Whilst we can never truly know the true extent of the head gasket failure endemic in the MGF (and indeed in the Landrover Freelander and Lotus Elise), it is interesting to see that Rover Powertrain Ltd (the company responsible for building and developing Rover's road car engines) has not been sitting on its hands in recent years - and indeed there have been a number of interesting developments in the cooling system department, particularly with the introduction of the MG TF. The most significant of these is the Pressure Relief Thermostat (also known as the PRT).

Quick links to features on this page: the PRT; aftermarket remote thermostats; electric water pumps

The pressure sensitive thermostat

How does the PRT work?

Conventional thermostats - as fitted to all MGFs and early MGTFs - open purely in response to temperature on the inlet side. As temperature of the engine increases, the thermostat bulb moves to allow flow through the thermostat to permit the entry of cold water into the engine. The PRT is a little different. A PRT thermostat, in addition to opening in response to temperature, will also open its value in a manner related to engine speed - which is rather given away by its name (PR = pressure relief). In effect, this is predicting a temperature rise, as increasing engine speed means that the engine is producing more power, and more power invariably means more heat that the cooling system will have to deal with. This opening of the PRT is accomplished by a simple comparison of inlet and outlet pressure, which is achieved mechanically by a balancing spring. When the engine's water pump spins faster (with higher engine rpm - remember it is mechanically driven off the cam belt) its inlet pressure is pulled down a bit and its outlet pressure rises. Therefore a pressure gradient through the entire cooling system develops and increases as engine speed increases. As the thermostat sits in the middle of that system, it therefore sees a higher pressure difference as speed increases. In order to minimise pump inlet depression the PRT therefore opens at a trigger pressure drop (determined by the aforementioned valve spring) - which for a particular system this pressure drop will be found at a fairly constant engine speed. Different engines will use different springs to suit the application - so using a PRT from a completely different engine will not be advantageous, but one developed for any 1.8 litre K-series ought to be fine for any other...

How does the PRT fit into the cooling system?

The figure below summarises some of the changes that have come about with the introduction of the TF, and with the pressure sensitive thermostat. It is interesting to note that the original thermostat housing is kept, albeit with an open 'ring' in place of the original 88C thermostat, whereas the new thermostat is mounted remotely, close to the under body coolant pipes.

Installing this new thermostat system IS an option for earlier cars, but the amount of additional pipe work is extensive, and potentially expensive. A more economical answer would be to locate a scrapped TF (or even a recent 4-cylinder petrol Freelander that uses the same 1.8 K-series as our beloved MGs) and obtain the complete cooling system from that car.

How do I fit a PRT to my MG?

I haven't yet undertaken this task, nor am I aware of anyone who has yet published a full on-line guide (let me know if you've found one though!) - but Carlo Santeroni, an Elise enthusiast, has an excellent website that covers the PRT installation very nicely for the Elise which is similar (but definitely not identical!) to the MGF that could be used as a useful guide. Click here.

An alternative is to mount a thermostat remotely - near the radiator. This has a number of advantages, not least because it eases access to the thermostat, as all you have to do is remove the front bulkhead trim from under the bonnet to view it! Read more here - along with the tests that prove that it works! We aim to repeat some similar tests on the PRT too!

Aftermarket alternatives

An aftermarket option that could offer a very similar advantage is the QED remote thermostat. (Thanks to Thierry Zoller for the images below)

Remote thermostat kit, as supplied by QED; a very similar kit is offered by Elise Parts - see Engine specialist listing for links

A close up of the remote thermostat housing

The remote thermostat kit is the kit that Simon Scutham was instrumental in developing for his racing Elise (see more here).

A rather obvious question is: "Which thermostat option is best for my car?" Unfortunately, I can't answer this at this time, although there are plans to further investigate the thermal cycling problem, and we may look into the relative merits of the PRT and remote thermostat.

The electric bypass water booster pump

Another potential cause of head gasket failure, and an important cause of cavitation in an alloy head, is coolant boiling. Coolant boiling is a phenomenon that is especially prone to occur after the ignition has been switched off. It is not a particularly common problem on cars used on the road, but those cars that have been adapted for "Fast Road" use, or those that partake in trackdays are more prone to this problem. In essence what happens is that once the engine is stopped, the mechanical water pump that is driven from the cam belt, also halts. Result: water becomes stagnant in the engine - the only circulation that occurs is due to thermosyphon (the way that coolant was often circulated in pre-war Vintage cars - so quite a reasonable way of moving water - and this is the reason why boiling tends not to be a problem on 'unstressed' engines). What can then happen in an engine that is very hot, the coolant may actually boil in certain hotspots around the head, despite the high pressure of the coolant system and the 'antifreeze' additives that raise the boiling point of the water comfortably above 100C. When this happens, steam becomes trapped in small pockets, and can corrode or soften the aluminium head and fatigue the head gasket.

So what is the solution? Interestingly, Lotus foresaw problems with the very high performance derivatives (VHPD) of the Elise and Exige. Although Lotus kept the high capacity mechanical water pump (capable of moving over 100L/min), they added an additional 'booster' pump in the bypass circuit. This is pictured above right.

Unfortunately, the pump available from your local Lotus dealer is prohibitively expensive - something in the order of 150-200 GBP... However, salvation may be at hand in the form of the significantly cheaper Davies Craig Electric Booster pump...

The Davies Craig Electric Booster pump

Davies Craig is an aftermarket manufacturer of automotive water pumps, who's current aim is to develop water pumps with the ultimate aim of selling them as OE to manufacturers. Unfortunately, the capacity of electric water pumps have not yet reached the level achieved by engine-driven mechanical pumps on modern engines, although Davies Craig pumps have reportedly been used sucessfully on a number of 'classic' racers (Triumph Spitfires being one example). Dave Livingstone and I were approached by an importer of the Davies Craig, MAW (http://www.MAWsolutions.com/) seeking to receive an endorsement from the MGF Register for the use of one of their pumps in MGs. We weren't entirely happy to do this without proper consultation with specialists - and we felt that their then solution could be improved. A little more research however indicated a staggering similarility between the Bosch manufactured pump specified by Lotus and the Davies Craig electric booster pump (see technical drawings, opposite right)... We were suitably intrigued by this, and in collaboration with Techspeed are currently investigating the installation of the Davies Craig in a fashion similar to that employed by Lotus...

It is not complete plain sailing however, as there are some important differences between the two pumps. Whilst both pumps work on 12volts over a similar current range, and work at the same peak pressure (10kPa, or 0.1bar), maximal flow characteristics show that the Bosch is capable of pumping 20L/min against the Davies Craig EBP's 13L/min.

A comparison of the two pumps current/ pressure/ flow characteristics are shown in the figure opposite. The question is: are these differences going to have a dramatic impact upon the effectiveness of a Davies Craig pump on heat soak following ignition-off?

This is the question that Dave and I are keen to get answered - and hopefully will be reporting on soon...

Water pump logic - as used on the Lotus pump - is shown below...

Read more here...