higher pressure (200 kPa) increases wear on hoses and seals over time. I may be overly cautious, but there are plenty of stories in this and other forums, reporting on the disastrous effects of leaking coolant pipes.

 

Is it normal to get a little “hiss” from the cap when removing? (200 cap)

 

But I hope you are not just using water in your cooling system.

And “yes”, it is a closed system, only venting is through the cap should the pressure exceed the rating. That hiss merely shows a slight increase in pressure compared to ambient.