Starting with GMHTP's July issue, we've been slowly feeding our readership (or, at least the open-minded portion thereof) details of a turbo system installation on our non-factory-supercharged 2007 Cobalt SS. Well, ladies and gentlemen, hunger no longer: this month, it's finally time to finish off the installation and put this little blue 4-banger through its paces!
For those who may have missed our prior installments on this FWD alt-hotrod in the making, here's what we've been up to. Not satisfied with a bolt-on-enhanced 148 whp and accompanying mid-15-second timeslips, we decided that to get anywhere near V-8 territory with a 2.4L engine, we'd need to go boosted. Enter Hahn Racecraft's "Stage 2" turbocharger system (Part No. H TS-2910, MSRP $4,199), a kit designed for 2006 and later manual- and automatic-equipped Cobalts and Pontiac G5s with the LE5 engine. Don't get us wrong: we are in no way attempting to duplicate the fantastic all-around performance of the LS/LT/SBC-powered RWD GMs to which this magazine owes its dedicated fan base. Instead, we're toying with an alternative, relatively low-dollar route to high-performance-and one that should make for a very sensible, economical, worry-free daily driven ride. We'd more or less polished off the turbo kit install at the Hahn facility last time, save for fuel system modifications.
So first and foremost, let's get Hahn's BoostFueler on the car before moving on to the really good stuff (i.e., power and performance results). We've mentioned this component in our previous installments, but never went into the details of how it works. (As a side note, Hahn Racecraft had used the BoostFueler on its Solstice and Sky turbo products-they use the LE5 mill as well-so by the time the Cobalt project rolled around, it was a proven setup.) As the name implies, the BoostFueler allows boost-proportional fueling in a form that works well with the ultra-sensitive ECM of the LE5 engine (we'll delve into that can of worms in a moment). But there's more to it than just that, as it also allows some end-user tuneability. According to company proprietor Bill Hahn Jr., "The field adjustability is a very strong value-added feature because it gives folks the ability to make adjustments that suit their particular condition-climate, equipment combinations such as different exhaust systems or boost pressures, etc.-that are hard to achieve in a solid fashion that suits every possible customer with just a single reflash approach." The installation of the BoostFueler is very straightforward, and the gist of it is this: as with most vehicles manufactured in the past few years, the stock Cobalt fuel system is returnless (primarily a concession to cost savings), so Hahn Racecraft converts it to a return-style setup that can vary the flow of fuel that is permitted to exit the fuel rail, thereby modifying fuel pressure at the rail as a function of manifold vacuum or boost. See our photo captions for more info on how the BoostFueler works and how easy it is to adjust.
In short, the BoostFueler is one of the main showpieces of ingenuity that allows the Garrett turbo kit on which this system is based to play with the 2.4L engine. So why exactly is the LE5 such a PITA to tune? In essence, it's the Variable Valve Timing (which is not used on any other Ecotec engine save the new 2.0L factory-turbo LNF). Hahn explains that GM's VVT is a closed-loop feedback system analogous to the O2-controlled fueling schemes we know so well. It continuously monitors cam positions and alters oil flow from the control solenoids to achieve the cam phasing it is programmed for. "It adds two magnitudes of complexity to the calibration because it adds two more axes to the fuel modeling that takes place in the ECM," says Hahn. In other words, take that pretty 3D fuel map from your custom Camaro tune and instead draw it up in five dimensions-needless to say, it's impossible for humans to even visualize, and the calculated airflow through the engine at any given time becomes extremely mathematically complex. Talented as your local LS tuner may be, this is a whole different ball game, and many a shop has been handed a boosted LE5-equipped car only to send the customer packing shortly thereafter. Hahn deserves major props for cracking this-so let's see how it turned out!
Nearly two weeks had passed since first dropping the car off at the Hahn facility (the kit install can be completed in a matter of hours; the added time spent here was for development and tuning on this pre-production kit). Now it was finally time for some behind-the-wheel action. The first thing I noticed when driving the car (other than the exhaust note being much quieter, which is a definite plus) was that a lot less throttle input was required to get the car moving. Talk about a change in response! Initial drives with Bill riding shotgun doing some laptop tweaks on ice-cold Illinois roads showed a huge lack of traction below 50 mph, thanks to the combo of FWD, skinny tires, and some serious newfound cojones under the hood.
Just how serious? Back in New Jersey, I headed to TT Performance Parts (Passaic, New Jersey) to get a solid comparison to our pre-install baseline. If you look at the accompanying dyno graph, you'll note almost 222 whp-73 more than our previous intake/exhaust combo (and just shy of 80 more against the exhaust-only situation-equating to 100 hp at the flywheel!). Obviously, values like these can be dyno-dependent, so the real world was the only way to get some true numbers-and that means hitting the 1320.
Quarter mile results were had on an early April day at Raceway Park (Englishtown, New Jersey). Two sets of back-to-back runs were performed. For the first pair, the tire pressures were a bit high, yet I was still able to nail a best pass of 13.59 at 103.8, on a 2.25 60-foot. This pretty much surprised everyone in attendance-including some magazine editors who shall remain nameless-and was only a few ticks off from the e.t.'s being turned during grudge races between a near-stock Pontiac G8 and Mustang GT! After the car cooled a while (during which time I dropped the tires to 30 psi), I did a second set of back-to-back passes, and the last was the best: 13.57 at 102.7. Clearly, I had nailed the launch better than before (2.19 to the 60-foot mark), but the air apparently just wasn't there by that time of day to support the same mph or a significant improvement in e.t. Considering a race weight of 3,090 pounds, this Cobalt is not a featherweight, and I can imagine what a stickier set of tires and/or some sort of a burnout would do to those timeslips. Low 13s anyone?
My experiences have revealed that there's much more to a Hahn-equipped Cobalt than just straight-line smackdowns. We want readers to know that Hahn Racecraft put very extensive development time into its final product, and the result, simply put, is about as close to OEM reliability and mannerisms as a turbo system can be.
We've already mentioned that a whole lot less throttle is needed to move the car. The other obvious part-throttle difference is the crisper shifts of the tranny, which are firm and occasionally hard enough to yield a little squeak underhood (presumably from gently rocking the engine/trans assembly on its mounts). But Hahn did a good job with the calibration of this electronically controlled slushbox, whose running parameters were halfway decent from the factory (typical of most GM 4-speeds). Shift points are basically unchanged at part-throttle, which is great, as the rpms stay low to take full advantage of the turbo torque. It's not kickdown-happy either, so you can, say, remain in 4th and build some boost up steep highway inclines (whilst enjoying the easily audible turbo whistle, mind you). At full throttle, shifts are instantaneous but definitely not neck-snapping. Driving pleasure, as with all cars, must be greatly improved with a manual (remember, this kit works on 5-speed cars as well), but there's nothing inherently bad about the 4T45-heck, it's even stout enough to have survived well in heavy, 3800 V-6-powered, full-size FWD GMs. Bill says the reliability results from Cobalts in the field thus far have been "stunning:" zero reports of tranny slippage or failure, and none expected at these power levels. In fact, the manufacturer's torque rating is actually higher for the 4T45 than it is for the Getrag manual; and for those who are stick-inclined, Hahn has taken those trannies into the low-11s in the quarter with zero attrition-that's with 600hp engines and slicks!
Worried about turbo lag? Forget about it. At part throttle, sure, you can roll into it gradually and let the turbo spool-but anything more than, say, half pedal movement gives instant gratification, with the kind of instantaneous go-power you might find in an N/A or supercharged engine. This has to do with the highly appropriate sizing of the turbocharger. "The GT2860RS is a great middle-of-the-road choice, which at its outer limit can support about 400 engine hp," says Hahn. "It has outstanding low-speed response, and begins to spool up hard in the 2,500-rpm range on these cars. It's a testament to modern turbo and turbo system design, in that there really is no lag-that's an outdated misnomer, born of an era of inefficient designs, when the body of knowledge and capability was not there. These days, I like to say that turbos give you 'continuously variable' power-it's like having a variable-displacement engine that's infinitely adjustable based on the position of your right foot!"
As to other driving dynamics, the traction control still works, and kicks output a good bit when in full activation (it is, after all, closing the throttle for you). But if you roll into it on a slippery surface, it functions pretty well with you to accelerate the car. And as typical of a high-powered FWD'er, torque steer is definitely present, but it's by no means uncontrollable-just be ready for it when you stab the throttle. As long as you're prepared to make slight corrections to the wheel, you're in business (my advice: give whoever's going to borrow your ride a heads-up about this, along with the more general concern that this is no longer your typical sub-180hp econo-car!)
There have been a few minor negatives that, in the interest of completeness, we ought to mention. On some full-throttle bursts (especially down the 1,320), the yellow SES light starts flashing on the dash; but rather than indicating an actual problem, this is simply a result of the additional energy needed to light the spark plugs tricking the ECM into thinking the ignition coils are not firing-whereas actually they are-and throwing a P0300 "misfire" DTC. (I'm personally planning on working with Hahn to turn down the sensitivity of this code-Bill is adamant about not simply "switching off codes on a wholesale basis," as can be typical in the tuning industry.) I had to make a few minor adjustments to cure some underhood component rubbing, and in some places zip-ties were utilized to keep vibrations at a minimum. Some non-factory noises still emanate from underhood occasionally, but they are definitely livable, especially for an aftermarket turbo system. Speaking of noises, the sound of water rushing through the turbo took a little getting used to (it's easily heard since it's right behind the firewall), and once in a while there's a hard cold start, but it never takes more than two key turns to get the Ecotec purring.
If you're wondering about emissions legality of this kit, this question doesn't have a definite answer. While the system retains all required factory equipment, Hahn says that laws vary so widely across the U.S. that he leaves up to the customers to know their local regulations and conform to them. Legal technicalities like CARB E.O. numbers (which would be very difficult to attain on a standard-location turbocharged car thanks to cold cat lightoff concerns) aside, Hahn assures us that the kit actually decreases overall emissions of both greenhouse gases and other pollutants, along with enhancing catalytic converter life (by allowing it to run cooler). As an example, hydrocarbon counts are generally lower thanks to a more effective "afterburn" of the fuel.
As to CO2 emissions (the reduction of which is all the rage these days), it's a simple matter of less fuel burned per mile. Indeed, I've seen increased gas mileage since installing the kit (I have found the average economy dash readout to no longer be accurate, by the way). Now, don't expect Prius-esque economy numbers: all in all, the increase is probably just sufficient to offset the additional price of now-mandatory premium gas. I've yet to fill up and show less than 25-27 mpg, even with substantial amounts of around-town driving. Chock it up to the turbo reducing pumping losses, even at cruising speeds-it recovers some otherwise-lost energy and adds very slightly to the volumetric efficiency of the engine. (This is one reason you'll be seeing a whole lot more turbocharging from OEMs in the future, BTW.)
When all is said and done, the result has turned out to be an economical, quick daily ride, and with my WS-6 free to get even more well-deserved beauty rest in the garage, I don't need to fret so much about it getting stolen, smashed up, or likewise being sent to a premature F-body afterlife. As the saying goes: "Mission accomplished!"
The Fun Has Only Begun!
We're calling this project a done deal for the time being, but want to let you know that Hahn is still working on ways to make even more Ecotec power for those interested, and some of the products needed to attain this have already been released.
The system we've just installed is pretty well optimized the way it comes. But even with the kit as-is, Hahn says there's an additional upward ceiling of about 2-3 psi of boost, as the turbo and intercooler can handle them. After this, changes such as larger injectors would be needed. Hahn tells us higher-powered iterations are in the works for future release.
Yes, Bill assures us that if people want further increased power levels out of these engines, it's definitely there for the taking. If you want to start really getting serious, there's Hahn Racecraft's so-called Hi-Boost Engine System for 2.4L engines that Hahn says "enables people to take advantage of the very robust LE5 bottom end (which features forged rods, oil-jet piston cooling, and the like), but corrects the aspects that prevent these engines from being readily exposed to very high boost numbers." The basics: heavy-duty valve springs, head studs, and a thicker MLS head gasket that drops compression from the stock 10.6:1 to 9.1:1-enabling a claimed 18-20 psi on 93 octane! Also included are associated doodads to get it to work with GM's sensitive factory VVT system.
It doesn't end with the LE5, either. As to other Cobalts, Hahn has products in development for the '08 SS Turbocharged, and right now offers complete turbo systems for '05-'07 LSJ-equipped cars that replace the factory supercharger system with a turbo system. This kit will produce 320-360 whp on the bone stock LSJ engine with pump gas. Plus, there's a Hahn kit for 2.2L Cobalts-check out the company website for more info.
Hopefully, you're starting to see that the Ecotec is a power-potential-rich engine family-it can do a lot of the same things our V-8s can, though admittedly with a far less gnarly exhaust note. Still not convinced of the worthiness of an I-4 GM, and its suitability to create a badass, fuel-sipping daily ride when turbo'ed? Don't worry, because Hahn Racecraft assures us it has kits for the RWD LS-powered GMs we know and love in the works as well. And since you're reading GMHTP, you already know where to look for 'em!
The BoostFueler adjustments are best performed on a dyno-here we double-check Hahn's work back at TTP. A boost gauge is hooked up (none is provided with the Hahn kit, but feel free to mount one in the cockpit so you can get thrills out of seeing its 8-9 psi). The large Allenhead atop the unit is adjusted until the fuel pressure gauge reads 25-30 psi at idle. Then, during a full-bore pull, the small brass control valve we've mentioned earlier is twisted to yield 70 psi. Be careful not to turn this latter knob too far or you'll prevent the fuel pressure from bleeding off when you let off the throttle. In this way, the BoostFueler system can be easily adjusted to match the exact setup of a given vehicle (no ECM tuning required or allowed here-Hahn mails it back to you in a "locked" state, i.e., its calibration cannot be altered.)
In addition to showing us DTCs (like the intermittent P0300 code we've already mentioned), our VCM Scanner software also allowed us to see (and log) parameters like knock sensor behavior. Truth be told, a few degrees of knock retard is now more or less standard fare on our LE5 when in the boost, but Hahn explains that this is typical for a forced-inducted Ecotec: the engine family's knock sensors and characteristics are such that it's actually one indication of optimized ignition timing, and one must resist the temptation to draw hard-and-fast comparisons to what knock retard indicates on an LS or other GM V-8s. Plus, we've never actually heard the slightest smidgen of any unusual combustion events from underhood, so that's great news.