Try as they might, no computer-generated audio can replicate the sound of a real engine. Forza developers know that, which is why they get their audio for exhaust noises the old-fashioned way: by recording it first-hand.
The Forza crew recently uploaded a video to its YouTube channel documenting exactly how it captures the ultra-realistic-sounding car audio for its upcoming open-world Horizon 5 video game. How it works is, developers bring each subject car to a wide-open closed course such as an airfield or race track, then strap a bunch of microphones to the exterior and interior. There are several by the exhaust pipes and some in the cabin, there to capture the car’s sounds from different perspectives. This makes sense, since what you’re hearing from the driver’s seat will differ from what you’re hearing from the outside of the car. For the full story and video, check out this article from Road & Track.