The new Subaru WRX is like the preppy kid in school who can really let it rip on Saturday nights. Then there’s his bad-boy brother, the WRX STI, who’s rowdy all the time.

Subaru within two months introduced ’15 versions of both redesigned sports compacts. They share family resemblances, but their personalities differ.  

Both hold birthrights to belong to the performance-car clan.

The STI sports a spoiler; the WRX doesn’t. The STI rides on larger wheels (18-in. vs. 17-in.), and its badging is bolder.

Otherwise, the two cars share the same exterior styling, which includes a wide-body stance, crisper lines, a hood intake scoop and a new nose-cone front. The overall design looks more refined than the outgoing generations dating to 2009.

The main differences between the cars are driving dynamics and power. Neither car is wanting, but the STI is more powerful and aggressive. It should be, because it costs about $8,000 more. It’s the star athlete of Subaru, an automaker that prides itself on engineering above all else.

The WRX’s high-compression 2.0L 4-cyl. boxer engine features direct-fuel injection, a twin-scroll turbocharger and an intercooler. It produces 268 hp at 5,600 rpm, accompanied by a broad torque curve that peaks at 258 lb.-ft. (350 Nm) across a 2,000-5,200 rpm range. Torque tip-in comes quick and often.

The FA20 engine also powers the Subaru Forester CUV, but the WRX version uses different camshafts, higher-rate valve springs and other enhancements to extend performance and broaden the torque range. The WRX engine redlines at 6,700 rpm, compared with 6,000 rpm for the Forester.

The ’15 WRX comes with a new 6-speed manual transmission plus Active Torque Vectoring for improved agility. After a 5-year absence, the return of an automatic-transmission option on the WRX aims at broadening its market appeal. The CVT lets a driver using shifter paddles choose the equivalent of 6- and 8-speed gear modes.  

The track-ready WRX STI flexes more muscle than its sibling because of its 2.5L turbocharged and intercooled 4-cyl. boxer engine. It reaches 305 hp at 6,000 rpm and 290 lb.-ft. (393 Nm) of peak torque at 4,000 rpm. The engine is exclusive to the WRX STI.

The STI engine is based on a specially reinforced semi-closed deck block with high-strength cast pistons. That setup offers cooling efficiency when the driver is going full throttle, often the case with STI owners.

The STI’s exhaust system emits a distinctive growl through signature quad tailpipes. Engineers usually work to keep engine sounds from entering a car’s interior. But a level of “desirable” engine noise is pumped into the STI people compartment. It is music to the ears of STI buyers.

And that spoiler is functional, not just a fashion statement. It’s designed to generate stabilizing down force on a car with rally-racing roots. 

The WRX STI is equipped exclusively with a specially reinforced 6-speed manual transmission, which includes an STI short-throw shifter in the Launch Edition model. An automatic transmission is not an option. Nor should it be one for this speed racer.

So the STI wins the power contest, by design. It’s the most powerful engine in the segment which includes the Mitsubishi Lancer Evo and Volkswagen Golf GTI.

Subaru also bestows the STI with high-grade equipment for outstanding driving dynamics. That includes Active Torque Vectoring, an enhancement of the Vehicle Dynamics Control system, for improved cornering.

The turbocharger is under the engine of both cars. That low center of gravity does its part in contributing to driving stability.  Both vehicles offer that. They’re on a new platform offering a stiffer, more nimble chassis.

The chassis engineers really went to work, especially on the STI, making greater use of high-tensile-strength steel and stiffening elements at key locations. The result is incredible agility.

Improved steering responsiveness is a standout feature on both vehicles. The turning ratio is quicker on the STI compared with the WRX (15:1 vs. 13:1). But the sharper steering on both earns Subaru bragging rights. The automaker gladly accepts those. A Subaru engineer draws comparisons to the steering attributes of a Porsche 911. That might be pushing it, but not far.