Javelin Cooling

[David Morris]

Hi All,

I recently watched a YouTube article, where the presenter was dealing with a classic car that was overheating. He found that a previous owner had removed the thermostat.

He demonstrated that, when he fitted a new theromostat, the engine temperature returned to normal during subsequent driving. He put this down to the fact that the flow of the cooling water had been too rapid without the thermostat, and the hot water flowing through the radiator had insufficient time to become cooled. When he replaced the thermostat, his hypothesis was that the water flow rate was then reduced, with the result that the water now being returned to the engine was being cooled correctly by the radiator. Anyway, it seemed feasible and I wondered if this was a logical suggestion?

Turning to the Javelin's cooling system, I remember that Jowett's described the system they had designed for the Javelin was, in their words, a 'Thermosyphon, Pump-assisted, Cooling System'. I believe that Jowett's had never introduced a water pump in any of their previous designs, so this was a new solution for them? In hindsight, I think they should have revisited the design, as the pump is probably not the best it could be? To me, it always looked like an after thought?

All the best,

David

[Keith Clements]

I find the hypothesis hard to validate. But without knowing the exact configuration of the cooling system it is hard to investigate.

The gauge sensor in the Javelin is situated opposite to the inlet so is hit by the water straight from the pump. This would be hotter than normal with the thermostat out, since the thermostat would restrict the flow until the engine had reached temperature. The flow is still more restricted with the thermostat in and the thermostat open than it would have been with the thermostat out, so the indicated temperature would probably be higher with the thermostat out. This could be interpreted as the engine is overheating.

The job of the water is to take the heat from the cylinders and radiate or convect the heat out of the engine compartment. It will 'obviously' carry more heat if the flow rate is increased. Or is it obvious? The heat transfered from the cylinder will depend on the temperature difference (Fourier's law), thus the temperature of the incoming water. The heat radiated or convected by the radiatior will depend on the average temperature between bottom and top of radiator. If the flow rate is small the temperature at the bottom of the rad will be less than if the flow rate is high since the water will have more time to loose its heat as it passes through the rad. Conversely, if the flow rate is high, the average temperature of the radiator will be higher so more heat should be expelled.

I guess this balancing act is possibly something Gerald did not fully deal with. He did say to me that the cooling system was his worse bit of design but blamed Jowett for constraining him.

There is one other consideration. I have used the heater to expell more heat when the engine is overheating, so possibly flow through the heater is less with the thermostat out.

And yet another....the water pump on the Javelin is badly positioned on the hot side. The back pressure of the thermostat might reduce cavitation both in the pump and at the cylinder liners.

[David Morris]

Hi Keith,

Yes, agree completely. Better to leave the thermostat in, rather than be tempted to take it out.

Interesting that Gerald considered the Javelin/Jupiter cooling system to be his 'Friday afternoon' bit of design work! He must have done the Rear Timing Case design on Friday morning then?

All the best,

David

[trymes]

I can say that my MGA Twin Cam came to me with no thermostat installed, and it indeed overheated. I installed a thermostat and it no longer overheated. However, in that case, I think the issue was that the lack of a proper thermostat meant that the cooling system bypass was not blocked when the engine was hot. Thus, hot water did not pass through the radiator and directly returned through the head.

I don't think the bypass is part of the Javelin cooling system, but I mention it as a general note, as many British cars do have a bypass built into the cooling system (the MGC comes to mind).

Tom

[David Morris]

Hi Tom,

Thanks for your post. Yes, as Keith says, on my Javelin, I too have used the car's heater to lose some of the heat, before I changed the radiator. Wow, didn't we get hot! However, replacing the radiator with a five row one solved the problem and she never exceeded 75 degrees C thereafter. Once in good condition, the Javelin's cooling system works really well.

All the best,

David