Does anyone know where I might be able to find an expansion tank cap that will fit our cars that is rated higher than 100kpa? I would hope there are compatible Fords or other Jags that have a similar design cap (with different pressure ratings) that I can try out... I recently installed a Chevy 408 stroker motor in my XK8 and it requires a higher rated cap pressure. I have searched everywhere and can't find one.

Thanks!

 

I'm confused. Why would you need a higher head pressure in the system? It's only purpose is to reduce the boiling point of the coolant.
Your stroker shouldn't be running at a significantly higher temp than a normal engine (I would think) unless you are finding the stock (?) radiator is insufficient. But that's a whole other issue.

 

mhminnich - why does the corvette require a 20lb cap and the jag only 14.5lb... and why does the X150 take a 140kpa (or 20lb cap).. and the 2000 camaro takes an 18lb cap. The current 14.5lb (100kpa) cap doesn't seem to be holding the pressure (and it is brand new) so I just want to go higher (16 - 20lb). Any suggestions on a replacement?

 

Not a clue to either the 'why' or 'where'.
My asking you 'why' is out of an abundance of curiosity over the very topic since the pressure in the system is a function of coolant expansion, not pump output. The cap just decides when to relieve the system. Maybe larger capacity systems have greater expansion and require a higher pressure cap to maintain the capacity at the desired temp? I dunno.
Here is a read that explains it in a different way.
Pressure cap discussion

 

Yeah I am not sure either... but I did go from a 4.0L to a 6.7L engine. I found a cap "7047" from a Ford Escape that is rated at 18lbs so I'll see if I blow anything out with it I took it for a quick ride this morning and there were some leaks around the heater valve (hopefully just the hose clamps)... maybe that was causing my issues with the 14.5lb. If I get all of the minor "leaks" tightened then I can try to go back to the 14.5lb OEM cap.

 

I agree that the only reason for the Pressure in the system is to increase the boiling point of the coolant. IIRC, 16 lbs = 288 degrees, so the 14.5 may be around 270 degrees. If it gets that hot, I would want to shut it down anyway. It may also be that the pressure relief is also a safety valve on the other parts of the system ( heater, etc) where a higher pressure might cause leaks.