Radiator cap pressure

Discussion in 'Street/strip 400/430/455' started by papa roger, Apr 17, 2017.

  1. papa roger

    papa roger Well-Known Member

    I need someone to educate me on how the radiators pressure cap affects the coolant temp. in the radiator. I understand the basic law of physics that says the more pressure you put on a liquid the hotter it gets and enough pressure will cause water to boil, so will going from a 16# cap to an 8-10# cap create a lower coolant temperature all other things remaining the same or does it just change the pressure at which the coolant will flow into the overflow tank ? I know some guys who race at the track that run a lower pressure cap but never gave me a reason way. Thanks Roger

    On The Eighth Day God Created Buick
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    No, that is incorrect. The reason for a radiator cap is so the system runs at pressure. Water at higher altitude, where atmospheric pressure is lower, boils at a lower temperature. Conversely, water under pressure boils at a higher temperature. System pressure is responsible for the boiling point elevation, more so than the ethylene glycol coolant. The pressure rating of the cap will have no effect on the coolant temperature. You may be thinking of Gas Laws. The more you compress air, the higher it's temperature. That's how diesels work.

    Water like other fluids, for all intents and purposes is not compressible. That is the principal behind hydraulics.
     
    Last edited: Apr 17, 2017
  3. Mark Demko

    Mark Demko Well-Known Member

    Raising cooling system pressure (to a point) will raise the boiling temp.
    At atmospheric pressure, water will boil at 212 deg.
    At 15 lbs ( as in your cooling system) the boiling temp is raised quite a bit.
    I don't know all the science behind it, but in simple terms, the pressure keeps the bubbles from forming:eek:
    Larry beat me to it!
     
  4. papa roger

    papa roger Well-Known Member

    Thank you Larry and Mark, I now understand how it works and why. Roger

    On The Eighth Day God Created Buick
     
  5. BuickV8Mike

    BuickV8Mike SD Buick Fan

    So the guys at the track? Are they trying to keep the pressure lower in the cooling system or is it something else that they run a lower pressure cap?
     
  6. Mike Trom

    Mike Trom Platinum Level Contributor

    I thought the pressure rating of the cap dictates the max pressure the system will run at? If not, what is the cap pressure rating for?
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Correct, the higher the pressure, the more boiling point elevation you get, but that has no effect on how hot the engine runs.
     
  8. BuickV8Mike

    BuickV8Mike SD Buick Fan

    Larry, Wouldn't a higher pressure cap allow the motor to run slightly hotter and at a higher pressure before overflowing? Is the only intent of a lower pressure cap to minimize the maximum cooling pressure or is there something else? Why do they make 8 psi caps?

    Cheers,
    Mike
     
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The only reason to run a cooling system at pressure is boiling point elevation. Yes, the cap would bypass fluid into the overflow at a lower pressure, but again, that does not determine how hot the coolant gets. Assuming your coolant system is in good shape and efficient, the thermostat determines how hot the coolant gets. The thermostat opens to varying degrees to allow circulation through the radiator and should maintain the temperature within 5- 10* of the thermostat rating. If you have a clogged up radiator, or not enough airflow, then nothing can prevent the temperature from increasing. The GM system was made to run at 15 psi. Not sure why there are 8 psi caps unless someone has a system that is potentially leaky and want to limit pressure. Bottom line, the lower pressure won't help if the engine is running hotter than you want it to.
     
  10. BuickV8Mike

    BuickV8Mike SD Buick Fan

    Thank Larry.
    The scenario was hurting my brain.........As usual you explained it perfectly.

    Thanks,
    Mike
     

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