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XPAG COOLINGEric Worpe delivered another of his superbtechnical presentations to the attendees at theMGCC T Register’s ‘Rebuild’ seminar back inthe Spring. I’m most grateful to Eric for takingtime to write up his presentation and for takingphotographs of those wonderfully explicitdrawings of his which help to make the subjectmatter ‘live’.It’s probably as well that we choose not to agoniseabout the lack of efficiency of an internalcombustion engine as we strive to improve theperformance of our T-Types. The realisation thatsome 75% of the fuel’s energy is wasted as lostheat would be quite dispiriting.By way of an example, even running the engine atits greatest efficiency (coinciding with its peaktorque output at around 2,800 rpm) which gives auseful power output of about 34 BHP, theequivalent of 100 BHP is generated as heat; some50 BHP through the exhaust and 50 BHP throughcooling. That’s sufficient energy to enable one tomake a cup of tea every second.heat expands the incoming air and thus reducesthe density of air available for combustion. Drivingon cold mornings usually generates a muchbroader grin, not so much from clenched teeth dueto the cold, but from the enhanced performance ofthe engine. At the same time, heat from theexhaust ports and valves also needs to beremoved, so most of the coolant flow is directedthrough the cylinder head.Engines can be subject to external temperaturesranging from -10 to 40 degrees centigrade andhave to cope with widely varying engine loads fromidling to full power. Trying to maintain a suitablystable engine temperature, despite such diverseoperating conditions, has resulted in someingenious ideas from temperature controlledradiator grill slats to the ubiquitous thermostat.The skirted thermostat in the XPAG is quitesophisticated, compared with everything else,Figure, 1.Internal combustion engines burn fuel attemperatures that can melt cast iron, which ratherfocuses the mind on finding ways to dissipatewaste heat. Removing high levels of heat from aconcentrated source would need massive coolingfins radiating out from the cylinder head if direct aircooling were to be used, plus a generous flow ofcool air. Not a problem with a propeller stuck infront of an aircraft engine.The alternative is to use a high specific heatcapacity liquid to remove heat from the confinedsource and then dump the heat in an efficientlydesigned device for dissipating heat into an airstream. Water, which is usually used as the liquidmedium, boils only slightly higher than theoptimum temperature for engines to run at. Thisleaves little head room for overheating beforesteam is produced.However, although generating steam absorbsconsiderable heat energy for a very small rise intemperature, the steam once generated is unableto absorb much heat, causing a rapid rise in thetemperature of the engine block. Any sudden risein the temperature of the block may not initially beapparent as the steam condenses when in contactwith the coolant in the radiator’s header tank andescapes as water vapour through the overflowpipe under the car.Fortunately, the XPAG’s cooling system wasdesigned with a generous cooling capacity, able tocope with most situations. The design of thecooling system should prioritise the inlet ports andvalves; these need to be as cool as possible as8Figure 1 – Original skirted thermostat.When the engine is cold and the thermostat’svalve closed, all the coolant is directed through theby-pass loop. This maintains sufficient flow toscavenge out any hot-spots in the cylinder headand helps even out any temperature variationswithin the engine block. Differential temperaturesand hence expansion effects, stress the cast ironblock which is weak in tension and thus prone tocracking.As the by-pass loop coolant flowing through thehead reaches the operating temperature of thethermostat, the valve starts to open and at thesame time, an attached skirt begins to shut downthe by-pass loop. Ultimately when the engine isgenerating sufficient heat, the thermostat valve willbe fully open and the by-pass loop closed off sothat all the coolant is pumped through the radiator.However, before reaching this stage, thethermostat apportions what coolant flows throughthe radiator and what portion flows through the bypass loop so as to maintain a constant enginetemperature whilst achieving the maximum flowrate. This is a feature not provided by substituting“modern” thermostats for the skirted type. Figure 2.

directed through two narrow vertical slits behindthe pump, to cool the front of the cylinder walls.This area would otherwise be exposed to theaccumulated heat picked up by the coolant as itflows forwards through the head and on to thethermostat.Figure 2 - Modern thermostats in originalhousing.The need for skirted thermostats in modernengines is obviated by the use of Nodular cast ironin the engine blocks. Nodular iron differs from greycast iron in that the free graphite is present ascompact, isolated nodules rather than as flakesdispersed throughout the cast iron. This reducedpotential cracks propagating along the fault linescreated by the alignment of graphite flakes.This improvement in casting during the mid-1950swas due to the addition of Magnesium and specialheat treatment of the cast iron. Consequently, fullflow through a by-pass loop was no longeressential due to the increased strength andductility of Nodular cast iron.The heat generated within the block is carriedaway by a thermo-syphon action in such a waythat both head and the upper block end up runningat a similar temperature, leading to reduceddifferential expansion and thus less stress withinthe whole head and block, enabling it to be madelighter.This may seem over-protective, but experiencegained with early air- cooled aero engines, showedthat rapid cooling of just part of the engine during anon-powered dive, could lead to fractures in thecylinder head.Figure 3 shows a schematic of the coolant flow.The temperature differences are exaggerated toindicate the problems due to low flow rates.An absence of turbulence in the coolant flow couldencourage wide temperature differences within theengine, so a high flow rate is desirable, confirmingthe need for an effective by-pass loop duringwarming up when the thermostat valve is closed.The thermostat isn’t just there to allow the engineto warm up quickly, but to stabilize the enginerunning temperature at an optimum level and thusreduce stresses within the cast iron block.Original skirted type thermostats are nowavailable, but I’ve discovered that their assemblylacks the correct adjustment that synchronises theopening of the valve with the closing of the bypass loop. Looking through the by-pass portshould reveal the skirt’s top sitting level with theopening. However, the skirt on some appears to sitsome 4mm below the opening, which ultimatelyprevents it completely closing off the by-pass portat the correct operating temperature. The actualthermostat bellows may soon be available as aseparate item for self-assembly in existinghousings.Coolant Flow PathsMuch credit is due to Claude Bailey, designer ofthe X-Series engines. He paid particular attentionto having full flow filtered lubrication and a pumpedcooling system that prioritised the cylinder head,whilst also cooling the area of the bores swept bythe piston rings.Coolant flow is “assisted” by the water pump’simpeller to flow along a duct under the manifold tothe rear of the block, where it rises up to the backof the cylinder head through an elongated hole inthe gasket, if fitted correctly. Some coolant is alsoFigure 3 – XPAG coolant flow.Pumped Flow RatesIt’s at this point that I admit to indulging in somehome spun theory concerning coolant flow rates.Consider the radiator in a near maximum heatgenerating situation, powering uphill on a hot day.Totally T-Type 2, August 2017 9

The thermostat would be almost fully open ataround 74 deg. C. A fast coolant flow would resultin a reduced temperature difference across bothengine and radiator. What heat the coolantabsorbed whilst flowing quickly through the blockwould have little chance to cool down during itsbrief period in the radiator. The whole radiatorwould sit at an elevated average temperature, withits maximum temperature of 74 deg. C. beingdefined by the thermostat.Such a situation actually enhances the coolingability of the radiator as Newton’s law of coolingstates that the degree of cooling depends on thetemperature excess.A slow flow rate would result in any specificvolume of coolant absorbing more heat in theengine and conversely would also allow theradiator to dissipate more heat from the samespecific volume, resulting in a greater temperaturedifferential.As mentioned, large temperaturedifferentials within the engine increase expansionstress levels.Fig. 4 illustrates the benefits of high flow rateswhen the engine is working hard (thermostat fullyopen). It seems six bladed impellers could offerimprovements in cooling.average air displacement of the fan would beabout 5,300 ft/min. In effect, the fan is onlydisplacing air at around 1,300 ft/min.(*) The air velocity through the radiator is anunknown, any air flowing through the radiator finsexpands due to the absorbed heat, resulting in anincreased velocity of the air leaving the radiator.An added bonus is the forward thrust generatedwhich can, with careful design, equal the drag ofthe radiator. This is known as the Meredith Effect.Somehow, I doubt if Morris Radiators would haveconsidered such an issue in their design.Over Heating1. Loss of coolant: leakage or evaporationdue to open vented system.2. Furred-up radiator: Is this due to calciumor rust deposits in the narrow brass tubes?For a thorough de-scale, the radiator wouldneed to be removed and a pumped recirculating flushing loop set up.For rust deposits, mix Phosphoric or Oxalicacid (available as powder) with hot water.For calcium deposits, mix Citric acid(available as a powder) with hot water. Analternative is Muriatic acid or Spirits of Salt,which is a dilute form of hydrochloric acidand therefore hazardous.After flushing with acids, use a neutraliserflushing solution with Baking Soda andthen follow up with several plain waterflushes. The final

to Hagerty International Classic Car Insurance and I must thank this company and NTG Motor Services, who have sponsored the road books and the rally plates for this tour and for the last four. We really do value their support. Loo