BMW M4 mule with aero mods caught in testing

Could a BMW M4 CS, or perhaps even a new CSL, already be in the works?

BMW has been testing a mysterious M4 mule with a number of subtle modifications over the still-fresh Competition version. While a hotter CS variant is almost a certainty for the M3 and M4 twins in the future, these even-higher-performance models usually come along much later in the model cycle, making this prototype a particularly interesting spot. 

The most obvious alteration on the mule is a new boot lid with an M3 CSL-like moulded ducktail that’s completely different to the standard M4 spoiler. However, recent CS models have featured a stick-on carbon lip rather than an entirely new panel design, which tells us this test car might well be something different. 

Also new on the prototype is a deeper front splitter, capped at either end with low-mounted flicks. Other CS-specific elements such as a bespoke bonnet and a unique wheel design seem missing, though, and the skirts and rear diffuser are standard items, but the test car is running the carbon-ceramic brakes and Michelin Pilot Sport Cup 2 tyres that are already optional on the current Competition.

The chance of this prototype being an even more hardcore variant than a CS, like a GTS, is unlikely, largely due to the lack of even more aggressive aerodynamic paraphernalia such as a stacked wing or the addition of a roll-cage. 

Any upgrades to the engine are typically impossible to spot from prototype cars like this, but the M4’s closed-deck S58 3-litre turbocharged straight-six is ripe for further modification despite its already strong 503bhp peak output. The question of whether a future M4 CS or CSL will retain rear-wheel drive or pick up the Competition's optional all-wheel drive is also a question we’re looking forward to being answered. 

Whatever this mule ends up as, those in the market for an even more dynamic iteration of the M4 won’t have to wait long, and with the standard Competition having made an excellent first impression, the foundations are already in place for an even hotter version to be something truly special.

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