For most BMW fans, the E46 M3 CSL is the crown jewel in the brand’s performance car history. Not only was it one of the best looking cars BMW ever made but it was potentially the most thrilling to drive. However, what most fans don’t know–and almost no one on earth knew until just today–is that BMW actually made an even better version, albeit just one of them. It was a prototype E46 M3 CSL V8. As an experiment, potentially after seeing how good the E46 M3 GTR race car was, the engineers at BMW M decided to stuff the 4.0-liter V8, engine code S65VB40, under the CSL’s hood.

The E46 chassis was never designed to fit a V8 engine, so the stonking free-breathing eight-cylinder engine barely fit. It also added quite a bit of weight to the car, nullifying much of the 110 kg (246 lbs) removed from the standard M3 to make the CSL. However, that weight gain–admittedly in the worst place to add weight, over the nose–was itself nullified by the big bump in power.

Back in 2003, the E46 M3 CSL made 360 horsepower from its 3.0-liter naturally aspirated inline-six. The big V8, though, made 430 PS (424 horsepower), which is an amount mathematicians refer to as “a whole effing lot of extra power.”

The engine itself is an interesting one, though. It’s a pre-trial carrier based on the S62 8-cylinder that was used in other BMW models. The spec-sheet BMW provides in the video claims it t be an “S65VB50” which is an engine code we’ve never seen before. According to BMW, the M3 CSL was donated to BMW M by the press department after the launch. It became a prototype with the S65VB40 engine and the denomination BMW M3 CSL V8 and remained a one-off. The insights gained flowed into the M family of engines, which consisted of the completely newly developed S85 (V10) and S65 (V8).

We do know it sounds great, with a snarly, motorsport V8 growl. I also love how the V8 needed extra air , so the CSL V8 prototype had two symmetrical circular air intakes in the bumper, rather than the asymmetrical intake of the standard car. Can you imagine driving an E46 M3 CSL but with a stonking V8 under its hood?