Radiator caps and PSI...Why?

[Brian]

Finishing up my coolant system and I see that radiator caps are available in a variety of psi ratings from around 9 to 30 psi. Is it correct to assue they somehow are matched to the thermostat and releive pressure at cooler temps? What's the standard one for a small block Ford putting out modest hp on a carb?

Thanks
Brian

 

[Russ Noble]

Brian,

Traditionally radiator cap pressures are generally in the 6 to 15 psi range. I've no idea what is listed for SBF.

Higher pressures are used to increase the boiling point of the coolant under arduous conditions i.e. racing or desert conditions. The addition of AF coolant to the radiator water will also delay the boiling point further but don't go over 30%. In this case more is not necessarily better....

Hope that helps.

Cheers

 

[Jac Mac]

Brian,
Raising the pressure also raises the temp at which the coolant will boil. It also reduces the likelihood of localised hot spots within the coolant passages of the cyl block & heads which can lead to formation steam pockets and therefore air locks in a system typical of rear engine / front radiator setups. The only downside if any is the hoses etc need to be in good order with decent fittings to prevent failure at joints.
It also means that you can run a higher operating temp which can be a help with power output, particularly if clearances in the engine build are configured for these temps .

Jac Mac

 

[Malcolm]

I have been running 15 psi ok on my engine if that helps Brian.

 

[Rick]

289's & 302's in the 60's & 70's ran 13# caps. 302's in the 80's & 90's were running 16# caps. They used reinforced rubber hoses for upper and lower radiator connections no metal tubes. I dont think a typical 1 1/2" or 1 3/4" OEM rubber hose would withstand more than about 16#.

 

[John Fitzpatrick]

I have heard that higher pressure will also increase the RPM that cavitation within the water pump will occur. Someone more knowledgable want to chime in on that?

 

[Phil]

Brian
Typically each pound of pressure over atmospheric will increase the boiling point of water by 3 deg.. This means doing the math that a 15 lb. cap will add 45 deg. to the boiling point of your coolant if it was straight water. (This is at sea level) A motor designed to run a 180 deg. thermostat will definitely see localized hot spots that can exceed the boiling point therefore adding 45 deg. of headroom is desireable. Also newer engines running higher pressure caps are typically designed for a 205 to 210 deg. thermostat, which was necessary to get the higher temps to energize cats and keep emissions in check. I would think some of those higher pressure caps as the 30 lb. are not necessary, especially since glycol also raises the boiling point somewhat and if you reach a point that a 15 lb. cap and 50 % glycol is not sufficient you probably have other issues like air pockets or insufficient cooling capacity as Jac Mac stated.
Like Russ and Jac Mac said a 15 lb. cap is fine keep your connections good, and purge any air and I would bet you have good results. Also consider a newer style cap that can take advantage of a coolant recovery bottle to insure that cycling the cooling system continually purges air by returning coolant as the system cools instead of creating a vacuum that can suck new air into the system
Hope some of this helps
Cheers
Phil

 

[Brian]

Many thanks for the info and advice guys. Lets get ready to make some noise in New England......

Brian

 

[Mike Drew]

Also FWIW, running a super-high-pressure cap moves the weak link in the system elsewhere, i.e. your hoses or your radiator or your heater core (GAK!)

13-16 psi is all you'll ever need.