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Please Explain- Radiator Cap Lbs

Discussion in 'The Hokey Ass Message Board' started by fordstandard, Aug 1, 2013.

  1. I'd like to know more about selecting -what Lb Radiator Cap is to be used in a 289 SBF Motor.

    How many options are there in radiator cap "pounds" and what determines which Lb one uses?
     
  2. mustang6147
    Joined: Feb 26, 2010
    Posts: 1,847

    mustang6147
    Member
    from Kent, Ohio

    Here is the way I do it... I use 10 - 12 lbs as the norm on an older vehicle. Reason being newer vehicles run hotter and need higher lbs to keep antifreeze from boiling.

    If you have an older radiator, it may not be able to hold 12 lbs, so perhaps 10.... if you have a leak repair substance in you radiator perhaps 7 lbs is what you want till you get it fixed....
     
  3. I used a 13 LB radiator cap on my302 sbf. HRP
     
  4. daddio211
    Joined: Aug 26, 2008
    Posts: 6,012

    daddio211
    Member

    Each pound of pressure increases the boiling point of the coolant X degrees. So water under a 10 lb. cap will boil sooner than water under a 12 lb. cap.

    I just don't remember HOW many degrees difference there is. As stated, most older radiators (and probably water pumps and gaskets) can't handle the higher pressure of today's cars.

    My '27 T Roadster build: http://www.jalopyjournal.com/forum/showthread.php?t= 734383
     
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  5. VoodooTwin
    Joined: Jul 13, 2011
    Posts: 3,453

    VoodooTwin
    Member
    from Noo Yawk

    Here's a good explanation:

    http://www.heat-transfer-fluid.com/pdf/techpapers/pressure-boiling-point.pdf
     
  6. Dwhip361
    Joined: May 18, 2011
    Posts: 1

    Dwhip361
    Member

    I'm pretty sure that it's 3 degrees per lb. of pressure.

    Posted using the Full Custom H.A.M.B. App!
     
  7. plym_46
    Joined: Sep 8, 2005
    Posts: 4,018

    plym_46
    Member
    from central NY

    If its the stock radiator, you need to determine if it was pressurized and to what extent. Many cars through the late 40's and early 50's used zero pressure systems. An easy way to tell is the location of the overflow tube. If its below the gasket of the rad cap gasket it is likely 0 lbs. If its in the filler neck but the port is above the gasket it a presurized system. But even through the late 50's not very many exceeded 7 lbs. Essentially making the system work like a pressure cooker. It keeps the coolant locked down but it the heat and pressure raise to high it over comes the spring in the cap to allow the pressure out the overflow. Too stiff a spring will blow the rad before it allows the tube to be accessed.
     
    neilswheels likes this.
  8. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    You need to know what year car it is to know what cap it is supposed to have. I like to back off one size on old cars, if it takes a 10lb I will use a 7 or whatever the next lower pressure is.

    I do this to prevent leaks and blown rads in older cars. As I live in the north overheating is seldom a problem.

    This does not apply so much on the newest cars with air conditioning and 16 Lb caps, you have to stick with the factory cap there.
     
  9. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,717

    1971BB427
    Member
    from Oregon

    Where do you find a 7 lb. radiator cap? Or even a 10 lb.? Can't remember ever seeing less than 12 lb.
     
  10. i work at carquest. we keep them in stock.
     
  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    Yes the boiling point is raised 3 degrees for every pound of pressure on the cap.

    Fordstandard, you don't want to rely on what engine you have but pay more attention to what radiator you have. As mentioned previously by a couple of the guys if you have the original radiator that came with the car it most likely will only handle a 7 pound and maybe less cap while a new custom radiator or a radiator transplanted out of a mid 60's and up car can usually handle a lot higher pressure rating.

    Any "real" parts house should be able to get you the cap you need.
     
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  12. as a rule of thumb, We used 2 degrees per pound of cap on roundy-round dirt track cars. coolant was not allowed , and these run hot ,4- 5000 rpm for 20 minutes at a time

    So, when we used a 15 pound cap , our boiling point was 30 degrees above 212 or 242 degrees on pure water . we added products like wetter water , which increased boiling point another 20 degrees , (product claims 40)
    Anyways , this was our opinion and we did pretty well with it , won best engineered car, A shroud , and a clean radiator , inside , and out , is also a must , Joe
     
    Driver50x likes this.
  13. GassersGarage
    Joined: Jul 1, 2007
    Posts: 4,727

    GassersGarage
    Member

    I had a new aluminum radiator with a 20 lb. cap. Even at 215 degrees, it would not boil over. The old radiator in my '49 Chevy with a 216 had a 7 lb. cap. It never ran hot.
     
  14. DocWatson
    Joined: Mar 24, 2006
    Posts: 10,273

    DocWatson
    ALLIANCE MEMBER

    The cap info posted is all good, one thing that often gets overlooked is the internal health of the cooling system. If you are running a used radiator it is well worth the expense of having a good clean out done, not just flushing. Have the top tank removed and the core "Rodded out" this removes any build up that may inhibit flow and give a purchase for more gunk to build up.
    It goes without saying that a used motor needs a good clean out before installation.
    A quality coolant/rust inhibitor is a must, also if the motor is running aluminium parts such as heads, water pump basically anywhere in the cooling system you may also want to consider a sacrificial anode in the system. The galvanic reaction between the iron block and the aluminium parts can expedite corrosion in the system, the anode will corrode before any of your motors parts. This is a good idea for either a used motor or a new build.

    Doc.
     
  15. VooDoo Child 56
    Joined: Mar 5, 2010
    Posts: 49

    VooDoo Child 56
    Member
    from Jersey

    At Lincon Tech the teacher put a cup of water under a heavy glass dome sitting on a piece of rubber. He then hooked a vacuume pump to a port at the top of the dome, with minutes the cup of water was at full rolling "boil" at room tempiture. Boiling does not always mean hot.
     
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  16. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,657

    Rusty O'Toole
    Member

    The first pressure caps were only 4 lbs, I bought one for a 1950 car not long ago.
     
    hemihotrod66 likes this.
  17. Dane
    Joined: May 6, 2010
    Posts: 1,351

    Dane
    Member
    from Soquel, CA

    The cooling system does not know what lb cap it has and really it's not that important unless you run on the edge of boiling over all the time. What goes on the car these days is usually determined by what's available. I've run 3lb to 12 lb caps with no noticeable difference. I'd run the lowest available unless you know your cooling system is up to the task. Installing a recovery tank to keep the system full of coolant is more important than what lb cap you have imo....
     
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  18. 40FordGuy
    Joined: Mar 24, 2008
    Posts: 2,907

    40FordGuy
    Member

    Yes, it is 3 degrees for each 1 psi pressure. That's with a 50% water / anti freeze mix.

    4TTRUK
     
    uncle buck likes this.
  19. Inked Monkey
    Joined: Apr 19, 2011
    Posts: 1,834

    Inked Monkey
    Member

    So if somebody is running a newer motor but still using the stock radiator, should you get a higher or lower lb cap?
     
  20. plym_46
    Joined: Sep 8, 2005
    Posts: 4,018

    plym_46
    Member
    from central NY

    low as you can go.
     
  21. 53sled
    Joined: Jul 5, 2005
    Posts: 5,817

    53sled
    Member
    from KCMO

    I replaced my 15 lb cap with a 1 lb cap to save weight. Picked up 3/10 in the quarter.
     
  22. Canus
    Joined: Apr 16, 2011
    Posts: 102

    Canus
    Member

    Radiator cap MUST be sized based on the radiator NOT the engine. Older radiators can not handle the higher pressures modern cars can. If you put a higher pressure cap on an older radiator don't be surprised if you blow out the radiator.
     
  23. Ozzy39Merc
    Joined: May 4, 2022
    Posts: 28

    Ozzy39Merc
    Member

    What's your take on a 15lb rad cap for an original Radiator on a 39 mercury? 2 ins and 2 outs. V8 A99 FlatHead.
     
  24. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 31,174

    Jalopy Joker
    Member

    No , run 7lb if all is good with cooling system
     
    Budget36 likes this.
  25. I run the heaviest one I can to keep the water in
     
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  26. Moriarity
    Joined: Apr 11, 2001
    Posts: 30,755

    Moriarity
    SUPER MODERATOR
    Staff Member

    lothiandon1940 likes this.
  27. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 6,250

    Ebbsspeed
    ALLIANCE MEMBER

    I run ZERO PSI on our '56 New Yorker, and use a coolant recovery system. Radiator and 354 Hemi are stock, and the car has never gotten close to overheating. I believe they were originally a 7 PSI system, but if I can get by with running no pressure in a cooling system I do it.
     
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  28. jimmy six
    Joined: Mar 21, 2006
    Posts: 14,795

    jimmy six
    ALLIANCE MEMBER

    I size it to the weakest link. With a new US Radiator in my 56 I still would not go over a #7 because of the original heater core. When I install an Old Air Products AC/heater I went to 14#.
    If you don’t use a recovery system leave the coolant 1” down in the tank. This allow air in the system and really isn’t good for contamination.
    We have a cap tester for our dirt track car and test every cap. We run a 30# on a double flow aluminum AFCO radiator and have run 260* and lost no water which is distilled only.
     
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  29. Ned Ludd
    Joined: May 15, 2009
    Posts: 5,025

    Ned Ludd
    Member

    What are the radiator tanks made of? When I bought my Morris Minor it had too heavy a cap on the radiator, and that had blown the soft copper top tank up like a balloon.

    If you're willing to take the risk, you could test your radiator by sealing the inlets/outlets except for one place to supply air under pressure, and reading the pressure at which it looks like it wants to start expanding. Be careful: you want to stop adding air while you're still in elastic strain, and hard-to-repair plastic strain can happen very abruptly. Subtract about 20% from that pressure to get a safe cap pressure.
     
  30. I have a new aluminum radiator in my car and run a 12-15 PSI cap.
     

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