Higher pressure radiator cap?

[Chris142]

Originally Posted By: 97 GTP
Where can you find lower pressure caps?

My OEM cap is 16 lbs, I would like a 8-12 lb cap but at AutoZone I got the blank stare.
Caps are market on them what pressure they are.

Might have trouble finding a lower pressure cap if you have a pressurized bottle with a screw on cap.

FWIW
Stant # 10228 is a standard size 7lb cap and 10229 is a 13 lb cap.
#10227 is a 13 lb small cap and 10241 is a 13 lb "stupid cap" as we call them.These usually come on Honda's and Toyotas.

 

[Gary Allan]

I think Chris is saying (and correct me if I'm wrong) ..that there are two size caps on most radiators (with or without coolant recovery - which I haven't seen since my 1978 BMW 320i). The size that we're used to seeing ..and smaller.

The rest is the pressure setting. Then it's basically asking for a xxlb cap.

 

[ehall]

A higher cap (eg 16 instead of 13) is probably the technically correct preference. I don't think it matters that much though.

This page says water boils at 206f @ 13 psi and 216f @ 16 psi, so if you have a 195 thermostat then you can expect normal operating temperatures of 190-220f and the higher cap will keep the water from boiling off as soon. Having said that, the boiling point of green coolant is 387f so if you have a good mix you're going to be exceeding the cap factor anyway. A lower cap will allow the water to boil a little sooner (10 degrees) but again the coolant is going to be preventing that anyway.

Either way, water expands 1700 fold when it turns to steam and the cap isn't going to be holding that back for very long either. A lower cap will allow the pressure to escape sooner, which may be the difference between overheating and radiator collapse.

So what it comes down to is if you run more water than you should then you ought to run a higher cap. However a lower cap and the right coolant mixture is likely to put less stress on your coolant system.

FWIW I run 13psi caps on my cars, both of them call for 15psi in the owners manuals but I can only find 13 and 16.

 

[Gary Allan]

Those numbers are @ PSIA, not PSIG (absolute vs. gauge). You're at zero "gauge" @ 212F at sea level. In the chart's case, you would have a zero pressure system at some elevation above sea level to boil @ 13psia and 206F.

Check out the steam table here on page one http://www.boilerroomservices.com/Facts/SteamTables.pdf

That all said, I do wonder why MOPAR spec's such a high pressure cap on my jeep engines (18lb). I've run 7lb on other applications without issues, but I have pondered the localized boiling hot spot as a potential unseen liability. I imagine that it's also not an issue except in extreme situations, but that's just reasoning with limited dimensional view.

 

[Chris142]

Originally Posted By: Gary Allan


That all said, I do wonder why MOPAR spec's such a high pressure cap on my jeep engines (18lb). I've run 7lb on other applications
I ran my 4.0 Jeep all last year with a 7lb cap. Nothing out of the oridinary happened.

One trip through Death valley had me really watching the temp guage. With a 7lb cap it will boil @ 233F. It was 125F that day and the engine was running 225F.