headers and heat soak

[JJDraper]

Don't forget that at low rpm the pump is shifting a lot less water... I have no data on the volume/rpm curve on a Lotus water pump, but if its anything like commercial pumps (heating etc) the volume shifted will fall off a cliff at low rpm. At high rpm you will get cavitation - like everything, there is a balance or sweet spot. Idling is not the sweet spot for cooling...

Some physics - heat transfer is related to temperature difference (Delta T - heat moves from hot to cold), the insulating properties of the material and time. The greater the Delta T, the greater the rate of heat transfer. The higher the insulating properties of the interface, the slower the rate of heat transfer. More time - more energy, as heat, is transferred. In an engine, all these factors are at work with varying effects, across multiple interfaces. Combustion gases to metal; metal to coolant; coolant to air, along with physical translocation of the transfer medium. The hot gases are moved out of the engine, the water circulates and the air passes over the metal radiator (a misnomer as most heat transfer is conductive in this case).

Sludge and scale acts as insulation between metal and water, as can carbon and deposits in the combustion chamber, allowing heat to build up in the metal. Slow water flow reduces the rate of heat translocation from the head to the radiator allowing heat to build up in the coolant in head. Too much heat and flash steam is produced, which acts as another layer of insulation, making heat build up in the metal even more likely.

All these factors (and more - don't forget oil) contribute to the temperature observed via the gauge. This is not the only measure of engine thermal health. You can hear, see and smell other things - although, if its got to the smell stage its getting bad...

Getting a bit carried away here.. back to the case in hand. Will the temp of the headers affect the observed coolant temp? Most unlikely; reason? Delta T of the underbonnet temp to head is low and may be negative, i.e head is hotter than the air - there is a radiative component, but this is likely to be small, unless the pipes are red hot, but in this case, the air movement will be high - if not, turn ignition off, run away and find a fire extinguisher. Most likely reason for high observed coolant temp at tickover is poor coolant circulation leading to local build up of heat in the head. Blipping the throttle helps, as does an electric fan, but in general not much you can do apart from fitting an auxiliary pump.

Happy New year all

Jeremy

[Elanconvert]

or fit an ewp.........

fred

[jimj]

Never mind the physics, if radiant heat from the exhaust manifolds was affecting temperature gauge readings, someone else would have noticed, sometime in the last 40 years.
Jim

[RichardHawkins]

Bob,

I think I have read all the posts on this subject, and agree that heat from your exhaust is unlikely to be the cause.

I suggest that you remove the sender from its position in the head, and dip it into a container of hot water water with laboratory type thermometer (mercury or alcohol in glass type). The temp gauge in the car and the thermometer should be nearly the same.

This way you should know whether you have hot coolant or a suspect coolant temperature measurement.

Happy New Year

Richard Hawkins

[gus]

Find someone with one of the calibrators shown here:

http://www.britishcarforum.com/bcf/show ... -very-well