Radiator Cap Pressure 7lbs vs 15lb

rambos_ride

Of course now I know the Hudson needs a 7lb cap.

But thought we ought to talk more about why and not let Parts Guys try and tell you any different.

The system is pressurized to increase the boiling point of the coolant, by doing so you allow the engine to run hotter without boiling off the coolant thru the overflow.

For each pound of pressure you increase the boiling point by 3 degrees.

So even if the seals in the water pump could handle a larger psi cap - you'd still be running the engine 20'ish degrees hotter with a 15lb cap vs 7lb

And the only time I think that would be a good thing would be the dead of winter

nick s

if i remember correctly 15psi pushes the limits of a good heater core. really dont want that extra 20 degrees spraying at you feet in the dead of winter

SuperDave

I agree with Nick, The heater core is probably the weak link. BUT.. Just because you have a higher pressure cap doesn't mean the engine will run hotter. It just won't boil if it does get hotter.

rambos_ride

LOL - that would require that there "IS" a heater core in place...not the case with my 51 at the moment!

GrimGreaser

I run a 16 lb cap for a little extra help since these flat sixes run hotter and have more iron to cool. The higher pressure not only raises the boiling point, but helps the heat transfer, keeps localized hotspot boiling from occurring, and reduces cavitation. It also lets you run more water, or straight water +waterwetter for better cooling. I have a modern heater core and radiator too.

Chris Smith

check out this stuff, expensive but amazing also www.evanscooling.com

[Deleted User]

There goes the head gasket. That is why Hudson used a 7 pound cap to start with . Walt

GrimGreaser

Never had a headgasket issue ever. YMMV.

[Deleted User]

Do you drive this car on a hot day for about 100 miles? Walt

rambos_ride

Hi Walt - No, don't trust it enough (yet) to drive that far - and that may be all I do with this car, this year..short trips as I shake things out.

[Deleted User]

Good think you only do short trips, for with that 16 lb cap, on a hot day you will either blow the top hose off, the seams on the radiator top tank or the head gasket. Please put a 7 lb radiator cap on and be safe. Walt.

rambos_ride

Walt - I do have a 7lb cap on my replacement radiator. the radiator before had the 15lb cap and kept blowing the upper hose off.

What I'm wondering is what you think about the bypass,.. I just blocked off the square opening with the gasket I made for the thermostat housing.

I used a sheet of rubberized fiber gasket material and didn't include a cut-out for the square port for the bypass.

DavidC

Or you could pick up Walt's metal bypass kit and not worry about it...

Ol racer

Do like Walt said and Dont use higher than 7 lb Radiator Cap if running hot. Instead, correct the problem using an additional fan, shroud, or larger radiator..Back when raced Hornets wide open 15- 20 minutes, sometimes longer in Feature Events, I never used higher then a 7lb cap.....

StayBrown

@rambos_ride, I know this post is few years old, but i want to ask you if you can provide the part  number and manufacturer of the 15 psi radiator cap?

rambos_ride

Staybrown, I'm not sure I have the cap around, I'll check and get back to you.

Ric West IN

Don't put a 15 lb. cap on a Hudson Radiator!  Would be ok for a modern manufacture aluminum of brass. 

RichardD

Am running Griffin radiator and using a 7# cap; risky on gasket unless a 7X engine.
My thought anyway~~
{Love the drag & drop for photo install !!}