One thing that actually annoyed me about the Forza 4 system was how "all or nothing" it was. For a distressing amount of cars, engine swaps included one extremely high performance road engine, and one literal racing engine; rather than swaps that were possible for them to include and relatively common (even staying in house for manufacturer) in real life. Most immediate example that comes to mind is the Mustang II. The emissions strangled boat anchor of a V8 could be swapped pretty much directly with one of later Windsor engines from a Fox body Mustang if you didn't want to spend the time building it up, and you could use that to build a relatively balanced vehicle. And, what a coincidence, there are two Fox bodies Mustangs with decently more powerful Windsor V8s in Forza 4. But instead the Mustang swaps jump all the way up to the SVT Cobra R engine, which would have no chance in hell of fitting normally and which triples the power with no steps in between.
Ditto with the Pinto, whose engine was eventually used as the basis for the Sierra RS Cosworth's (and the later Escort RS Cosworth) humongously successful racing powerplant. And what do you know it, Forza 4 has both of those cars as well as the Pinto. Match made in heaven. They'd both only add about 120 horsepower. So why does the engine swap list go to the SVT Focus engine (which... is kind of odd, but alright) all the way up to the GT500 KR supercharged V8 (which would fit even less in that than the example above and adds five times the horsepower)?