Section : Engine

Radiator Cap for 13/60 7lb or 13lb? 

Posted By : Simon Forsdyke, 14-Jun-2007, 07:44pm

• Radiator Cap for 13/60 7lb or 13lb? 

  Posted By : Steve Smith, 14-Jun-2007, 08:02pm

  I'd go with the 13 lb if things are in good shape. If your rad, hoses or water pump are knackered it'll show itself but it's worth it for the improvement in resistance to overheating.

• Radiator Cap for 13/60 7lb or 13lb? 

  Posted By : Chris Taylor, 14-Jun-2007, 08:14pm

  The pressure of the radiator cap will make no practical difference to the temperature the engine runs at. As water boils at a higher pressure as pressure increases, allowing the engine to run at 7psi or 13psi could allow the system to run at more than 100 degrees without actually producing steam, but was actually done to prevent localised boiling at the hottest spots in the block or head. This would ensure consistent heat transfer to the water and prevent vapour bubbles interfering with coolant flow. Since the temperature of the coolant should be in the low 90's, this raising of the boiling point is really only a reserve feature for extreme situations. It is perhaps most often needed just after switching off the engine after a long fast run when the block and head are still fully hot, but coolant circulation, and hence cooling through the radiator stops.
  If the temperature rises too high, steam will be produced and the radiator cap will open to release excess pressure.
  It is the thermostat, the effectiveness of the cooling system in circulating coolant, and the radiator in transferring heat from the coolant to the air, as well as the amount of heat the engine produces, that determines how hot the cooling system runs at.
  If the coolinmg is marginal AND the radiator loses coolant via the overflow pipe, then fitting a 13psi cap might help reduce the coolant loss, but it won't help actually reduce the temperature.

  • Radiator Cap for 13/60 7lb or 13lb? 

    Posted By : Steve Smith, 14-Jun-2007, 08:37pm

    Geez, I knew that was coming....All of the above is of course true. And of little consequence in this case. If the man is running a bit hot and allowing boling in the head and losing water then he's gonna walk home right? Theory is freakin' lovely but in the Mojave desert of Nevada, I'm going with a thirteen pound cap, reduce nucleate boiling, not lose my water and drive home every damned day of the summer.

    • Radiator Cap for 13/60 7lb or 13lb? 

      Posted By : Steve Smith, 14-Jun-2007, 09:18pm

      Hmmmm, someone seems a bit grumpy today. Forgive me. Maybe it's because it's 106F outside today.

    • Radiator Cap for 13/60 7lb or 13lb? 

      Posted By : Chris Taylor, 14-Jun-2007, 11:51pm

      If you do not want a rational explanation of why a piece of advice is given, please just discount it rather than ridicule it. Some advice on the messageboard is, frankly, treating symptoms rather than causes and based on flawed logic or a misunderstanding of design. If others seek to disagree with my suggestion, or have better ideas, at least I have explained why (or tried to). In the UK, a 13/60 with a cooling system in good condition (especially if fitted with a wider radiator) should run happily on either 7 or 13 psi. Changing one for the other will make little noticeable difference. If the car overheats, then I suggest the causes lie elsewhere.

      • Radiator Cap for 13/60 7lb or 13lb? 

        Posted By : Steve Smith, 15-Jun-2007, 00:38am

        Chris, this isn't personal, I apologized for being grumpy.
        So, you demonstrated a very profound understanding of the cooling system and the educational contribution is precious if folks will really read it and understand. Much more likely on your side of the pond then mine. But the original question was whether to fit a 7 or 13 lb cap. My opinion, based on years of driving Triumphs and Jags in the desert, was simply to fit the 13 Lb cap if the rest of the system was sound. Bottom line, round trip versus a theoretical perfect system. Of course a 7 lb cap will work. They drove for years without pressure caps at all. Why did they improve the system with a pressure cap? A greater tolerance of fault. I run a 21 Lb cap on the racer in +100F races. mores better. I run 16 lb caps on the street cars, Jags and Triumphs. The seals will take it and if I don't nucleate and blow off precious water than I'm winning. I'm driving home rather than walking. In the end, that's the point.
        
        BTW, it's 76F in the garage with the swamp cooler running so I'm in a better mood now)

      • Radiator Cap for 13/60 7lb or 13lb? 

        Posted By : Leon Guyot, 15-Jun-2007, 01:35am

        Hi Chris,
        
        I am intrigued, your explainations on this board are always very educational and greatly appreciated by many of we mere mortals
        
        What is your background?
        
        Are you, or were you an automotive design engineer?
        
        Anyhow, thank you again for all your input.
        
        And, for that matter, thanks to Steve Smith and others...a board like this with no input would be pointless, obviously.
        
        All I have to contribute is my experience, and understanding such as it is.
        
        Best Regards
        
        L&195;&169;on
        
        

        • Radiator Cap for 13/60 7lb or 13lb? 

          Posted By : Clive Senior, 15-Jun-2007, 07:50am

          You say overheating, is this all the time, when run hard, after a hard run, and is the needle a bit over normal, firmly in the red or what? And is there water loss??
          
          Avoiding the pressure cap thingy, other causes could well be dodgy temp sender or gauge (I tried 3 or 4 of each on my vitesse, each change gave a different reading) or if the fuel gauge over reads too the voltage stabaliser may be iffy.
          To get an honest temp reading I use a capillary temp gauge now. Easy to test as just dunk the end in boiling water and if it reads 100 it works perfectly!
          
          Other things worth doing include flushing the engine block/heater matrix/rad through properly, ideally using some radflush or whatever. Can help cooling a lot. Also do you have the correct thermostat fitted? Not sure which one you should have, but easy to check and worth swapping if required.
          
          

          • Radiator Cap for 13/60 7lb or 13lb? 

            Posted By : Simon Forsdyke, 15-Jun-2007, 08:02am

            Wow, this has provoked some interesting feedback.
            The car tends to run about 3/4 or slightly more on the gauge and on a hot day is touching the red. The gauge is okay as the fuel one works fine.
            I am losing very little water. I topped it up for the first ime in months yesterday and it only needed about 1/2 pint to get it above the fins which was how I noticed the cap.
            It is hard to tell if it is genuinely running hot or not. Silly question maybe but should the rad be a lot cooler on the fan side?. It is very noticeable on mine if you feel the front on the rad even when it has been idling for a while which also makes me wonder about the flushing suggestion.
            

            • Radiator Cap for 13/60 7lb or 13lb? 

              Posted By : Clive Senior, 15-Jun-2007, 08:32am

              Should be pretty even I think. The water flows top to bottom, so should be the same temp to all fins at the same hight, give or take. Maybe the fan side of the rad is a bit gunged up, stopping water flow? A thought.
              
              And just because the two gauges appear normal only discounts the voltage regulator! As I said, try different guages and you will get different readings! same with temp senders.
              
              I would be inclined to do the flush. Chuck in some radflush. Leave for the recommended time or a bit longer, and then flush everything. (engine cold good idea!)
              Disconnect heater hoses, rad hoses and takeout the block drain plug (rear side of engine, below manifolds). I start by hose in top of rad, then bottom exit and reapeat a few times until no more crud comes out each time. Then each heater hose a few times (valve open!) and see the crud appear! Then the engine block. I push the hose against the drain plug and then the front of engine, both bottom and top hoses. Repeat several times until water come aou freely from everywhere and is clean. The block drain plug is often VERY blocked with crud and needs a bit of wire poking to get things moving.
              All this cannot hurt and may well improve things a lot. Be warned though, if a rad is badly blocked it may need a recore, but try other (cheaper) fixes first. I still like the capillary gauges!

              • Radiator Cap for 13/60 7lb or 13lb? 

                Posted By : Paul Dearing, 15-Jun-2007, 10:02am

                I suspect the thermostat every time. They are in a hostile environment and are a fairly low tech design in the first place.
                
                If an engine is running hot, the reasons can in basic terms be broken into two areas: too much heat or not enough cooling.
                
                Too much heat can be a mixture/timing thing but there are usually other symptoms such as power loss involved as well.
                
                If there is not enough cooling it can be
                - not enough coolant (fairly obvious)
                - coolant not going around engine properly. Blockage in water channels, dodgy thermostat, water pump failure. I had a water pump that looked fine but the impellor was separated from the shaft. Collapsed hoses.
                - radiator not transferring heat away. Blocked cores, blocked vanes.
                
                You can probably thinks of others as well.
                
                I keep an old thermostat that has been fixed in the open position. Useful for testing out problems. Its also good practice to remove the thermostat housing every so often as they tend to sieze onto the studs and a good dosing of grease/vaseline each time stops this happening.
                
                In your position, I would replace both the theromstat and the cap, flush the system thouroughly and check the hoses. Use a paint brush and detergent and clean out the radiator cooling vanes. Do NOT use a jet wash on the radiator. I did once and it disintegrated (the rad, not the jet wash!)
                
                If the replaced components are OK, then you have a spare which is always good.

              • Radiator Cap for 13/60 7lb or 13lb? 

                Posted By : Peter & Mick Lewis, 15-Jun-2007, 10:05am

                and dont forget to check that the stand off of the seal is correct for your rad if its a deep seat and short cap it wont seal so any psi spec. is irrelevant, a deep cap on a short seat wont fit so its easy to have the wrong one and not realise it,,,just another thought ...I agree back to basics as if the system is working OK you should be able to drive for miles with the cap in the glove box and not loose any coolant and if you look down the spout you should see the coolant flow and stop as the stat opens and closes as it modulates the flow worth a quick look Peter