WESTLAKE VILLAGE, CA – Traffic permitting, the sensible route from Pismo Beach, CA, to Los Angeles is a straight shot down Interstate 101 to historic Highway 1, which shadows the coast through Santa Barbara and other quaint seaside communities.

However, those who love to drive know a better way: Exit the freeway north of Santa Maria onto Route 166, the Cuyama Highway, and head east on a relatively deserted flat 2-lane road that connects with Route 33.

Also known as the Maricopa Highway, this delightfully slithering passage bends and pitches its way south through the Los Padres National Forest before exhaling into Ojai.

There’s no better place to experience BMW’s first 4-cyl. offering in the U.S. since 1999, when the gasoline 318ti left the market.

This all-new 2.0L DOHC direct-injection gasoline I-4 (internal code N20) first appears in the ’12 Z4 sDrive28i and enables the low-slung roadster to devour a steady diet of switchbacks in a fashion befitting the Ultimate Driving Machine brand character.

U.S. product planners surely lost sleep wondering if luxury roadster buyers would balk at a 4-cyl. engine. They can rest easy now because the Z4 is a better car with the N20.

Built in Munich, the I-4 is the most important offering on this mid-cycle enhancement of the Z4.

Rated at 240 hp and 258 lb.-ft. (350 Nm) of torque, the turbocharged TwinPower is an ideal match for the 3,263-lb. (1,480-kg) Z4, easily pushing it to the limits of adhesion and yielding little to the highly regarded naturally aspirated N52 inline-6 it replaces in the Z4.

Although the I-6 produces an extra 15 hp, the N20 4-cyl. trumps it with a noticeable 38 lb.-ft (52 Nm) of additional torque, and the peak is available much earlier – at 1,250 rpm – and doesn’t let up until 4,800 rpm.

That broad powerband makes for effortless and exhilarating rowing through six speeds manually or eight speeds with the sensational automatic transmission that proves its versatility beyond the more staid 7-Series.

The 6-speed manual is standard, and the 8-speed automatic with its taller cruising gear is a no-cost option that, when mated to the N20, is expected to boost fuel efficiency about 20% over the N52 I-6.

Standard on the 4-cyl. Z4 with the manual gearbox is a stop/start system that shuts down the engine when the vehicle comes to a complete stop, then kicks back on with a slight burble when the brake is released.

Already installed on U.S. versions of the high-performance M3, as well as 5-Series sedans and the 640i coupe, stop/start is becoming common on German luxury performance cars as auto makers seek ways to reduce fuel consumption in the face of pending government regulations.

In the case of the Z4, BMW says the device boosts fuel economy some 3%, although a button on the dashboard allows the driver to bypass those savings, altogether.

Observed fuel economy after 90 minutes of thrashing on the Z4 and its resilient I-4 were impressive: 25 mpg (9.4 L/100 km), according to the vehicle trip computer.

Official Environmental Protection Agency numbers will be released in the coming weeks as the vehicle arrives in U.S. showrooms.

An estimated 20% improvement over the N52 I-6, based on the European driving cycle, would peg the N20 at 22/34 mpg (10.6-6.9 L/100 km) in city/highway driving.

Those numbers top the Audi A4 and its excellent high-volume 2.0L direct-injection turbo I-4 that pioneered the notion of 4-cyl. power in a luxury car.

That engine is rated at 22/30 mpg (10.6-7.8 L/100 km) and badly trails the Bimmer with only 211 hp. But the Audi engine, which won five straight Ward’s 10 Best Engines awards between 2006 and 2010, goes head to head with the N20 on torque, at 258 lb.-ft.

The TwinPower name for the N20 comes from the pairing of high-pressure direct injection and BMW’s Valvetronic intake control with a single twin-scroll turbocharger.

Valvetronic, a variable-intake valve-lift system that regulates how much air the engine needs, does its job without the throttle valve typical in most engines.

Instead, the system better manages combustion, minimizing pumping losses and boosting efficiency.

’12 BMW sDrive28i
Vehicle type Front-engine, rear-wheel-drive, 2-passenger roadster
Engine 2.0L DOHC DI turbocharged all-aluminum I-4
Power (SAE net) 240 hp @ 5,000 rpm
Torque 258 lb.-ft. (350 Nm) @ 1,250-4,800 rpm
Bore x stroke (mm) 84 x 90.1
Compression ratio 10.3:1
Transmission 6-speed manual
Wheelbase 98.3 ins. (250 cm)
Overall length 166.9 ins. (424 cm)
Curb weight 3,263 lbs. (1,480 kg)
Base price $48,650 (plus $875 handling charges)
Fuel economy NA
Competition Mercedes SLK, Audi TT, Porsche Boxster
Pros Cons
Fabulous engine, execution Audi did it years ago
25 mpg despite thrashing I-4 should come with price cut
N20’s next stop: 528i Where’s the 4-cyl. diesel?

Fuel injectors are centrally positioned between the valves, spraying a fine high-octane mist into the combustion chamber at a pressure of 2,900 psi (200 bar).

The injected fuel creates a cooling effect that allows for further efficiency improvements and a higher compression ratio of 10.3:1, although slightly lower than the 10.7:1 compression ratio for the outgoing N52 I-6.

Exhaust gases leave the engine two cylinders at a time, and each stream is kept completely separate as they flow through the exhaust manifold, taking a spiral path to the turbine wheel.

BMW opts for this design to minimize exhaust backpressure at low engine speed and maximize rotation of the turbine blades, meaning quicker engine response. As for turbo lag, there is none.

In casual mode, the N20 is just as pleasurable as during spirited driving. Noise, vibration and harshness is more than acceptable, and the exhaust is tuned to be a bit raspy, just to let all comers know this engine means business.

The all-aluminum N20 I-4 is about 22 lbs. (10 kg) lighter than the N52. Overall, the engine weighs 304 lbs. (138 kg).

Launched this year, the N20 appeared first in the X1 small cross/utility vehicle in Europe. The 4-cyl. engine is an excellent match for the X1, based on a Ward’s test drive of the vehicle at the Robert Bosch test track in Germany this past summer.

Bosch supplies the solenoid-based fuel injectors and high-pressure pump, as well as the engine controller, air-flow meter, sensors and other components.

But don’t count on the X1’s arrival in the U.S. anytime soon. The auto maker says demand is so strong elsewhere in the world that the plant in Leipzig can’t keep up. Canada already gets the X1, with the 2.0L N20.

The engine also appears this fall in the 528i sedan and is expected to be rated above 32 mpg (7.3 L/100 km) on the highway. When the all-new 3-Series arrives next year, count on it in that engine bay as well.

In the Z4, the engine can be had (for an extra $3,900) with the M Sport package, which includes adaptive suspension, aerodynamic body elements, 18-in. alloys, sport seats and aluminum carbon interior trim. A fetching new Canyon Brown leather package also is offered for the first time.

A 7-speed dual-clutch automatic transmission also is available with the uplevel Z4 sDrive35i, which propels the 300-hp N54 3.0L twin-turbo I-6 to 60 mph (97 km/h) in 5 seconds, a half-second faster than the I-4.

BMW has owned the market for European luxury roadsters since the Z3 arrived in 1996 and ended up in 15,000 U.S. garages that year.

The segment understandably fell on hard times during the recent recession, but the appearance of an impressive new Mercedes SLK and the N20 in the Z4 promises excitement.

For the first time since 2005, the SLK is within striking distance for leadership in the sector: Through August, the Z4 has held a narrow edge over the SLK – 2,792 units to 2,343, according to WardsAuto data.

The N20 is a welcome addition to the BMW powertrain family, despite a starting price (of $48,650) that represents a premium of $1,275 over the outgoing I-6, due to the incorporation of “new equipment,” engineers say.

Not to worry: Other less-expensive BMWs will come along that will prove the value of this engine, especially on roads such as Route 33.

If there’s a better mass-produced 4-cyl. engine, we’ve yet to see it.

tmurphy@wardsauto.com