Why I dislike IL-2 games..

[Robotic Pope]

Well there you go. I learned something new lol.

Did a bit of searching and reading. Can't find any mention on why the allison engine suddenly didn't need to have then airscoop on top rather than underneith the engine. It seems to me to be part of the modification that gave the allison a two stage supercharger though. I now know that most the Twin Mustangs went back to using Allisons (even though they were weaker) for political reasons. I expect these fake engined Mustangs are from old F-82's and are all in the US then and why Ive never come across one.

One thing I can't understand. I still count 6 exausts in the photos?

Production F-82Es were slow in arriving. The problem was the updated Allison V-1710 engine. The Allison was selected because Packard had to pay Rolls-Royce a $6,000 USD royalty for every V-1650 the company produced. During the war, Rolls Royce had been lenient about license fees, but after the end of the conflict Britain's economy was in the dumps and the royalty fee skyrocketed. There was also the fact that General Motors, which owned Allison, had a 40% share in NAA. GM had not been happy with the Mustang's switch to Merlin power during the war, but demand for Allisons by other aircraft such as the P-38 Lightning was strong and GM had not been in a position to protest. With the war over, aircraft production took a dive and GM wanted to sell more Allisons. There were few other reasons to use the Allison engine, since even the two-stage supercharged Allison V-1710 was inferior in power-to-weight ratio to the Merlin.

The souped-up Allison engine was also temperamental and unreliable, and Allison couldn't deliver product in quantity. NAA had completed the 100 F-82E airframes by April 1948, but wasn't able to deliver them all for another year due to engine shortages. In service, the Allison-powered F-82Es were marginally slower than the Merlin-powered F-82Bs and the reliability problems persisted. The V-1710 became known as the "Allison time bomb" due to engine failures. Spark plug fouling from backfiring was particularly acute, and spark plugs were often swapped after a single flight.

NAA engineers modified some Allison engines with Merlin components and fixed most of the problems, but Allison took a "not invented here" attitude and insisted on applying their own fixes, which never quite worked. Given the small production run of the Twin Mustang, even the steep license fee Rolls Royce was demanding for the Merlin was a bargain compared to all the troubles the USAF had with the Allisons.

The F-82Es were externally indistinguishable from the F-82As except for minor details. One of the giveaways was the engine exhaust stacks. On the Merlin-powered aircraft, there were six exhaust stacks on each side. On the Allison-powered aircraft, there were duplicate exhaust stacks for each cylinder, for a total of 12 exhaust stacks on each side.