From: Headquarters, Air Defence of Great Britain.

To:      Headquarters, Allied Expeditionary Air Force.

Date: 16th June 1944.

Interim Report – Service Trials of Merlin 66
Engines operating at + 25lbs. Boost Pressure.

1.      Early in March, 1944 Service Trials with Spitfire IX L.F aircraft operating at + 25 lbs./sq.in boost and using 150 grade fuel were commenced at Milfield. Nine aircraft were involved, six being fitted with S.U. Fuel Injection Pumps and the remaining three with standard Stromberg carburetors. The trials were extended in the first week of April to include an aircraft with an S.U. Pump at the A.F.D.U. and twelve standard Spitfire IX L.F. aircraft at No. 17 A.P.C Southend. At this time, the only plugs cleared for operation at these high boost pressures were the Lodge RS.5/5 and the K.L.G. RC.5/3 types. It was found necessary to service these types of plugs at 15-hour intervals, because of lead fouling that was experienced. Previous experience with 130 grade fuel had proved that the Lodge R.S. 5/6 type of plug was less prone to lead fouling than the types mentioned above. A request was made that the R.S. 5/6 plug should be cleared for use at + 25 lbs. boost in an endeavor to improve on the servicing period of 15 hours. This approval was given by the Ministry of Aircraft Production on the 6th April 1944 and since that time the R.S. 5/6 type of plug has been used.


          In the early days of the trials, there was one case of an engine developing violent vibration at high boost. This engine was returned to Messrs. Rolls-Royce, but they were unable to reproduce the symptoms and the engine was returned to service and has since run satisfactorily. During the week ending the 14th June 1944, one case of backfire occurred at Southend, which resulted in excessive damage to the intercooler and air intake cowling. The cause of this backfire has been established and was due to collapse of the boost capsule.


2.      In view of the success of the trials at Milfield, Wittering and Southend, the trials were extended on the 3rd may 1944, to include all aircraft in No.1 and 165 squadrons, at Predannack. During the first week of the trials at Predannack, four cases of backfire occurred. In all cases, it was found that the fuel pressures were too low and it was assumed that this was the reason for the backfires occurring. Fuel pressures in all aircraft were adjusted to 16lbs and it was hoped that this would effect a cure. Since that time four further cases of backfires have occurred and the cause of these remains obscure. The engines have been returned to Messrs. Rolls-Royce for investigation, but bench tests have been unable to reproduce the backfires or to provide a reason for them. Messrs. Rolls Royce now propose to install some of these engines in aircraft and endeavor to reproduce the backfires and establish the cause.


3.      1,200 hours flying has been done at Milfield, Wittering and Southend and 1,900 hours flying has been completed in the squadrons at Predannack. Experience with the R.S. 5/6 plug to date, indicates that these plugs will be satisfactory for at least 30 hours, and it is possible that with further experience this period will be extended to the normal service time of 40 hours.


4.      In an endeavor to eliminate plug fouling and backfires, the following action was taken:

  • (i)  Pilots were instructed to clear their engines every 15 minutes, by opening up to 2,800/3,000 R.P.M. and 12lbs boost.
  • (ii)  Great care is being taken in the fitting of drop tanks to ensure that air cannot be sucked into the fuel system.
  • (iii)  12 Modification sets which chamber “D” of the Stromberg carburetor have been fitted. This modification is standard practice on American built Merlin engines and it was thought that its introduction would help to eliminate air in the fuel system. To date, there is no definite evidence that this modification is essential, and it is significant that cases of backfire have occurred in engines, which this modification was embodied.
  • (iv)  An improved volute casing drain has been fitted to 12 aircraft. It is known that during idling conditions, that fuel and oil build up in the supercharger casing and when the throttle is opened a rich oily mixture results, which soot’s the plugs and aggravates lead fouling. The improved volute casing drain rectifies this state of affairs and a recommendation has been made that this modification should be applied to all aircraft.
  • (v)  The gap of sparking plugs has been increased to .020”-.022”.
  • (vi)  A new type of sparking plug (lodge 949/5) has been produced for use in Merlin 66 engines at high boost pressures. This plug has a similar heat factor to the R.S 5/6 plug, but has a more robust body and incorporates a different central electrode assembly. 4 sets of these plugs are at present on trial at Southend and if these preliminary trials are satisfactory, the plugs will be brought into use at Predannack.
  • (vii)  Samples of 150 Grade fuel have been taken from Predannack and a test report on these samples is expected in the near future.
  • (viii)  The existing throttle quadrant is thought to be unsatisfactory in that the movement allowed between the closed position and the +18lbs boost position is to small and further the arrangements for gaiting the throttle to 18 lbs boost is unsatisfactory. A request has been made for an improved quadrant to be fitted so as to give the pilot a more sensitive control of his engine.

    • 5.      All pilots report most favorably on the value of the high boost pressures and state that the manual supercharger change over switch is acceptable and presents no difficulties to the average pilot. Attached and Marked “A” is a copy of a report received from the W/Cdr. Flying at Predannack. This report can be taken as representing the views of all pilots and of the Air Staff at this Headquarters.


    6.      Attached and marked “B” is a summary of the trials with 150-grade fuel as at the 14th June 1944.


    7.      The modifications necessary, and the man-hours involved, to modify aircraft to use + 25 lbs boost are shown at Attachment “C”.


    8.      To date, 9 aircraft have been lost over enemy territory, but there is no evidence to show that any of these losses were due to engine failure. It must be assumed, however, that the enemy are now in possession of the aircraft and a most cursory examination of the cockpit would indicate that +25 lbs boost pressure was being employed.


    9.      From the forgoing, it will be seen that pilots are impressed by the performance resulting from the use of 150-grade fuel and consider that the time is ripe for its general introduction. This view is not shared by the Technical Staff of this Headquarters, who consider that before the fuel is introduced on a large scale that the causes of backfires must be established and that at least 12 engines should complete 200 hours each, so that the effects of the high lead content of the fuel be known.


    Air Vice-Marshal
    Air Officer i/c Administration.
    Air Defence of Great Britain.