The sport compact segment has grown competitive in recent years, from entry-level “tuner specials” (Scion tC) to delinquent “hot hatches” (Mazdaspeed3), all vying for the hearts, minds and pocketbooks of young enthusiasts.

Nissan Motor Co. Ltd., which has been notably absent from the segment, enters the fray in 2007 with a 2-pronged attack based on its new Sentra sedan.

Encompassing fourth-generation variants in SE-R and SE-R Spec V guise, the new models eschew rabid performance for affordability and packaging. Unfortunately, the tepid enhancements pale in comparison to those possible through modern aftermarket tuning.

At the core of the new car is a new QR25DE 2.5L high-output 4-cyl. gasoline engine producing 177 hp in the SE-R and 200 hp in the Spec V. Both feature variable intake valve timing, counterweighted crankshafts and dual balance shafts for extra smoothness and efficiency.

However, the Spec V mill adds a reworked valvetrain, a higher compression ratio and a less restrictive exhaust system.

Both units feel stout yet smooth and eagerly rev to the redline, especially in the Spec V. The more aggressive Spec V dons a 6-speed manual, but the regular SE-R is saddled with Nissan's Jatco Ltd.-supplied Xtronic continuously variable transmission (with six pre-set ratios operated manually by steering-wheel-mounted paddles).

On mountain roads outside Los Angeles, the Spec V proves nimble and athletic, despite its archaic torsion beam solid rear axle. The ride is a little soft but very composed, with a neutral personality that can provide either understeer or oversteer, depending on the driver's commands.

However, the standard SE-R, which foregoes most of the Spec V's dynamic additions in favor of a more “daily driver” demeanor, feels wholly inadequate when pushed hard.

The biggest detriment is the CVT gearbox (a first in the segment), which Nissan says is designed to offer drivers a more convenient option for long freeway commutes and unenthusiastic spouses.

This configuration is fine for normal Sentras, but the elastic nature of the CVT saps the SE-R's responsiveness and, in normal mode, leaves it hunting around for the proper ratio when the driver needs it most.

Making the SE-R the range-topping regular Sentra and designating the Spec V the standard SE-R would have better served Nissan, while also leaving room for a more track-focused (and more powerful) Spec V.

Also problematic is the styling. Body mouldings and badging do little to improve Sentra's bulbous proportions.

Inside, occupants find black sport seats with red stitching and red seatbelts (Spec V only), six airbags and active head restraints.

Nissan adopted a $100 per horsepower figure and pledges a “200 hp for $20,000” benchmark for the Spec V. Exact pricing will be released prior to the models' March 8 on-sale date, the auto maker says.

Nissan expects the SE-R models, built alongside regular Sentras in Aguascalientes, Mexico, to account for about 10% of U.S. Sentra sales, or approximately 12,000 to 15,000 units, with the mix split 50/50 between the SE-R and Spec V.

Although not poseurs, Nissan's new SE-Rs lose some performance credibility in their quest to attract a larger customer base, which likely would have been equally satisfied with a single, better executed model.

However, 200 hp for $20,000 in Spec V dress undercuts almost every one of the SE-R's rivals and positions the homely pocket rockets near the top of the attainable performance ladder.