Section : Cooling

Which Pressure Cap? 

Posted By : Martin Bryant, 26-Mar-2005, 10:00pm

• Which Pressure Cap? 

  Posted By : Doug Paterson, 26-Mar-2005, 10:37pm

  
  13lb

  • Which Pressure Cap? 

    Posted By : Martin Bryant, 26-Mar-2005, 10:46pm

    oh, what would happen if I don't change it back to 13lb?

    • Which Pressure Cap? 

      Posted By : William Davies, 26-Mar-2005, 11:34pm

      The engine will run hotter, never a good idea on a 1500.
      Which radiator did you fit, full width or a (standard type) small one? Either way you should use a 13lb cap, but it might be more urgent if you have a small radiator....
      Cheers,
      Bill.

      • Which Pressure Cap? 

        Posted By : Martin Bryant, 26-Mar-2005, 11:47pm

        I fitted a full width rad, now I understand!
        
        Why did I chuck the old cap in the bin
        
        
        Thanks guys
        
        
        Martin

        • Which Pressure Cap? 

          Posted By : Tim Everitt, 01-Apr-2005, 01:31pm

          I'm not sure I entirely agree with everyone here. There is no reason I can think of why the engine should run hotter with the lower pressure cap. What will happen is that it will boil over at a lower temperature - not the same thing at all. I think if you have a brand new radiator and the rest of the engine is in reasonable nick I'd see how it goes with the lower one. When I bought my last radiator but one I was told to fit a lower pressure cap to avoid putting too much strain on ageing hoses and joints. So I did and it worked fine for me for a good number of years.

          • Which Pressure Cap? 

            Posted By : Martin Bryant, 01-Apr-2005, 06:14pm

            "What will happen is that it will boil over at a lower temperature", that's true.
            
            There is no harm in using it is there.
            
            
            Thanks Tim
            
            Martin

            • Which Pressure Cap? 

              Posted By : John Davies, 01-Apr-2005, 10:05pm

              Martin, All,
              I think there is good reason why a higher pressure cap should be used and it's all down to the physics.
              
              A liquid boils when its vapour pressure reaches atmospheric pressure. At that temperature, bubbles of pure vapour, in this case water vapour, appear in the liquid. Water is a good conductor of heat, but water vapour, any vapour, is not. So less heat will be conducted away from the block, into the circulating coolant and out via the radiator. Also, as those above have pointed out, the bubbles push all the water out of the radiator, which is not good for cooling either, even if it goes into the catch bottle, as it then can't take part in heat exchange.
              
              Very early in the history of automobiles, the significance of this was realised and cooling systems were pressurised. Now that the water is under a higher pressure than atmospheric, it will not boil until it reaches a higher temperature, preserving it's heat conductivity. That higher temperature is atmospheric plus the "13lbs/sq.in" on the cap. Normal atmospheric pressure is 14.7psi, so the boiling point of water under a 13lb cap will be about 250F or 121C (Compare 212F & 100C)
              
              And I'm sorry, if your hoses etc. are so old and manky that they won't stand that pressure, you need new hoses, not a lower pressure cap.
              
              JOhn

    • Which Pressure Cap? 

      Posted By : Doug Paterson, 26-Mar-2005, 11:37pm

      If you keep with the 7lb cap, it will allow water to be expelled into the expansion bottle sooner (And in greater quantity) than the 13lb bottle and essentially this can have the effect of allowing the car to run hotter - cooling on a Spit 1500 isn't great at the best of times so sticking with the 13lb cap is best.

      • Which Pressure Cap? 

        Posted By : Stewart Sloan, 31-Jul-2005, 09:42pm

        Interesting discussion but what about my overheating spitfire 4 which is designed to have a 7psi cap?
        
        Stewart

        • Which Pressure Cap? 

          Posted By : William Davies, 31-Jul-2005, 11:11pm

          Hi Stewart,
          Plenty of Spitfires run perfectly happily with a 7lb cap, it was just one factor in the overall design of the cooling system. There are any number of reasons why it might be overheating, what have you eliminated to date?
          Cheers,
          Bill.

          • Which Pressure Cap? 

            Posted By : Stewart Sloan, 05-Aug-2005, 05:45pm

            Havent really started yet, due to gainfull employment getting in the way! Intend to check thermostat and have been advised today that inspite of no external evidence, pump impellors do corrode, so it is worth removing. Previous owner (recently) fitted a container to collect the "overflow" but it is currently set too low so the hose does not remain in the water. Will change this also. Any other suggestions or priorities?