Since its inception in 1992, Saturn hasn't exactly flown the performance flag for GM. Not that it was ever meant to. The company has brought its share of design, manufacturing, and sales innovation to the market, and Saturns have a reputation for being well-built and easy to buy. But in terms of actual, get-your-adrenaline-pumping thrills, I suppose there's not much one can expect from a car company named after a Roman god of agriculture. Heck, until recently, presumably the only "rush" Saturn owners got was while viewing an unscathed plastic door after a good whack from a shopping cart.
Glimmers of hope for leadfoot junkies emerged in model year 2004 with the new Ion Red Line. With its supercharged 2.0L engine, the 205hp Ion introduced the Red Line edition of vehicles to Saturn's lineup and the first bit of excitement Saturn drivers had ever known. But it still lacked a lot in the styling department and, perhaps more significantly, a true performance-oriented driveline layout. Plus, 205 hp, while a big improvement over the standard 2.2L motor's 140, didn't exactly create class-leading performance.
Model year 2007 builds substantially on this slow-to-get-going excitement theme at Saturn with the introduction of the Sky roadster. One of the coolest looking cars introduced in recent years, the Sky--along with its Kappa platform sibling, the Pontiac Solstice--is poised to compete with successful convertibles like the Honda S2000 and the Mazda Miata (come on, nobody calls it the MX-5). With a base price of $23,690, the Sky has been on showroom floors since late spring and is selling well. But its 2.4L, 177hp LE5 engine leaves something to be desired for those of us seeking serious speed. Fortunately, the automotive aftermarket has already started to catch on to these vehicles, with exhaust systems, turbo kits, and the like. But for a car with such a capable chassis and looks to kill, you'd think GM could cook up something a little more impressive under the hood.
Enter the Sky Red Line. Scheduled to be introduced this fall, this car's subtle styling upgrades over the already-gorgeous standard Sky do tasteful justice to the real news for enthusiasts: a turbocharged 2.0L engine with the first example of gasoline direct injection in GM's North American market. The performance industry has been following this vehicle's pending release very closely, and while the 260hp and 260 lb-ft (both coming at 5,300 rpm) originally quoted by GM were "pending certification" with the new, rigorous SAE J2723 engine test, we're happy to report this turbo'd mill passed with flying colors.
We're going to introduce you to the amazing engine technology you'll see when this new car debuts, and discuss the vehicle's other improvements over the standard Sky model. We'll also talk a bit about the possibilities for the involvement of the automotive aftermarket, and finally give some predictions--ahem, caveats--for the success of a car that could very well be the next big thing in performance, GM or otherwise.
Over the years, GM's Ecotec line of engines has by and large been seen as nothing more than an economical and reliable underhood offering. But with its "Gen II" revision that debuted with the 2.4L LE5 engine, GM drew upon experience racing the Ecotec engine family and built in strength to handle extra horsepower, with thickness being added to the main bearing bulkheads, cylinder bore walls, and other areas. The stage was set for a badass, forced-induction four-banger, and here we have it with the turbocharged, 260hp 2.0-liter known as the LNF.
Built for toughness, the LNF features a steel crankshaft and forged connecting rods. The pistons, while of a cast aluminum variety, feature a built-in oil galley and nifty jet-spray cooling system. We spoke to GM's Ed Groff, Assistant Chief Engineer for the LNF engine program, about this and other aspects of the design. "During development, we attached thermocouples to the pistons to allow us to analyze temperatures while the engine was in operation. Piston temperatures are a function of power level, and based on what we saw initially we decided that for the 260hp target power level, we were closer to our in-house limit than we wanted to be. This jet spray technology allows us to cool the piston by between 40 and 50 degrees C (104-122 degrees F), opening up a large safety margin for the engine."
For power, this DOHC engine features variable valve timing on both the intake and exhaust, and a "dual-scroll" turbocharger that delivers 20 psi of boost with minimal lag. This is achieved through some pretty badass technology. "We separate the exhaust passages of cylinders 1 and 4 from those of cylinders 2 and 3 all the way from the exhaust ports to where they enter the turbocharger," says Groff. "So you actually have two nozzles feeding the turbine wheel; hence the term dual-scroll. This creates somewhat stronger exhaust pulses as different cylinders are not interfering with one another as the engine goes through its firing order, which helps the turbine. Also, going off of conventional four-cylinder tuning during the valve overlap period, the cylinders are not experiencing pressure pulses coming back from the exhaust. Because of the combination of the dual-scroll and the VVT, we are able to design in a lot of valve overlap at low engine speed, which helps low-end torque."
But the real story on the LNF is gasoline direct injection, the first system of its kind offered in North America by GM. Instead of squirting fuel into the intake port upstream of the intake valves, fuel is instead injected directly into the cylinder during the intake stroke--at an amazing pressure of up to 2,250 psi. This results in superior fuel atomization and a more efficient burn. To accommodate such a SIDI (Spark Ignition Direct Injection) system, a special intake manifold, cylinder head, high-pressure mechanical fuel pump, and variable-pressure fuel rail are used. The SIDI system pays many dividends, including allowing a higher compression ratio as well as a leaner air/fuel mixture.
Also noteworthy for the LNF-equipped Sky is that fuel efficiency is improved over the 2.4L model, with an estimated 21 city/30 highway with the manual tranny versus the standard model's 20/28. Although clearly this difference has to do in part with the 0.4-L decrease in displacement and efficient direct injection system on the Red Line's engine, it probably is also due to the fact that GM engineers cut the final drive ratio from 3.91 to 3.73 for the car's turbo version. The two models share similar axle assemblies borrowed from the Sigma platform (CTS-V), so this is at least encouraging news for Red Line owners looking to swap to a lower final drive ratio for quicker acceleration.This turbocharged 2.0L mill will also be used in the 2007 Solstice GXP and Opel GT. And as if 260 hp isn't enough, look for substantial power increases on this engine in model years to come!
Engine Electronics: High-Tech
In addition to the advanced electronics needed to run the new SIDI system, the Red Line's new E69 ECM takes charge of other neat powertrain control functions like electronic throttle control. The ECM provides spark signals through a Quick-Sync 58x ignition system, and the cam and crankshaft position sensors are digital. Another cool feature is that despite the forced induction character of the LNF, premium gasoline is recommended, but not required. "We've tested this engine on all lower grade fuels and confirmed that the control system is able to prevent detonation," says Ed Groff. "The two knock sensors on the engine alert the ECM and it will take steps to control it, it's a computerized learning process. We've tested this engine all the way down to the 85 octane fuel you find in high elevations. But to get the full 260 hp, the engine should be operated on premium fuel."
Perhaps most interestingly, the LNF sports something that GMHTP readers know all too well: a wide-ratio (or, as we like to say, wideband) oxygen sensor. We talked to Ed Groff about why the design team chose to go this route over a conventional narrow-band unit. "The wideband is pretty uncommon in a factory application, and we use it because it gives advantages during the high level of scavenging that we use to increase the turbo response. Particularly at low speed with all the valve overlap, there can be a lot of fresh air going through the engine. So the mixture you sense in the exhaust can actually be lean while you are stoichiometric or even rich within the cylinder. We found the accuracy of the wide-ratio oxygen sensor to be beneficial in this particular application."