In Shacklady´s " Hawker Hurricane"... "The cumbersome Vokes (desert filter) was installed but this was to restrict performance to such an extent that the fighter was handicapped when meeting its old enemy, the Bf 109. In an attempt to boost performance the number of guns was reduced to two. No attempt was made to install the local modification, desert filter ( Aboukir ) as fitted to the Spitfire." Any idea why not? Problem with fitting it or not enough for all planes...??
According to this both types were used on the Spitfire. Start reading about 25% down the page. Perhaps one plane had the Merlin and the other had the Griffin ? Just a wild guess. Supermarine Spitfire variants - Wikipedia, the free encyclopedia Also the Me-109 had to have a bulky air filter also and perhaps the RAF thougt it would reduce the 109's performace too. Again, just a guess.