Hi Tomo,
I see your statement that the myth of British fuels being significantly different from U.S. fuels is dead and buried. I have not read anything that supports that other than posts in here, and have first-hand testimony from several P-38 pilots that the early P-38s DID have fuel-related problems that were cured. They were told it was the gasoline after the cure was effected. It took some 8 - 10 months because nobody sent any British fuels back to the U.S.A. for analysis. When they did, the difference in aromatics became known and the carburetor jets were changed, eliminating that issue. After that, it wasn't an issue as long as the fuel to be used was specified.
To be sure, the P-38 had other issues, each of which were also solved except for the low critical Mach number. Part of the cure may well have been the turbulators installed in the Allison intake manifolds as well as new jets, but the combination DID fix the issue when the aircraft were used in the ETO or MED. PTO operations used U.S. fuel, so they never had the issue, and were also mostly conducted AFTER the original issue was found and solved. When I was at an Allison shop, we used ONLY the intakes with the turbulators for aircraft engines. When I was there, we didn't make any new tractor or boat engines, so I can't say for sure about them, but many tractor guys in Europe run fuel injection anyway, so the intake wouldn't matter since it is eliminated in an injected or boosted-injected setup.
I have no real dog in the hunt since the pilots I know who fly warbirds can't get either U.S. or British WWII fuels anyway, and they have to fly on the best they can get today. But the first-hand testimony indicates the situation could have been a real one. It WOULD be nice to know for sure, but I have no definitive source.
Hi DarrenW,
In WWII, aromatics were mostly Benzene and Benzene derivatives, used mainly to increase the Octane number. After an Octane rating of 100 (101 - 150), the term is "performance number." In WWII, we had gasoline with two ratings, the lean rating (cruise condition, leaned out), and the rich rating (full power, full rich). We started the war running 80 / 100 fuels (generally) and ended with fuels called 115 / 145, sometimes mistakenly called grade 150 fuel. Nobody makes 115 / 145 fuel anymore except by special order.
If you cut down the aromatics, the base Octane number dropps unless compensated for by adding other additives, such as tetraethyl lead (hence the obsolete term "ethyl" for high-octane gasoline for cars). European fuels generally had more aromatics than U.S. fuels due to the sources of their oil. If the U.S.A. had sourced the base oil from the same places as, say, Germany, our fuels would have had higher aromatics, too. Of course, the refineries can ADD Benzene and derivatives to make any desired mixture. Current U.S. 100 / 130 fuel is 5.0% aromatics, as is 100LL. WWII U.S. 100 / 130 avgas had 3.0 ml/gal tertaethyl lead, and 115 / 145 had 4.6 ml/gal. The gallons are, of course, U.S. gallons.
To answer your question, we used different additives to get high-performance number fuels and used less aromatics than did Europe. Neither is good or bad, just a fact. But the two different routes to higher Octanes burn differently when the mixture changes, and it isn't important which fuel you use as long as you know and jet for it. Bottom line: if the performance number drops, you get detonation at a lower temperature. When that temperature drops down to within the range of what is seen in the cylinders, you get destructive detonation, and pilots HATE it when they go down and get captured due to easily-preventable failures.