Pressure caps for radiators

[Rodney Bullock]

I have a aluminum radiator in my 55 chevy, I was told by this radiator guy that all 350 need a cap rated at 16 pounds. I had been running a 13 pound cap all along. I put the 16 rated cap on my car and a couple of days later went on a cruz. The lower radiator hose blew off and I lost all coolant. I had been running my car for the past 5 years and had no problems. If my thermostat is a 180, it's going to open at or around that temp, what then is the cap rating and how does that effect my system. I am running a 7 pound cap right now and am afraid to run anything higher. I put that cap on on the way home and the temp stayed at 185 all the way home, with no fluctuation. Can some one tell me the deal?

[Lou Earle]

For each pound of pressure the boiling point is raised 3.25 from 212 degrees. A 15 pound cap will raise boiling to about 260. Caps on 55-57 Chevy's were probably around 8 pounds like most cars of that era.

I think your adviser is well how shall I say it with kindness and candor - He wrong wrong wrong- and full of it. I really do not believer a radiator of that time would ever hold 18 pounds. I run a 350 in my 48 phantom with a 12 pound cap all day no problems.

Lou

[48mirage]

Water under pressure has a higher boiling point. The higher the pressure the higher the boiling point. It sounds like your 13 lb cap was good enough when you went to the higher pressure cap the mechanical connection couldn't stand the increased pressure. You are using a 7# cap now. As long as you are not losing antifreeze through the cap relief you are fine.

[Rodney Bullock]

The cap I am using doesen't have a relief flap and I have a overflow tank so I'm I still ok? I have not driven the car since I cleaned everything up and replaced the hose( I never use a hose after it blows off)

[greg g]

Rodney, way back when some Italian guy named Bernuli came up with a lot of observations and principles regarding the behavior of water and its interaction with the atmosphere. As you know water's boiling is alway noted at sea level and normal atmoopheric pressure 14.7/15 lbs per square inch is 212 degrees. If you change altitude, pressure drops and the boiling point decreases, I suppose if you were driving in under the ocean, the pressure would increase and the boiling point goes up. So by that observation you can change the boiling point by changing the atmspheric conditions in which the water resides. So if you remember you mother or grandmother canning vegetables (wondered why canned veggies always ended up in jars) they used a pressure cooker to seal things. The pressure cooker had a device on the top that traped the steam and increased the pressure in the cooker raising the boiling point. So the cap allows the system to build presure to the point indicated on the cap. which raises theo boiling point above 212 as does the additon of antifreeze. So for arguments sake lets say you 50/50 mix, and a 13 lb cap would allow the coolant temp to get to 220 degrees before it began to boil, and or the system would achieve 13 psi before the cap opened allow coolant into the overflow. I suppose this give you a greater margine from normal operating temp to Hot, but doesn't effect the normal running heat of your engine. I suppose if you lived about 2 mies high, the cap and system would be necessary to keep the coolant from boiling at say 190 degrees, but last time I looked Maryland was only about 150 or so feet above sea level. But since you car might have been driven by some on living 2 miles high, the engineers at GM needed to design one system for all applications, and you got a 16 lb recommendation. This of course presupposes al you hoses are in tip top shape. You know the deal where the chain is only as strong as its weakest link. If the hose blew off, guess where the weak link was???? Either the hose or the clamp, either one couldn't deal with the 16 psi.

[55 Fargo]

Rodeny, if it ain't broke don't go fixin with it. Sounds to me like you had everything going right with a 13 lb cap, the temp was running around 185. A 350 engine will probably do just fine with the way you had it set-up, no reason necessarly to go with a 4 or 7 lb cap as was used with a 1950s style set-up.........Fred

[Rodney Bullock]

Fred, I am with you, however that Greg is a science professor That was the must complete and perfect explaination I have ever heard. I must not have been paying attention in school cause I missed alot. I totally understand what you are saying, now tell me have you ever been stranded on a desert island with ginger, mary ann and the skipper.......... Thanks greg:)

[greg g]

Would that I were with Ginger and Mary Anne!!! My answer to the age old question would have been "Both in equal measure!!" (You loose if you choose)

Actually I have been watching "are you smarter than a 5th grader" and that answer is depends on what night...... Sometimes I am amazed at the stuff rumbling around in my noggin. Glad my effort turned on your light. As always it's a pleasure to be of service.

G

[Tim Frank]

The boiling point of a 50/50 mix of Ethylene glycol and water is 225 deg F.

The cap is really a non-event, and contributes little, unless something else is going on....the lower the number the better.