Please Explain- Radiator Cap Lbs

Do not use a cap that high.
I’ve run a 4 lb cap on early flathead radiators with no issues.
When pressurizing them you also need to pressurize the overflow tube with a pressure relief valve close to the same poundage.

 

I just bought a 4 lb. cap at O'Reilly's last week. I had a little drip on the floor every morning (that I could not find) with a 7 lb. cap and replaced it with a 4 lb. cap and it quit.

I don't have any overheating problems and I don't like high pressure cooling systems.

 

I run what the OE ran
saves over thinkin

 

Look at it like this, newer cars run the higher pressure/high operating temps because they are mandated to do so by emission requirements. There is no reason to want to make your hot rod emulate that lead. Higher operating temps MAY produce more complete emissions burning but thats in computer controlled vehicles where spark is continuously advanced and retarded by listening for knocking. Hamb vehicles don't (knowingly) have those controls to keep the engine safe. (most late models also employ very high compression ratios)

On a carbureted street driven on street gasoline engine, usually compression ratios are going to be much lower as there aren't any knock sensors saving your engine from catastrophe. What someone should realistically do is decide what operating temperature they want their engine to operate at. Probably for most engines operating at around 180 degrees is good/sufficient. That means you are operating 32 degrees below the boiling point of pure water. With a 50/50 mix of ethlene glycol the boiling point becomes 223 degrees. So the engine has 43 degrees before it boils and thats with no pressure cap.

At that point someone should begin to worry about the health of their engine should they continue driving it. Adding a high pressure cap on top of that only provides you with the opportunity to damge your engine by continuing to drive it. Your coolant system may be made to continue function at higher temps, but your oil will begin to break down and you may blow head gaskets or even cause detonation.

My goal in setting up a coolant system would be to find an operating temp near 180 with a low pressure cap (5/7 lb) and make that work by insuring there is plenty heat transferring area supported by a good fan. High temp and high pressure are not needed or wanted in my view.

 

The 180 sounds good on flatheads.
My caddy runs 180/185. I don’t like that. Plan to up the thermostat. My 350 ran 210-215 all day.
Ran great. An occasional vapor lock in traffic. Fuel plumbing improvement would have cured it.
Plugs and timing play a part.
Yep, new EFI engines run hotter and have technology to detect knock issues and make corrections.
We have guys here worried when temps rise over 180 and I’ve never understood that. That’s almost too cold in my world. I build to run the hottest thermostat I can get for it.

 

There is a good chance that your eighty year old rad has an internal vent running from the top tank. Then it wouldn't matter what cap you are running. When you pressurize the rad, you add more stress to the rad giving more chances for the old soldered seams to let go and the tanks may crack from work hardening over countless years of flexing from heating / cooling cycles. Things to consider.

 

1955-57 Thunderbirds were equipped with a 3-row radiator and a 13-lb. cap.