Lower pressure radiator cap, anyone run that?

Discussion in 'Troubleshooting & Diagnosis' started by 2ndOwner78, Jun 14, 2021.

  1. 2ndOwner78

    2ndOwner78 Veteran Member

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    I just dropped off my original radiator at an old school radiator shop for a flush and inlet fix. They have decades of experience so I am thinking I should open my ears when they have advice to share.

    When I asked for his advice on a radiator cap he mentioned that unless I am running in the mountains (higher elevation) it may be beneficial to run a lower psi cap, something like 5 psi. He said it would help take some stress off the cooling system in general. That is what he does personally on his older vehicles. He mentioned it is easy enough to carry a 16psi cap along and switch it out if ever needed.

    This made sense to me. Anyone running with a lower psi cap like this. If so, which one?

    Btw, my local elevation runs in the 800-900 foot range for the most part. My upcoming trip to Sturgis, SD I'll be in the 3000-5500 foot range. So maybe run a lower cap locally and switch out to the standard (16psi?) when I'm cruising around the Sturgis area?
     
  2. camarochevy1970

    camarochevy1970 Veteran Member

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  3. COPO

    COPO Veteran Member Lifetime Gold Member

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    I always use a 15 psi cap with a 180* stat and the temp never goes above 180* including when outside temp is 90+.
     
  4. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    You should be fine running the lower pressure cap until you get your A/C working again.
    (The higher boiling point might be needed on hot days, with the condenser dumping heat and partially blocking air flow through the radiator.)

    Personally, I would make sure the system can handle 15-16 PSI after the radiator fix, by driving it locally, before you raise the system pressure for a long road trip.
    Stress test it before you leave, not during the long haul across the middle of South Dakota.
     
  5. 2ndOwner78

    2ndOwner78 Veteran Member

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    Nice well written read, thanks!

    I have the new Stant 180 stat and 16psi (SAE Range 14-18 psi) sitting and ready for install. I'll just stick by the factory specs and not monkey with it.

    Good point about the A/C too. I'll get it out and about for some long runs before we head out after I get the radiator back and have the new stat and cap on. I'm also installing a second temp gauge (sensor next to the intake water outlet) to supplement/verify the factory gauge (sensor on the head). Some say the headers can heat up the factory sensor in the head and give a misleading reading, so this will help me feel better.

    Thanks for the quick feedback guys!
     
  6. Gary S

    Gary S Administrator Lifetime Gold Member

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    All good points. I used to have my radiator repairs done in a local shop here. The guy running it also suggested changing from 15psi caps to 7psi caps. His reasoning was that you can expect fewer pressure related failures over time running less pressure. I believe that is correct, but as the above article states, you don't have as much "reserve" cooling in extreme conditions with the lower pressure caps.
    Everything seems to be a tradeoff. Aluminum radiators should cool better than copper ones as aluminum dissapates heat better, but copper is a stronger material that should have fewer pressure failures in the long run. Lower pressure systems should have fewer pressure related failures, but possibly more heat related failures if extreme conditions are encountered.
     
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  7. 1978 Z/28

    1978 Z/28 Veteran Member Gold Member

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    50% water and 50% ethylene glycol has a boiling point of 223F. Each 1PSI will add 3.25F to boiling point. So a 7 lb cap will boil at 246F, a 15 lb can will boil at 272. If your engine gets to 246 you probably have a major problem.
    I think high pressures were needed more for old alcohol based coolants that would evaporate and cars did not have coolant recovery systems.
    You may want to watch your overflow canister closely at first when pressure increases above 7psi, coolant will expel from engine sooner when the engine heats up, but I do not think this would be a problem
     
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  8. berg2695

    berg2695 Veteran Member

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    Remember, too, that there are hot spots in the cylinder heads that are hotter than what the thermostat or temp gauge reads. The higher pressure will help keep steam pockets from forming there.
     
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  9. biker

    biker Veteran Member

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    Correct. The overflow tank will see more action, so make sure the hose is sealed well and the tank fitting isnt cracked so that the rad can suck the coolant back in when it cools.
     
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  10. kenny77

    kenny77 Veteran Member Lifetime Gold Member

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    It's funny to me when guys start trying to "Out Trick" the Small Block Chevy oiling and cooling systems.
    I have an aluminum radiator and went back to the stock HD cooling fan and clutch, 15 lb stock cap.....and all my A/C cooling problems in traffic went away.
    The factory guys knew what they were doing.
     
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