What rad cap?

I am running a brand new replacement an aluminum rad in the 55 with an overflow bottle so that it is a closed system. My old rad cap for the stock rad was a seven pound. What rad cap should I run now?
Thanks
Jay

 

Modern radiator so something around 15lbs ,13-17 should do fine.

 

If you still have the old heater core in it stick with the 7 pound cap or you will ruin your carpet and boil your p***enger's feet.

 

No heater core here!
Jay

 

I tried to find the largest rad cap that I could put on my Allstar aluminum rad and have had no luck on finding the proper rad cap to run on this. I do have a 15 pound. I just don't like when it came up to pressure that the upper and lower rad hoses were quite hard.
Jay

 

Pressure only raises the boiling point, if the system is in good shape, stick with seven, it'll work just fine.

 

Your radiator should take the pressure no problem. Kinda weird that your hoses got hard, are they standard molded rubber or something like the gates vulco-flex?

 

I would ask the radiator mfg.... Or why not split the difference, go 10 to 12 lb cap

 

I bought a desert cooler (copper) 2 or 3 years ago & was told to run the 7 lb cap

 

Good idea. I will contact them and see what they suggest.
Jay

 

The only purpose a higher pressure cap serves is that it allows the temperature in the cooling system to rise above the sea level boiling point with out boiling over. I have run several early Fords with Chevy motors and no pressure cap at all. I like my engines to run about 180-185. Water won't boil at that temp.

It doesn't make any sense to me to have a system that is capable of operating at 235 degrees with out boiling over if I never allow it to get that hot. I watch the temp gauge and I will be on the side of the road long before that temp is reached because something is wrong and needs to be addressed. How many times do you operate your hot rod at 235 degrees? I got caught in traffic at the Ocean City show one year and had to get out of the traffic and turn around to get the temp down. It never boiled over. I'd use the 8-10Lb cap. Anything higher is a waste for me.

 

running 12 lb cap on my 56,aluminun rad. sbc 4 speed, runs 182, 185 down road, never seen it over 192. 5 blade fan, good shrould.

 

Sounds good! I run just a little cooler than you so I am thinking a 12 pound might be good. What about a 9 pound? On a closed system? Would this work?
Jay

 

Ditto, check with the radiator builder ; They'll give you the straight info.

4TTRUK

 

I did check. They said I could run 22-24 pound rad cap? Wow... So how about running a nine pound? 15 just seems too high?

 

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

A 7lb cap will take the boiling point from 212 F to 233 F
A 15lb cap will take the boiling point from 212 F to 257 F

The main thing generally about a recovery system is typically if it is not losing water it is not getting too hot, in other words not boiling. The rating of the cap ***ists this process.

I have been in some cars where temps were as high as 210 F on a 100 deg day, but no loss of water.

 

I put a 9 lb rad cap on to day and went for a cruise. Seems ok. From cold temp. climbed to 180 degrees then as soon as the thermostat opened it dropped to 160 (coolant in the rad being transfered into the block) then came back and ran 165-175 degrees running down the road free air (no electric fan on) at stop lights came up to 180 degrees (electric fan on) then it keeps coming down to 170 mark (fan off) running down the road again. Seem ok? The rad hoses don't seems as tight either as when it had a 15 pound? hmmm? What do you guys think? Am I where I need to be?
Jay

 

Run a 15# cap.

 

Why?

 

Because it increases the boiling point.

 

Thanks for all your help! You can read!
Jay