“Airspeeds jumped by 30 to 40 knots, and rates of climb increased by 500 to 1,000 feet per minute.”

By Marc Liebman

IT’S MAY 1940 and Royal Air Force pilots are struggling to provide air cover over the British Expeditionary Force in Belgium and France. Unfortunately for the RAF, the German Luftwaffe Bf-109s could climb faster and higher and outrun the Hurricanes and Spitfires.

While British pilots could hold their own if they could force the Messerschmitts into a turning dogfight, they were at a distinct disadvantage when Germans had the initiative. And when Luftwaffe pilots found themselves in danger, they could use their 20 to 20 mph speed advantage to dash to safety.

Interestingly, it wasn’t aerodynamics, engines or even pilot training that gave the Germans the advantage. It was fuel.

In the Spring of 1940, RAF fighters were fueled with 81/87 octane; the Germans on the other hand filled their tanks with 91/100. The higher-octane fuel used by the Luftwaffe enabled their airplanes to generate more horsepower.

However, help was coming from two directions.

One was the Rolls Royce Merlin XII being fitted to Mk. II Spitfires beginning in May 1940, which was the first version of the famous engine designed to use 100/130 octane fuel that was arriving from the U.S.

The other were newer airplanes with engines designed to take fuller advantage of 100/130.

What’s in an octane number?

But what do these fuel numbers actually mean?

The first number in an aviation fuel rating is the level of octane at lean mixture settings.

The second number is the octane at “full rich” or maximum fuel flow.

An octane rating is the measure of the fuel’s resistance to premature ignition, also known as “knocking.” Knocking occurs when the fuel explodes before the piston reaches the top of its stroke. Prolonged premature ignition can lead to engine failure.

Higher octane ratings have two major benefits.

First, it enables the engine designer to use higher engine compression ratios (volume of air in each cylinder’s combustion chamber when it is at the bottom of its stroke compared to when it is at the top).

Secondly, high-octane fuels allow engineers to use superchargers and turbochargers to generate higher manifold pressure settings and more horsepower.

At sea level, a normally aspirated engine (one that does not have either a supercharger or a turbocharger) has a “normal” sea level maximum manifold pressure of 29.92 inches. As the airplane climbs, the manifold pressure drops by approximately one inch per thousand feet. At 10,000 feet, the maximum manifold pressure is 19.92 inches. Put another way, at 10,000 feet, the engine has lost 1/3 of its power.

Airplanes like the North American A-36 Apache, the ground attack

variant of the famous P-51 Mustang, and the Bell P-39 Airacobra, with their normally aspirated engines, were at a severe disadvantage when their pilots had to fight a Japanese Zero or a Bf-109, both of which had engines with two-stage superchargers.

Single and two-stage superchargers and turbochargers are game changes for airplane engines, just as they are for car and truck motors. While both work in different ways, they have a similar effect: They compress the air coming into the engine and make it denser. Aircraft engines with superchargers and turbochargers can maintain sea level power at 20 or even 30 thousand feet or higher.

At the start of the Second World War, better materials and engine designs allowed sustained manifold pressures two- and three-times sea level. For example, the Pratt & Whitney 18-cylinder radial engine in the F4U Corsair or a Republic P-47 Thunderbolt could safely operate at 65 inches of manifold pressure, or about 2.17 times a normally aspirated engine.

High octane fuel and higher compression ratios also raised the critical altitude, which is the height above sea level at which the engine will still produce 29.92 inches of manifold pressure.

The accompanying chart from the Pilot Flight Operating Instruction manual for the Grumman F6F-3 Hellcat dated Dec. 7, 1943, shows that in high blower (the second stage of its supercharger), the plane’s Pratt & Whitney R-2800-10 could still generate 30 inches of manifold pressure at 34,000 feet!

The “war emergency” manifold pressure for the Allison engine in the P-51 was, after 1944, 70 inches, or 2.34 times sea level. The enabler to higher manifold pressures was high octane fuel. All the above is why we see the R-2800 rated at 2,000 horsepower and the Packard-Merlin rated at 1,750.

By comparison, single-engine General Aviation airplanes such as the Mooney 231 and 252, the critical altitudes are much lower. The 231’s Continental TSIO-360’s was 15,000, and the 252’s was 24,000 at 2,400 rpm. Maximum manifold pressure is 36 inches for these engines which are optimized for using 100 octane low lead fuel.

The evolution of high octane

The quest for more powerful aircraft engines and fuels that would make unprecedented performance possible was spurred on by aviation racing in the 1920s and 30s. During this period, in the U.S., Shell Oil was experimenting with additives that would increase the standard aviation fuel from 80 octane to 100.

Jimmy Doolittle, who earned one of the first doctorates in aeronautical engineering, saw the performance improvement potential available by using higher octane fuels, but the economics of producing them were against him.

Shell would only invest in refineries capable of producing these new fuels if there was a market for 100-octane, which was 100 times more expensive than producing 80 octane automotive and aviation gas.

Doolittle lobbied his friends in the U.S. Army Air Corps to convince the service to make 100-octane fuel the standard for both the Army and the Navy. In 1938, the Army Air Corps and Navy agreed to standardize on 100-octane fuel.

Suddenly there was a market, but Shell and other oil companies still had to produce at a cost the U.S. military could afford. Until they adopted a process created by a French émigré, Eugene Houdry, refining crude into 100-octane was time-consuming and expensive.

Houdry came to the U.S. in 1931 after experimenting with processes that converted coal to gasoline. Working with Sun Oil, he developed a new method to economically “crack” crude oil and generate 100 octane gasoline. Then, with the addition of additives like tetraethyl lead, the octane rating zoomed to 100/130.

By the spring of 1940, U.S. refineries were producing enough 100/130 to export to Britain, whose refineries couldn’t make the newer fuel. The modifications to the Merlin engine to take advantage of the higher-octane fuel were known to Rolls-Royce and were incorporated into newer engines.

The difference between 81/87 and 100/130 in a Hurricane II or Spitfire II was transformative.

Depending on the altitude, airspeeds jumped by 30 to 40 knots, and rates of climb increased by 500 to 1,000 feet per minute. Luftwaffe fighter pilots escorting bombers over Great Britain during the Battle of Britain suddenly found that their performance advantage had evaporated.

In the U.K., improvements to the Merlin (and other engines) were coming fast and furious.

The Merlin XII developed 1,150 hp while the Merlin XX, which entered production in May 1940, was the first to have a two-stage supercharger and was designed from the ground up to take advantage of 100/130. It pumped out 1,480 hp.

As an aside, Rolls-Royce shipped a Merlin XX and a set of drawings to Packard in the U.S. It became the V-1650 of which 55,523 were built.

Enter Fuel 44-1

The U.S. military was still not satisfied with 100/130 octane fuel. In January 1944, the Army Air Force’s Materiel Command tested a fuel rated at 110/150 octane. In trials using this fuel in a Lockheed P-38J Lightning, a P-47D and a P-51B, the results were enough to convince the military to start using the new formula gas immediately. Each of the airplanes used in the test were at Wright Patterson and were relatively early production models and were not even modified for the tests.

The fuel also allowed the Allison V-1710-89 in the P-38J and the Allison V-1650-7 in the P-51B to use 75 inches of manifold pressure. For the P-47, the Army Air Force kept the maximum manifold pressure at 65 inches for its R-2800-63 radial engine.

Each airplane showed a noticeable increase in speed. The P-47D was, on average, 19 mph faster from sea level to 35,000 feet, and its rate of climb went up by 410 feet per minute.

The P-51B was 10 to 11 mph faster and climbed 560 feet per minute faster beginning at 2,200 feet and 580 feet per minute faster at 20,800. According to the report, the air force put two engines in the P-51B, a V-1650-7 and a V-1650-3. Carburetor problems caused the swap, but before it occurred, the P-51 with the -3 engine was 20 mph faster than the -7.

Of the three airplanes, P-38J benefited the most. Up until the test, it was limited to 60 inches of manifold pressure. Going to 70, speed increased by 20 to 25 mph. Rates of climb increased by 500 feet per minute.

By D-Day, 100/150 44-1 fuel had become the standard; the performance increases outweighed the wear and tear on aircraft and accompanying maintenance headaches. The biggest of these was a decrease in spark plug life. After only 12 to 15 flight hours, or two seven-hour or three, four-hour missions, the plugs had to be changed.

Rough running at low power settings (lean mixture, low rpm, low manifold pressure) caused by fouling plugs was also a problem. The solution was to run the engines at a “high” power setting for two to three minutes when the engine began to run rough. High power being defined as between 30 and 40 inches of manifold pressure and maximum rpm.

A third problem was that 100/150 ate up synthetic rubber parts into which it came in contact. This required careful monitoring and more frequent replacement of these parts. In an interesting note, the AAF noted that the high toxicity of 100/150 required careful handling.

Deposits on the valve seats, cylinder bores, and rings in the Allison engines was yet another problem. These could cause a loss of power and/or failure which resulted in more engine changes and overhauls.

Germany plays catch-up

The science of making higher octane fuels was well known to the Germans. By the end of the war, they were managing to make 91/100 out of coal, of which Germany had an abundance. But the process was slow, cumbersome and expensive.

Attacks on German fuel processing plants and the oil fields in Romania hindered fuel production even though the Germans had a compound called C-3. Added to its 91/100 fuel, the octane ratings increased to 95/130. However, the Germans could never produce enough C-3 to meet its needs.

Japan’s fuel woes

Japan had to import almost everything it needed to support its war effort, including refined petroleum products.

The U.S. submarine campaign against Japanese merchant shipping starved Japan of food, raw materials for its industries and crude oil. At the beginning of the war, the Japanese were using 87/91 octane fuel; in 1945, they were still using it.

Even if the Japanese petroleum engineers had enough crude oil, shortages of key alloys prevented the design of newer engines that could take advantage of the higher-octane fuels.

It was not until the Kawanshi N1K-1 Shiden and Mitsubishi J2M Raiden fighters came off their production lines in 1944 that the Japanese produced engines approaching the horsepower of their opponents.

While both the Raiden and Shiden were very capable fighters that approached the performance of the American F4U and F6F, Japanese production couldn’t match the output of U.S. factories.

By late 1943, the 100/130 fuel feeding American aircraft engines was only one of many advantages. By then, the average Japanese pilot entering combat had far fewer hours than his U.S. counterpart. This experience gave the Americans and later British Fleet Air Arm pilots a considerable and lethal edge in combat.

High-octane fuels continued in U.S. service for almost 40 years after the Second World War. Air Force A-1 Skyraiders with Wright 18-cylinder R-3350 flew until 1973 in Vietnam and the U.S. Navy operated T-28 Trojans with R-1820s in the Naval Air Training Command until 1984. Both engines burned 115/145 aviation fuel which was a descendant of 100/150.

Anyone who has ever smelled the exhaust of a radial engine burning 115/145 will never forget its distinctly sweet odor. And for those who fought in World War Two with engines burning 100/150, it was the sweet smell of victory.

Marc Liebman is a retired U.S. Navy Captain and Naval Aviator and the award-winning author of 14 novels, five of which were Amazon #1 Best Sellers. His latest is the counterterrorism thriller The Red Star of Death. Some of his best-known books are Big Mother 40, Forgotten, Moscow Airlift, Flight of the Pawnee, Insidious Dragon and Raider of the Scottish Coast. All are available on Amazon here.

A Vietnam and Desert Shield/Storm combat veteran, Liebman is a military historian and speaks on military history and current events.

Visit his website, marcliebman.com, for: past interviews, articles about helicopters, general aviation, weekly blog posts about the Revolutionary War era, as well as signed copies of his books.

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