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With a history of 50 years, BMW M has proven its worth as the high-performance division of the German automaker with the introduction of M cars.

But with several decades under BMW M's belt, there were only three CSL models that made the cut: the 3.0 CSL from 1973 (shortly after BMW M's birth), the E46 M3 CSL from 2003, and the recently unveiled M4 CSL.

The acronym – which stands for Competition, Sports, and Lightweight – has become synonymous with road-legal yet track-capable hardcore M cars, and their exclusivity has made them more desirable to enthusiasts and collectors alike.

But did you know that BMW has four more CSL prototypes that almost happened? BMW M takes us on a tour of a secret garage where these four cars reside, some of them the world has never seen before.

BMW M3 CSL E46 V8

Secret BMW M CSL Prototypes - BMW M3 CSL PROTOTYPE E46 V8
Secret BMW M CSL Prototypes - BMW M3 CSL PROTOTYPE E46 V8

Not to be confused with the BMW M3 GTR, the E46 M3 CSL V8 prototype remained a one-off but with a purpose. With a 430-horsepower (321-kilowatt) 4.0-liter S65VB40 engine under its hood, it was created by BMW as a pre-trial carrier of the V8 used in other BMWs. The insights from the M3 CSL V8 flowed into the M family of engines, such as the S85 (V10) and S65 (V8).

BMW M5 CSL E60 V10

Secret BMW M CSL Prototypes - BMW M5 CSL PROTOTYP E60 V10
Secret BMW M CSL Prototypes - BMW M5 CSL PROTOTYP E60 V10

Just like the M3 CSL V8, the BMW M5 CSL E60 V10 became a prototype for a newly developed, high-revving 5.5-liter V10 that put out 630 hp (470 kW) and peaked at 8,750 rpm. To go with the immense power created, BMW created a lighter M5 CSL with a carbon roof, Recaro seats, and no rear seats – saving 331 pounds (150 kilograms) in the process.

With more tuning involved including suspension and additional cooling, the M5 CSL lapped the Nürburgring in 7 mins 50 seconds. The BMW M5 CSL remained a one-off, celebrating BMW M's 25th anniversary.

BMW M6 CSL E63 V10

Secret BMW M CSL Prototypes - BMW M6 CSL PROTOTYP E63 V10
Secret BMW M CSL Prototypes - BMW M6 CSL PROTOTYP E63 V10

Differing from the M3 CSL V8 and M5 CSL prototypes, the E63 M6 CSL V10 became a test bed for BMW's active aerodynamics. Apart from the reduced weight, the M6 CSL V10 came with an automatic tail spoiler and a retractable front spoiler, designed to further improve the coupe's performance.

The BMW M6 CSL also featured the double-strut M exterior mirrors, found on the sportiest BMW models these days.

BMW M2 CSL F87

Secret BMW M CSL Prototypes - BMW M2 CSL PROTOTYP F87
Secret BMW M CSL Prototypes - BMW M2 CSL PROTOTYP F87

The most hardcore M2 was the 450-hp (336-kW) M2 CS, but did you know that BMW M simultaneously worked on an M2 CSL? Made to further amplify the performance of the M2 Competition, the M2 CSL had the same amount of horses and 406 pound-feet (550 Newton-meters) of torque, powering a lightweight two-seater with bucket seats, rollover bar, CFRP central console, carbon rear wing, and carbon-ceramic brakes.

Both the M2 CS and M2 CSL were presented internally, but the green light went to the M2 CS with its features more suitable for daily use. The M2 CSL remained a one-off.

Gallery: Secret BMW M CSL Prototypes

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