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In the past, there were many different grades of aviation gasoline in general use e.g. 80/87, 91/96, 100/130,108/135 and 115/145. However, with decreasing demand these have been rationalised down to one principle grade, Avgas 100/130. (To avoid confusion and to attempt to eliminate errors in handling aviation gasoline, it is common practice to designate the grade by just the lean mixture performance, i.e. Avgas 100/130 becomes Avgas 100). More recently, an additional grade was introduced to allow one fuel to be used in engines originally designed for grades with lower lead contents: this grade is called Avgas 100LL, the LL standing for 'low lead'.


80/87 was(is) red
100LL is blue
100/130 is green
and
100/130 is purple
 
 
The use of 100/150 fuel started with the 8th Air Force in March of 1944. Selected squadrons were used to test the fuel. Despite the increased engine maintenance, that is, increased fouling of spark plugs, all 8th Air Force fighter groups were converted to the fuel between July and late September of 1944. The fuel was given the nickname "purple passion", I guess due to the fuel's color. To reduce the plug fouling, ethylene di-bromide was added to the fuel in December of 1944. The 355th Fighter Group was given the task of testing this fuel. The test ended in February of 1945 and was declared a success. By March of 1945 all 8th Air Force fighter groups were using this new fuel, nicknamed "pep". This fuel mixture did reduce the number of fouled spark plugs, at the expense of incresed valve wear. Valve adjustments were required after roughly 25 hours of flight instead of the usual 50 hours. By the end of March 1945, the fighter groups were asking for the old 100/130 grade fuel, but what was available went strictly to the bombers.
It doesn't seem like anyone makes 100/150 grade fuel anymore, but there are still a few companies who manufacture 115/145 grade fuel (developed near the end of World War II) for air racers and vintage aircraft. One of those companies has an even higher grade fuel (127/160) for air racing.

 

Many parts of a successful aircraft are easily visible—the control surfaces, engines, wings, fuselage, and structure for instance. But the fuel that powers the engines is equally important, though not nearly as visible. Aircraft engines, from powerful piston engines to jet turbines, have always required a more sophisticated form of fuel than most ground vehicles, and the technological development of this fuel to power the engines is just as significant as other technological advances.

For the first few decades of flight, aircraft engines simply used the same kind of gasoline that powered automobiles. But simple gasoline was not necessarily the best fuel for the large, powerful engines used by piston-driven airplanes that were developed in the 1930s and 1940s.

Before World War II, Major Jimmie Doolittle realized that if the United States got involved in the war in Europe, it would require large amounts of aviation fuel with high octane. Doolittle was already famous in the aviation community as a racing pilot and for his support of advanced research and development (and would later earn even wider fame as head of the 1942 B-25 bombing raid on Tokyo). In the 1930s, he headed the aviation fuels section of the Shell Oil Company.

Fuel is rated according to its level of octane. High amounts of octane allow a powerful piston engine to burn its fuel efficiently, a quality called "anti-knock" because the engine does not misfire, or "knock." At that time, high-octane aviation gas was only a small percentage of the overall petroleum refined in the United States. Most gas had no more than an 87 octane rating. Doolittle pushed hard for the development of 100-octane fuel (commonly called Aviation Gasoline or AvGas) and convinced Shell to begin manufacturing it, to stockpile the chemicals necessary to make more, and to modify its refineries to make mass production of high-octane fuel possible. As a result, when the United States entered the war in late 1941, it had plenty of high-quality fuel for its engines, and its aircraft engines performed better than similarly sized engines in the German Luftwaffe's airplanes. Engine designers were also encouraged by the existence of high-performance fuels to develop even higher-performance engines for aircraft.

A major problem with gasoline is that it has what is known as a low "flashpoint." This is the temperature at which it produces fumes that can be ignited by an open flame. Gasoline has a flashpoint of around 30 degrees Fahrenheit (-1 degree Celsius). This makes fires much more likely in the event of an accident. So engine designers sought to develop engines that used fuels with higher flashpoints.

Avgas Grades

Avgas 80/87 is used in low compression ratio aircraft engines, and contains little or no lead (up to .5 grams of lead per gallon is allowed, but none is required). It is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.

Avgas 100/130 can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming hard to find.

Avgas 100 LL (the LL stands for "low-lead") contains two grams per gallon, half the lead contained in the avgas 100/130 it was designed to replace. It has research and motor octane numbers very similar to 100/130 avgas. The color is blue. This product sometimes has a high level of aromatics.

Avgas 115/145 was developed for high performance piston aircraft engines used in World War II and the Korean conflict. It is very hard to find today due to lack of demand, and is usually only produced on special order. The color is purple.

 
100/150 Grade fuel

Initial Testing and Proposals

During 1942 and 1943 the British started testing fuels that allowed for higher engine powers than were possible using the standard fuel of the time.

Testing of a Spitfire IX by Rolls Royce, Hucknall in October 1943 determined:

  • The increase of boost pressure to 25 lbs/sq.inch provides a considerable improvement in the low altitude performance of the Spitfire IX aircraft, the necessary modifications to achieve this being comparitively simple.

The same aircraft was tested by the Aircraft and Armament Experimental Establishment (A.& A.E.E.), Boscombe Down in November 1943, the conclusion being:

  • An increase of about 950 ft/min in rate of climb and about 30 mph in all-out level speed is achieved by the increase of boost from +18 lb/sq.in. to +25 lb/sq.in.2

The USAAF and Rolls Royce discussed the probable performance gains to be anticipated in the P-51B with improved fuel (160 grade) during a conference at Rolls Royce, Derby on October 23, 1943. These discussions took place six weeks before the P-51B first went operational. It was noted "that a maximum speed of about 450 MPH is predicted at 21,000 feet - optimum high critical altitude for use against the present FW-190 fighters.

Testing continued, with an Air Ministry Memorandum on Introduction into Service Use of 150 Grade Aviation Fuel of 25 January, 1944 summing up the position with respect to performance, production and requirements. The Memorandum concluded in part:

  • On the basis of the figures used, it is apparent that the earliest a substantial quantity of fuel can be allocated, i.e. 40,000 or 60,000 tons for Home Based Fighters is May/June. This would still permit the allocation of one month’s consumption and one month’s reserves, i.e. 73,000 tons to A.E.A.F. Fighters in June.

USAAF Material Command held a "Conference on National Advisory Committee for Aeronautics Test Program to Investigate 150 Grade Fuels" on 27-28, January 1944. It was concluded that "The program outlined should permit conclusive data to be obtained and should indicate the relative advantages of the various high octane fuel components for the preparation of satisfactory rich and lean rating fuels. It should also indicate the military value of these fuels for long range patrol or bombardment operation". It was recommended that "the program outlined should be carried out as expeditiously as is possible".

Using the fuel for Operation Overlord, the invasion of France, was being actively considered at the highest levels as of February 1944. General Dwight D. Eisenhower, commanding SHAEF, wrote to Army Chief of Staff General George C. Marshall and General Henry H. Arnold, head of the Air Force on 11 February, 1944 of the importance of using 150 fuel to the "fullest possible extent".5 On 13 February, 1944 General Arnold replied that Xylidine, a necessary component for 150 grade fuel production, was being drummed for shipment and that information available to him "indicates satisfactory operation of Merlin and possibly R-2800 engines on this fuel".

USAAF Testing

Material Command

As of 16 March 1944 "Accumulation of engine test data subsequent to the 4th of March 1944, has resulted in the clearing of the following engines at the specified power ratings in accordance with the requirements of 7-1/2 hour W.E.R. run:"

 
W.E.R. Tests

War Emergency Rating Tests of V-1650-7 Engines Using 44-1 Fuel, 19 April 1944.
Preliminary 7-1/2 Hour War Emergency Rating Test of the Allison V-1710-91 Engine Operated on Grade 104/150 Fuel, 27 March 1944.
Attempted 7-1/2 Hr. War Emergency Rating of Allison V-1710-89 as Installed in P-38J Airplane Operated on Grade 104/150 Fuel, 22 April 1944.
Preliminary Report of 7-1/2 Hour War Emergency Test of Pratt & Whitney R-2800-63 Engine using Power Plant Fuel 44-1, 12 May 1944.

Flight Tests at Wright Field beginning 20 March 1944 found:

1. Flight tests were started on P-38J, P-47D, and P-51B airplanes at Wright Field on approximately 20 March 1944 in order to measure the performance and note any effect on flight characteristics when flown with 44-1 fuel. Tests on the P-51B have been completed but tests on the P-38J and P-47D have not been completed to date.

2. All tests were flown with the airplanes loaded to their maximum combat gross weight. The P-38J airplane tested was P-38J-15, AAF No. 43-28392, equipped with Allison V-1710-89 and 91 engines with Curtiss electric three blade propellers. Gross weight at take-off was 17,360 lbs. with the c.g. at 26.72%. The P-47D tested was AAF No. 42-26167 and was equipped with Pratt & Whitney R-2800-63 engine and an A-23 turbo regulator. Gross weight at take-off was 13,320 lbs. with the c.g. at 29.5%, gear up. The P-51B tested was the P-51B-15, AAF No. 43-24777 and was equipped with a Packard V-1650-7 engine with a 11 ft. 2 in., four blade constant speed propeller. Gross weight at take-off was approximately 9680 lbs. The weight included 265 gal. of fuel, full oil, and no ammunition (85 gal. in auxiliary tank instead of ballast for ammunition).

3. There was no noticeable change in handling characteristics of any of the airplanes tested when operating at the higher powers which were obtainable with the 44-1 fuel. Only a slight increase in vibration was noted at the higher powers. On one long range test made with the P-51B, there was no apparent trouble due to the 44-1 fuel.

4. All performance data obtained on the P-51B is included in the attached curves. It will be noted that all tests were run with the wing racks installed. Speeds would be approximately 12 mph faster with the wing racks removed as shown by the dash line curve on the Speed vs Altitude Curve. Approximately 16 MPH increases in speed below critical altitude and approximately 600 ft. per minute increase in rate of climb below critical altitude was obtained by using the 75” Hg. Manifold pressure allowed by 44-1 fuel. No tests were made on this airplane with standard fuel.

A Material Command Memorandum dated 13 May 1944 on "Preliminary Flight Tests of Fighter Aircraft Using PPF 441 Fuel at Increased War Emergency Rating" concluded:

Based on these preliminary flight tests, it is established that satisfactory operation is experienced on the P-47D airplane at 65" hg. M.A.P., on the P-51B airplane at 75" hg. M.A.P., and on the P-38J airplane at 70" hg. M.A.P. except in the case of the P-47D airplane in extended climbs with water injection.

Another Material Command Memorandum Report on “Use of PPF 44-1 Fuel in Fighter Aircraft”, dated 13 May 1944, summarized the advantages and disadvantages of using PPF 44-1 fuel in fighter aircraft based on tests conducted to date:

Conclusions

1. Based on tests conducted to date, it is concluded that use of PPF 44-1 in fighter aircraft permits higher power operation which increases airplane performance.
2. Disadvantages resulting from the use of PPF 44-1 fuel in fighter aircraft may be summarized as follows:

a. Decreased spark plug life.
b. Increased rate of replacement of synthetic rubber parts in contact with the fuel.
c. Probable increase of spark plug fouling trouble under low power cruise conditions.
d. General increased engine flight line maintenance on all three engines probably resulting from the higher power operation.
e. Generally increased engine deposits and ring sticking tendencies particularly on V-1710-89 and -91 engines.
f. Higher relative toxicity of the fuel necessitates more careful handling.

"Flight Tests of the North American P-51B-15 Airplane, AAF No. 43-2477 Using 44-1 Fuel" as reported by the Flight Test Engineering Branch dated 20 May 1944 states:

Conclusions

A. Operation of the airplane at 75 in. Hg. manifold pressure in low blower increased speed in level flight 16 MPH over the high speed at 67 in. Hg. manifold pressure at altitudes from sea level to 7400 ft. A 14 MPH increase was obtained at altitudes from 16,000 ft. to 20,800 ft. in high blower.

B. The rate of climb was increased 560 ft/min. from sea level to 2,200 ft. by 75 in. Hg. manifold pressure operation. At 15,700 ft. in high blower the rate of climb was increased 580 ft/min.

C. From sea level to 10,000 ft. in low blower the 1650-7 engine gave increases in speed from 2 to 11 MPH over the 1650-3 engine. From 16,600 ft. to 24,000 ft. in low blower the 1650-3 engine was approximately 20 MPH faster than the 1650-7 engine. In high blower from 18,000 to 24,200 ft. the 1650-7 gave an increase in speed of approximately 10 MPH but at 29,200 ft. the 1650-3 engine was 6 mph faster. This increase of the 1650-3 engine over the 1650-7 engine should continue at altitudes above 29,200 ft. as is shown in Fig. 8 by the dash line. Due to malfunctioning of the carburetor, however, speeds of the 1650-3 fell off above this altitude and were as much as 12 miles slower than those obtained with the 1650-7 engine.

Recommendations

A. It is recommended that the war emergency rating of the V-1650-7 engine as installed on the P-51B airplane and using 44-1 fuel be increased to 75 in. Hg. manifild pressure and 3000 RPM.

"Flight Tests on the P-38J Airplane, AAF No. 43-28392 Using 44-1 Fuel" as reported by the Flight Test Engineering Branch dated 5 July 1944 states:

Conclusions

A. In level flight operation a gain of 17 MPH can be obtained by increasing the allowable power from 60 to 70" Hg. (W.E.R.).

B. In climb operation a gain of 500 ft/min can be obtained by increasing the allowable power from 60 to 70" Hg.

C. Cooling the airplane can be easily maintained at 70" Hg. However, maximum performance can only be maintained by strict maintenance on the duct system to prevent possible leakage.

D. The maintenance difficulties experienced throughout the tests were considerable. These consisted mainly of induction, exhaust system, and spark plug failure. However, these difficulties could not be attributed directly to any action of the 44-1 fuel.

Recommendations

A. It is recommended that the Allison V1710-89 and 91 engines be rated at 70.0" Hg. when using 44-1 fuel or its equivalent. Because of the mechanical and maintenance characteristics of the engine and the P-38J installation this rating should be limited to a very short time. Periods between overhaul should be shortened for the engines using this power.

"Flight Tests on the P-47D Airplane, AAF No. 42-26167 Using 44-1 Fuel" as reported by the Flight Test Engineering Branch dated 15 July 1944 states:

Conclusions

A. The R-2800-63 can be operated at 65.0" Hg., 2700 RPM, in level flight and climb without water injection when using 44-1 fuel. It can be operated at 70.0" Hg., 2700 RPM with water injection with 44-1 fuel. Climbs at high power must be limited because of high cylinder head temperatures and carburetor air temperatures. Short climbs can be made without dificulty.

B. A gain of 19 MPH can be realized by using 65.0" Hg., 2700 RPM over 56" Hg., 2700 RPM. 8 MPH can be gained at 65.0" Hg. by using water injection. With water injection at 70.0" Hg., 2700 RPM, 7 MPH can be gained over 65.0" Hg., 2700, water injection.

C. In climb operation a gain of 510 ft/min. by using 65.0" Hg., 2700 RPM over 56.0" Hg., 2700 RPM can be realized. 410 ft/min can be gained at 65.0" Hg., 2700 RPM using wate injection. No 70.0" Hg. climbs were made.

Recommendations

1. It is recommended the Pratt & Whitney R-2800-63 engines be rated at 65.0" Hg. with and without water injection when using 44-1 fuel or its equivalent.

2. It is recommended that the use of 70.0" Hg. be further investigated.

3. It is recommended that pilots using these higher powers be cautioned concerning the high cylinder head temperatures and carburetor air temperatures which may be encountered in extended climbs or level flights.

Proving Ground Command

An Army Air Force Proving Ground Command Report “Service test of Nominal 100/150 Grade Fuel” dated 7 July 1944 determined the effect of nominal 104/150 grade fuel on the performance and maintenance of P-51B, P-47D, and P-38J airplanes.

Conclusions:

a. In view of the inconclusive nature of test results, it is not possible to make any definite decision concerning the operational use of nominal grade 104/150 fuel and the attending higher emergency power ratings.
b. Only three of the nine original test aircraft finished the specified test.
c. At this station, only very minor malfunctions and failures were traced specifically to the action of the nominal grade 104/150 fuel.
d. Maximum performance of all three types of aircraft was aided materially by the new power settings permitted with the new fuel.

Performance gains. - Attempts were made throughout the test to determine the average gain in performance due to the increased power rating allowed by the special fuel. Speed runs and climbs were made by approximately twenty-five pilots of all grades of experience. Speed curves shown in Inclosure 3 are average curves drawn from all data obtained from all three airplanes of each type. Data are not reduced to standard conditions, but are plotted against pressure altitude from actual free air temperatures. All flights were made with full military load.

P-51-B-15 Airplane.
Increase of power from the standard war emergency rating of sixty-seven inches Hg. to the test rating of seventy-five inches Hg. resulted in an average true air speed increase of fifteen m.p.h. from sea level to the seventy-five inches Hg. low blower critical altitude (about 8000 feet). Speed increase was also approximately fifteen m.p.h. from fourteen thousand feet to the high blower seventy-five inches for critical of about twenty-one thousand feet. No measurable difference was found between airplanes. The aneroid controlling supercharger shifting point was reset at the begining of the test to shift from low to high blower at sixty-two inches Hg. in a war emergency climb. This change resulted in a blower shift altitude of approximately seventy-five hundred feet, so that it was necessary to select low blower manually for cruise at medium altitudes where the desired power was available in low blower. Climbs were made to thirty thousand feet at the standard, and at the test war emergency ratings. Climbs at seventy-five inches Hg. required about one minute less than was required when climbing at sixty seven inches Hg. All engine temperatures were normal during climb at the increased power.

P-47D-22 Airplane.
Speed increase from military power of fifty-two inches Hg. to the test war emergency rating of sixty-five inches without water averaged approximately twenty-five m.p.h. true air speed from sea level to about twenty thousand feet. Over the same altitude range, water injection at sixty-five inches Hg. gave a further speed increase of about ten to fifteen m.p.h., so that the total speed gain from military power to sixty-five inches Hg. with water was about forty m.p.h. A considerable amount of scatter was present in the obtained speed data, possibly due to varying induction losses. Engine temperatures and operating characteristics were normal in level flight runs at sixty-five inches Hg., both with and without water.

Only a few climbs were made at the sixty-five inches Hg. rating, with and without water injection. Both with and without water, cylinder head temperatures rose to the maximum allowable at the end of the high power period at eighteen to twenty thousand feet. Over-heating tendencies in climb were greater with the use of water, average comparable cylinder head temperatures being approximately 10 C. higher. This is believed to be due to the higher heat rejection necessary at the increased water injection horsepower. Climbs to thirty thousand feet at Military power (52 inches Hg.) required approximately one and one-half minutes longer than if sixty-five inches Hg. without water were used. Climb to the same altitude at sixty-five inches with water decreased the time about one and a half minutes. Due to improper throttle-turbo schedule, all flights were made with throttle and turbo levers used separately.

P-38J-15 Airplane.
Some difficulty was experienced in obtaining comparable speed data at the standard sixty inches Hg. and the test seventy inches Hg. war emergency rating. Speeds at both powers varied greatly due to carburetor, spark plug and induction system malfunction. Curves drawn through point concentrations indicated an average speed increase of twenty to twenty-five m.p.h. between sixty and seventy inches Hg. at three thousand R.P.M.

No climbs were made at seventy inches Hg.

 

Into Service with the USAAF Eighth Air Force

In late winter 1944, the Allied Expeditionary Air Force (A.E.A.F.) decided, pending further trials, not to employ 150 Grade Fuel for Overlord due to spark plug issues, however, it was intended that 150 Grade would be used when proved satisfactory. 17 Meanwhile, cross channel operations by two squadrons of P-47’s and one P-38 using 150 Grade fuel revealed an increase of speed and climb characteristics at the expense of spark plug difficulties. 18 The Production Division was directed on 28 March 1944, under the authority of the Commmanding General, Army Air Forces, to modify all P-38, P-47 and P-51 airplanes in the United Kingdom for the use of Grade 150 fuel, with the necessary modification kits to be shipped to the European Theater of Operations within 30 days.

Successful service tests led in May 1944 to the Eighth Air Force Fighter Command requesting that it "be supplied immediately with grade 150 aviation fuel for use in P-47, P-51 and P-38 planes". 20 Deliveries of Grade 100/150 aviation fuel to AAF Stations commenced within a week of the landings in France. The change over to 150 grade fuel necessitated the resetting of all aneroid switches on the P-51s.  

 

150 grade fuel continued to be used by 8th AF units through 1944. The WER engine limitation for the P-51 continued to be 72" Hg. Eighth Air Force Fighter Groups converted to a new blend of 150 grade fuel, with increased amounts of ethylene dibromide (1 T) in early 1945. P.E.P, as the new fuel was called, was tried in order to remedy lead fouling of spark plugs. While spark plug fouling was eliminated, PEP was found to have an undesirable effect on valve seats. As a result of excessive maintenance required on the V-1650 engines, General Doolittle of the Eighth Air Force decided in late March 1945 to revert to the normal 100/150 (1 T) grade fuel.

Technical Operations, Eighth Air Force issued a 4 April 1945 memorandum in which 100/150 grade fuel experience in the Eighth Air Force was summarized. It is reproduced in full below:

1. The following is a summary of 100/150 grade fuel experience in Eighth Air Force.

2. a. This fuel was first service tested by Technical Operations Section, this headquarters, in October 1943, said service test lasting through until March 1944, at which time it was recommended that if extra performance from P-38, P-47 and P-51 aircraft was desired it could be secured by the use of this fuel. It was pointed out at that time that the only apparent deleterious effect of this fuel on any one of the three types was the extra lead fouling of spark plugs.

b. A decision was made in May 1944 to have all fighter units supplied with this fuel no later than 1 June. As of that date operations with this fuel continued until approximately 1 February 1945 when all fighter units switched to “Pep” (100/150 plus 1.5 T’s ethylene dibromide). As of 1 April 1945 all units switched back to 100/150 fuel containing 1.0 T ethylene dibromide.

3. At the time the 150 grade fuel was first used all three fighter types listed above were in operational use by this Air Force. Shortly after June 1 P-38 units were re-equipped with P-51 type aircraft so that experience with 150 grade fuel in P-38 aircraft is limited. Gradually, conversion of P-47 outfits to P-51’s took place during the Summer and Fall of 1944, and as of approximately 1 November only one P-47 group remained in this Air Force.

4. Maintenance difficulties can be summarized as follows:

a. P-38 (V-1710 Engine).

Spark plug leading was increased. The extent of this leading was such that plug change was required after approximately 15 hours flying. This conditions was aggravated considerably by low cruising powers used to and from target areas, while trying to get the maximum range possible. It was found, however, that regular periods of high power running for a minute of two in most cases smoothed out any rough running engines unless the cause was other than leading.

b. A-26 (R-2800 Engine).

Spark plug fouling was the only maintenance difficulty encountered during the period in which 150 grade fuel was used. Spark plug life was reduced by about 50%, the same low power cruising as described above being the principle cause for the extra fouling. No deleterious effects on diaphragms, fuel hose or any other rubber of synthetic rubber materials were noted.

c. P-51 (V-1650 Engines).

The same type of lead fouling as described in a and b above happened in the case of the P-51 except that is was probably more serious than in either of the other two types. Using 130 grade fuel with 4 cc. of lead, the average operational P-51 could last 5 missions (roughly 25 hours) before the fouling required plug change. With 150 grade fuel containing 6 cc. of lead, 10 to 12 hours, or normally 2 missions, was the average length of time between spark plug changes or cleaning. At various times in the six months of operation of P-51 aircraft on 150 grade fuel many other maintenance difficulties were attributed to the fuel, but final analysis proved that the only real effect of the fuel was the lead fouling. Some units maintained that they had some deteriorations of seals, but this was not borne our throughout the command, nor was there any concrete evidence that it existed in the units.

The excessive fouling of spark plugs usually exhibited itself in roughing up of engines after a couple of hours of low power cruising. Periodic bursts of high power in most cases smoothed the engine out. However, if the engine was allowed to go too long a period without being cleaned out, the accumulation of lead bromide globules successfully withstood any attempts to blow them out. In some instances, long periods of idling while waiting for take-off and a failure to use high power on take off resulted in loss of power during take-off run and in some cases caused complete cutting out with subsequent belly landing. The cases of cutting-out on take-off definitely attributed to excessive fouling were comparatively few, although numerous enough to list it as an effect of the extra lead.

As a result of several months operational use with the fuel, an SOP – designed to reduce power failures on take-off, leading troubles in flight, and other things which were causing early returns and abortive aircraft – was published. This is inclosure no. 1. Almost immediately after this section published this SOP practically all of the troubles then existing ceased, although it was necessary to change plugs after each two missions or thereabouts.

In an effort to reduce the lead fouling, tests were conducted by this section with 150 grade fuel containing 1.5 T’s of ethylene dibromide. A total of about 120 hours was run by this section and the three squadrons given the “Pep” fuel for accelerated service tests. The results of these service tests showed a considerable reduction in lead fouling with no apparent effects otherwise. As a results, all fighter units of the Air Force were put on Pep fuel late in January 1945. About thirty days thereafter a sharp increase in valve trouble was experienced with the V-1650 engine. Inspection of engines at overhaul revealed that the hydrobromic acid was eroding the silchrome valve seat inserts to such an extent that after approximately 100 hours of operation all the valve clearance was gone. This 100-hours is the minimum life some engines going 170 to 180 hours before this condition prevailed. There are no other deleterious effects of this fuel noted. As of 1 April 1945 fighter units of the Air Force returned to the use of 100/150 grade fuel containing 1.0 T of ethylene dibromide.

Into Service with the Royal Air Force

Following successful testing, the Spitfire IX's Merlin 66 was cleared in March 1944 to use +25 lbs, obtainable with 150 grade fuel. In early May, No. 1 and No. 165 Squadrons comprising the Predannack Wing, were the first to convert their Spitfires to +25 lbs boost and employ 150 grade fuel on operations. Air Defense Great Britain (A.D.G.B.) shared a report, dated 16th June 1944 with A.E.A.F. summarizing the RAF's experience with using 150 Grade Fuel in Merlin 66 engines. All pilots reported most favorably on the value of the high boost pressures obtainable with 150 Grade Fuel, however, Technical Staff felt that before the fuel was introduced on a large scale that the causes of backfires must be established and that at least 12 engines should complete 200 hours each. Eventually the backfire problem was sorted out, see: Backfire trouble resulting from use of 150 grade fuel. 27 July, 1944 and Backfire trouble resulting from use of 150 grade fuel. 12 August, 1944

The increased performance obtained with 150 Grade Fuel was put to good use by Mustangs, Tempests and Spitfires in intercepting Buzz Bombs launched against Britain beginning mid June. Performance increases at sea level were as follows:

130 Grade 150 Grade
Spitfire IX 335 mph 358 mph +25 lb
Spitfire XIV 359 mph 366 mph +21 lb
Tempest V 372 mph 386 mph +11 lb
Mustang III (V-1650-3) 360 mph 390 mph +25 lb

The Royal Aircraft Establishment (RAE) reported in Technical Note No.Aero.1501(Flight) that a Mustang III (Merlin V-1650-7), flying at +25 lb./sq.in. as received from Squadron, obtained 382 mph at sea level. 404 mph was obtained at sea level after "cleaning up" the aircraft by removing the bomb racks and aerial bracket, repainting the wing's leading edge and rubbing down the aircraft. 316 Squadron was one of the Mustang units to convert to 150 grade fuel, their Operations Record Book stating for 1.7.44 "18 A/C test after modification to +25 lbs boost". 610 Squadron uprated thier Spitfire XIVs on 18 July, the ORB stating "the modification of the aircraft to take 21 boost continues". These squadrons did more that just chase divers as 315 Squadron demonstrated when they shot down 6 Me 109's, 1 Me 110 and 1 Fw 190 while escorting Beaufighters to Norway on 30 July 1944. By mid August the Buzz Bomb threat was largly eliminated with the advance of the allied armies beyond the launching areas. The ADGB squadrons that had converted to 150 grade fuel now found more time to operate over the continent. The Spitfire IX Squadrons were permanently pulled off anti-diver duty on 10 August and went over completely to escort work, sweeps and armed recces. 316 Squadron flying their Mustangs downed 3 Me 109’s and a Fw 190 five miles N. of Chalom on 14 August. 315 Squadron met with remarkable success on 18 August, claiming 16 Fw 190’s shot down near Beauvais with their boosted Mustang III’s (II./JG 26 recorded 8 killed and 2 wounded). The Spitfire XIV squadrons quickly got into the swing of it with 350 Squadron scoring on 19 August by shooting down a Ju 88 on the outskirts of Brussels. And so it went - so much for divers.

On 18 September 1944 A.D.G.B. very positively summarized the experience gained to date using 100/150 grade fuel. However, due primarily to logistical difficulties, such as the interchange of squadrons between A.D.G.B. and 2nd T.A.F., it was decided that UK based fighter squadrons should revert to the use of 130 grade fuel. 34 Its unclear as to the degree to which this decision was carried out as of November 1944 Fighter Command was apparently still using 2,000 tons of 150 grade fuel per month. Combat Reports show that the the UK based Mustangs of ADBG were in fact still running high boost, only made possible through the use of 150 grade fuel, on operations over the continent and Germany in 1945. 

The Second Tactical Air Force

Plans were being made in August to supply the 2nd TAF with 150 Grade Fuel. During November 1944 S.H.A.E.F cleared 100/150 grade fuel for use by the Second Tactical Air Force:
















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