There was a time when a midsize German sedan with heated seats and a usable back seat had no chance of humiliating exotics. Then BMW built one anyway. Now, though, a rough, high-mileage F10-generation M5 gets dragged back from the edge and pushed much closer to the car it always hinted it could be: a four-door missile with a twin-turbo V8, a luxury-car window sticker, and enough oomph to make modern supercars feel a little too pleased with themselves. This M5 Was A Mess, Which Made It Perfect Danny Z YouTubeThe car at the center of this rescue mission isn't exactly what you'd deem collector-worthy. It’s a 2013 BMW M5 that had been sitting around, smoking, leaking, and generally doing a fine impression of a very expensive bad idea. It shows more than 165,000 miles, and that tells you this has been around the block. It was bought because someone looked at a tired F10 and saw potential instead of a future therapy bill.In theory, that's a solid start. The F10 M5 has a great foundation, what with it marking a turning point for BMW M. It traded the old high-revving V10 formula for a 4.4-liter twin-turbo V8, and while purists groaned at first, this was a sedan with brutal midrange punch and effortless speed. Factory output sat at 560 horsepower, which was enough to make this thing absurdly quick when it was new and still more than enough to embarrass plenty of fast machinery now.This particular car had even more drama in its history, including an engine replacement after a dealership-related oil-loss disaster. So baggage is part and parcel of this particular build, but in the world of used performance cars, baggage is often where the plot starts. The Turbo Job Turned Into A Full Mechanical Adventure Danny Z YouTubeThe big job was replacing the worn turbos, which on this car had clearly seen better days. Oil where it shouldn’t be, chewed-up blades, smoke, and a hidden coolant leak all showed up once things came apart. BMW might’ve packaged the hot-side hardware in a clever spot at the top of the V8, but that didn’t make it simple. Apparently, the job is something a dealer could charge roughly $4,000 to $5,000 for, which is a nice reminder that fast German sedans age like avocados when the labor bill arrives. Proper Project Car Danny Z YouTubeStill, the fun bit's just about starting. The teardown keeps uncovering the kind of stuff that defines old high-performance BMW ownership: brittle plastic, awkward clamps, tight clearances, and just enough engineering brilliance to make you forgive it for a few minutes. A broken plastic coolant fitting got replaced with a brass piece, the engine mounts were swapped, and the whole thing slowly started shifting from abandoned headache to proper project car.Then the replacement turbos got lost in shipping, because of course they did. Used units eventually stepped in, which adds one more layer of glorious chaos to an already sketchy plan. On the whole, it felt like a real garage fight with a 4,300-pound executive sedan that still thinks it’s the hero. The M5 BMW Should’ve Built Danny Z YouTubeOnce the turbos were back in place, the upgrades kept coming. A valved exhaust system transformed the car’s voice, and once the vacuum-control issue got sorted, the result was exactly what this generation M5 always needed more of: theater. The stock F10 was never a bad-sounding car, but it often felt a little too restrained, like it had shown up to the party in a tailored suit and then refused to loosen the tie. A Proper Supercar Slayer Danny Z YouTubeThat changed in a hurry. Add freer-flowing intake hardware, more turbo noise, and the kind of aggressive exhaust note that makes tunnel etiquette impossible, and suddenly the car feels less like a used luxury sedan and more like a back-alley answer to a supercar. The builder even suggests the setup is now somewhere around 1,000 horsepower, which is... impressive, to say the least."This is by far the best sounding BMW I've ever owned." - Danny, the F10 M5's ownerAll in, the F10 M5 was always one of the great modern sleeper sedans, the kind of car that could haul adults in comfort and still terrorize the fast lane. This one just leans harder into the second half of that job description. Sure, it’s rough around the edges, and slightly crazy, but it's way more entertaining because of it.Source: Danny Z (YouTube).