[Eric64]

I was hoping someone would let me know the determing factor as to what lb. rating the radiator cap needs to be, I have a basically stock SBC and have a 16# cap on it now,seems fine I was just wanting to know would a 12# cap be better? less pressure in the hoses and on the pump. Also on an overflow tank, I realize that if the cap vents it goes into it but when the engine cools does it go back into the radiator? Thanks, Eric

[1ownerT]

The higher the pressure the system (engine, cooling system) can stand the better, higher pressure=higher boiling point.
I would stay with the 16 lb.

[adantessr]

Originally Posted by Eric64

I was hoping someone would let me know the determing factor as to what lb. rating the radiator cap needs to be, I have a basically stock SBC and have a 16# cap on it now,seems fine I was just wanting to know would a 12# cap be better? less pressure in the hoses and on the pump. Also on an overflow tank, I realize that if the cap vents it goes into it but when the engine cools does it go back into the radiator? Thanks, Eric

The higher pressure cap raises the boiling temp of the coolant . And yes, a properly designed radiator surge tank allows the radiator to draw the coolant back in as the engine cools . For example the small tank on my T-bucket is empty when the engine is cold and about 3/4 full when the eng is up to operating temp. The surge tank in the pic is the small chrome tube mounted vertical next to the fill cap .

[matt167]

yep. 3* higher boiling point per psi of preassure.. a 50/50 mix of antifreeze/ water and a 16 lbs cap is good to 357*.. however. your engine should never reach anywheres near much less over 250* which would be a danger zone.. you could go down to a 7lbs cap and see no adverse effects. if you really wanted to

[MARTINSR]

But lets get this straight, the engine has not a damn thing to do with what pressure you should run, its the rad and cooling system that needs to be "matched". You best not be running a 16 lb cap with a stock 50 Chevy radiator or you will be blowing it apart!

And what the pressure means to you is for every pound of pressure it raises the boiling point by 10 degrees.

Brian

[T-bucket23]

Originally Posted by MARTINSR

But lets get this straight, the engine has not a damn thing to do with what pressure you should run, its the rad and cooling system that needs to be "matched". You best not be running a 16 lb cap with a stock 50 Chevy radiator or you will be blowing it apart!

And what the pressure means to you is for every pound of pressure it raises the boiling point by 10 degrees.

Brian

Every pound is 3 degrees not 10

[MARTINSR]

Originally Posted by T-bucket23

Every pound is 3 degrees not 10

LOL, thanks, we have went thru this before and I am still saying the wrong number.

Brian

[Eric64]

Thanks you all for the answers, I knew that it raised the boiling point but I didn't know what you wanted to have for the rating, I do have a new aluminum radiator and and all new hoses so I figured I had that covered I just didn't know if I should stick with the 16# cap. It looks like I'm running between 180-190 deg. and it's 85 today, I did just get it on the road so I've been watching everything probably closer than I need to. I really do appreciate everyones help and this site is great for all sorts of info. Thanks again, Eric

[MattsonsCustomRad]

A 13lb cap will put less stress on your system once the vehicle is shut off vs. a 16lb. This is a good thing. Overflow bottles are used to keep air out of the system. When the car is driving, a vacuum is created. This is when the water returns through the radiator. Th-tathe-th-the-That's all folks...

[T-bucket23]

Originally Posted by Eric64

Thanks you all for the answers, I knew that it raised the boiling point but I didn't know what you wanted to have for the rating, I do have a new aluminum radiator and and all new hoses so I figured I had that covered I just didn't know if I should stick with the 16# cap. It looks like I'm running between 180-190 deg. and it's 85 today, I did just get it on the road so I've been watching everything probably closer than I need to. I really do appreciate everyones help and this site is great for all sorts of info. Thanks again, Eric

The water temp will be higher in different parts of the system so the pressure is needed. There is no downside to running the higher pressure cap as long as your other components can take the pressure.

[adantessr]

Originally Posted by MattsonsCustomRad

A 13lb cap will put less stress on your system once the vehicle is shut off vs. a 16lb. This is a good thing. Overflow bottles are used to keep air out of the system. When the car is driving, a vacuum is created. This is when the water returns through the radiator. Th-tathe-th-the-That's all folks...

Coolant expands as it gets hotter. See post #3 . I am not argueing with a radiator expert. Just my observation from 43 years of turning wrenches, and as a maintenance supervisor responsible for the maintenance of a fleet of 18 trucks valued at $2.4 mil. each, along with a host of other mobile equipment . (retired)

[oldschool hero]

I too have been at it over 40 years and built some pretty potent engines and never used over a #13 cap.

[matt167]

I had a '65 Rambler American. The old lady drove it until 2 years prior to my purchasing. I'm pretty sure she stopped driving it cause the radiator was about kaput.. bought it figuring I could still make it work. and I needed a cheap car and it was cheap.. When I got it home and inspected everything. I found that the Stant LeV-R-Lock on it was a 21 PSI cap. that would explain why the thing leaked about everywhere.. I put about 6 tubes of quick steel epoxy putty into that radaitor ( new was $600 ). till it did not leak a drop. then pulled the 4 PSI cap out of my cabinet that was for my '51 Chevy ( correct pressure for the Chevy low for Rambler ). that rad held togther for the summer I drove it and when I sold the car it was still holding.. never got hot ither..

[adantessr]

Originally Posted by oldschool hero

I too have been at it over 40 years and built some pretty potent engines and never used over a #13 cap.

Just a little side note : The previous $160,000.oo engines in our haul trucks ran a regulator from the trucks air system to keep the cooling system pressurized to 12 psi and had a 10.5 psi low pressure warning . They sure didn't want any air bubbles in the cooling system . The replacement engines have a more conventional cooling system . Nothing quite like the sound of a 12 cyl diesel with four turbos and 2500 hp @ 2100 rpm pulling a 830,000 lb gvw up a 6% grade. Oh yeah . Cooling system holds 80 gals, 6' radiator fan. Tires 134" tall. The engine turns a generator and the rear wheel drivers each have an electric motor inside a planetary gearing. I sort of miss that stuff since I retired Aug of 2009...Allan