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Coupe-like crossovers, carbon-bodied sportscars, urban-mode city hatchbacks--BMW can build them all, but when it gets placed in the balances and judged by its hardcore enthusiast audience, the only BMW that matters is the M3.

It's the critical nuclear core of the brand and a bellwether for Bimmerphiles and the wider auto realm alike. If a car is lithe and taut, it will inevitably be compared to the M3.

The M--and now the M4 alongside it--has to be right. The 3-Series may be BMW's sales heart, but the M cars is its soul.

Both are new for 2015, and both have some history to overcome. The last M3's been widely regarded as a hefty high-speed specialist that veered a little too far away from BMW's lightweight, nimble roots. The M4? It's an unknown.

We've now had time to drive the M4 and M3 on the track--Wisconsin's Road America, the Gigantor of woodsy thrill rides--and the M4 extensively as a part of our Best Car To Buy driving on Malibu's more challenging canyon roads. (Yes, we're now long after the first drives actually happened, but sometimes life happens.)

What did we find? A car that's astonishingly fluid transitioning between wide sweepers, one that's fearsomely fast, but something of a ditherer when the roads get really tight.

Adultsonly

The running gear underpinning the M3 and M4 occupies the adults-only corner of BMW's hardware store. The centerpiece is a 3.0-liter twin-turbo in-line six rated at 425 horsepower and 406 pound-feet of torque. Up 2 percent in power and 38 percent in torque, the S55 engine's peak torque arrives at a low 1800 rpm and sticks around through 5500 rpm--where peak power arrives and lingers through a soaring 7300 rpm.

The rev limiter body-checks throttle input at 7600 rpm--but you'll shy away from it before then. This engine just doesn't sound fluid or muscular in the way old BMW in-line sixes did, or the current crop of competitive V-8s do (RC F, Mustang GT), even with piped-in engine noises filtered and amplified for your enjoyment. The classic BMW ripple has turned into a wall of white noise.

Making up in crazy speed what it lacks in charisma, the M3 and M4's six pounds out 3.9-second 0-60 mph acceleration times like it's dealing poker. That's when paired with the M-DCT dual-clutch transmission. If you opt for the six-speed manual, that figure rises to 4.1 seconds--it's the price you pay for rowing your own gears, though you won't have to worry about rev-matching even if you pick the three-pedal 'box, as BMW has included automatic throttle-blipping that's shut off when the M's put in Sport+ mode. In either body style, top speed is capped at 155 mph, a number Road America brings within tantalizing reach.