[Tony Wheeler]

Fabulous to see the '44 memo Andrew. Wartime directives add so much to our understanding and appreciation of military vehicles and their history. This one is particularly interesting and useful for owners as you say. Evidently the 10 psi bypass valve did not provide sufficient pressure relief with the 70 psi motor when cold. The same issue would arise for early carrier owners fitting a post '44 motor, and such owners may be wise to follow the advice in this memo concerning cold start up. Adjusting the bypass valve to less than 10 psi would seem like the obvious solution, and presumably the British tried that initially, but the limitation then becomes the passage through the valve, which is quite small and probably insufficient to allow cold viscous oil to flow fast enough to cope with the extra 30 psi produced by the post '44 motor - thus giving rise to greater than 10 psi pressure drop through the cooler, with the attendant flexing stress that would cause, concentrated at the return bend. Furthermore, adjusting below 10 psi would lead to bypass flow when the oil is hot, thus greatly reducing flow through the oil cooler when it's most needed. The next solution would be to fit a higher capacity 10 psi bypass valve, which would cope with the required flow when the oil is cold and viscous, without effecting flow through the cooler when hot. However, while that would certainly keep the pressure drop through the cooler pipes to no more than 10 psi, it would not reduce the ABSOLUTE pressure within the cooler pipes, and it's this ABSOLUTE pressure which the British seemed intent on reducing, to somewhere below 70 psi. Perhaps they considered the potential existed for coolers to burst at normal operating temperatures also, and sought to guard against that possibility. However it's also true that by utilizing less than the full 70 psi pump output the pressure would be more constant through the rpm range, as the pump is always operating well within its capacity. In other words the external PRV is not only reducing absolute pressure in the cooler pipes, it's also dampening surge pressure, eg. during gear changes. As such it's a very well conceived mod IMO, which I'd expect to provide excellent protection for the oil cooler, superior even to the the standard 40 psi system.

It's also clear the British were intent on improving oil temperature control, otherwise they would have simply left the 10 psi bypass valve in place, rather than go to the trouble of fitting a thermostat. This is a vast improvement of course, particularly in cold climate use. Note however that British removal of the 10 psi bypass valve means ALL the oil to the motor must pass through the oil cooler first. That would explain why your oil cooler remains cold for so long Andrew - it's the engine oil itself which is remaining cold. In other words the cooler is cooling the engine oil unnecessarily. That can't be avoided as there needs to be sufficient flow to the motor at all times, which means the thermostat installed by the British must NOT restrict flow excessively when cold. That makes it even more important to fit thermostats in the heads, otherwise the oil will take far too long to reach operating temp, causing extra wear in the motor. Optimum oil temp is 220F or thereabouts, to drive off moisture accumulation due to crankcase condensation, and provide optimum viscosity for lubrication, and such temps cannot be reached until the motor itself reaches full operating temp. Of course if the oil cooler thermostat is stuck fully open it would exacerbate the problem, and this would be worth checking in due course. Firstly however you need to ensure thermostats are fitted in the heads and operating correctly.

Given that ALL the oil is flowing through the cooler at ALL times, the INCREASED flow observed into the breather standpipe when cold is explained by REDUCED flow through the motor when cold, due to oil viscosity. When the oil heats up and gets thinner it will flow through the bearings much faster, thus reducing overflow to the breather standpipe. Once again this another reason to ensure thermostats fitted in the heads, as it will greatly reduce the duration of high oil flow to the breather standpipe.

It's worth noting in this discussion that modern multigrade oils do not present viscosity problems when cold, certainly not to the same extent as single grade wartime oil, which in severe cold required dilution with fuel prior to next start up. I believe this applied also in desert use, due to cold overnight temperatures, with high daytime temperatures precluding the use of 10 SAE grade engine oil.

I agree with you concerning British PRV setting, certainly that would be my own preference in this situation, for the reasons you mention. As you say they had the expertise and no doubt consulted Ford on the question, plus they were able to trial in service. I'd be interested to see what pressure it runs at full operating temp, seems likely to me they would have chosen 50 psi or thereabouts, rather than drop it all the way back to 40 psi like the pre '44 motor. The memo suggests it was only a cold start up problem, and they'd be seeking to minimize overflow to the breather standpipe at normal operating temp, no more than necessary to solve the cold start up problem. Of course it's possible the use of modern multigrade oil would be sufficient in itself to solve the problem, and allow you to run the full 70 psi, but why take the risk, especially in your cold English climate! It is after all a 70 old oil cooler with a known tendency to burst during cold start up even when brand new.

Once again a fascinating MLU thread which has furthered my carrier education, which I confess is sadly lacking!