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2004 Pontiac GTO - Hahn Racecraft 2004 Twin-Turbo GTO

Arvid Svendsen Oct 23, 2006

Interested in a daily driver that has 476 rwhp on tap, can idle like a stocker in traffic jams, run 11-second quarter-miles, and get 26 miles per gallon on the highway? Working with a 2004 GTO, Rick Bottom and Bill Hahn have combined efforts to build a car that achieves performance and drivability goals that were at one time thought impossible. The partnership of Rick Bottom, of Rick Bottom Custom Motorsports (815/539-6487) in Mendota, Ill., and Bill Hahn of Hahn RaceCraft in Yorkville, Ill., has created this Twin-Turbo, LS1-powered GTO that represents the best blending of an excellent OEM platform and state of the art turbocharger technology. The result is a stunning, high-impact car that should appeal to many performance enthusiasts across the board.

The 2004-2006 GTO sets a high watermark in American musclecars because it is arguably the first thoroughly modern GT car. Its excellent LS1/LS2 V-8 power, Tremec six-speed, independent suspension, and rock solid chassis establish the GTO as a serious contender amidst the competition, domestic and imported. The GTO offers the potential for what can be considered a world class, sophisticated performance car.

The concept behind this Bottom/Hahn Twin Turbo 2004 GTO was to build the ultimate street machine that would catch the attention of the tuner/drifter crowd and concurrently gain the respect of the hardcore musclecar enthusiast.

The owner of the car, Rick Bottom, states, "A lot of people criticized the new GTO as being bland. But that's part of the appeal because it lends itself to personalization. GM is trying to get the younger guys on their side by appealing to the tuner/drifter crowd. Having done a few tuner cars, I really felt the tuner theme worked on the GTO. It's a modern car, one that doesn't lend itself to retro themes. There's nothing retro about this new GTO, except of course, the name. The GTO is aimed directly at a younger crowd that is along the line of Rhys Millen's drift car."

Questioned as to the name for this twin-turbo GTO project, he says: "The name 'Street Cleaner' originates from the turbo sound, because they make a 'whooshing' sound. That sound reminded me of a street cleaner. The car is so fast, it creates the picture of cleaning the streets of any and all competition."

The creation of "Street Cleaner's" tuner theme was achieved through a number of exterior modifications. In order to enhance the exterior appearance of the GTO, an RK Sport Body Kit was installed. A Webasto Sunroof was added for increased driving pleasure. The superior quality of the Webasto Sunroof explains why it is the Holden OEM supplier of sunroofs in Australia. The quarters were widened an inch on each side. An MPD Ram Air hood was installed that was custom built to channel air to the turbos. The GTO was painted using Glasurit paint products at Rick Bottom Custom Motorsports.

Front and rear suspension modifications include Eibach springs installed for a total two-inch drop. Stock rear struts were retained with the addition of Monroe air adjustable shocks in the rear keep a consistent ride height when passengers are on board. Axis Hiro 19x8.5 fronts and 19x9.5 rears were mounted on Dunlop SP9000 tires. Staggered-sized 235/35/19 fronts and 275/35/19 rears were installed in order to achieve a rake that would let people know that the car was a rear-wheel driver.

The high ride quality, as good or better than OEM, is the successful combination of the Eibach springs and Dunlop SP9000 tires. In the owner's view, the Dunlops provide better ride quality than any other low profile tire on the market. Bigger front 13-inch Stainless Steel Brakes with slotted rotors and three piston aluminum calipers replaced the stock units. The stock rear disc brakes were deemed more than sufficient for excellent braking. The Tremec six-speed benefits from the SPEC Stage 3 hybrid clutch and billet flywheel. A factory limited slip differential spinning 3.42 gears provides an excellent performance/cruising combination.

What would it take to get an old 1970 GTO to make almost 500 hp at the wheels? It would take a stroker 455 with Edelbrock heads, a lumpy cam, and a big 850-cfm carburetor. Will a typical Chicago hot rodder get in that car and drive it to California? Not likely. That old school technology is going to get 8 mpg, idle at 1,250 rpm, and break valvetrain components. Though big on the wow factor, it would be driven occasionally, and be a chore to maintain reliability. Modern technology provides a better alternative.

Bill Hahn, owner of Hahn RaceCraft, a recognized leader in turbocharger systems for a broad range of vehicles, was on hand to discuss the Twin Turbo GTO:

"At Hahn Racecraft, we wanted to show the capabilities of the GM LS1 and LS2 drivetrains. We wanted to build this car with serious impact, a tour-de-force to demonstrate our capabilities with some very tasty design and execution. The goal was to maintain OEM level engineering from a twin turbocharged engine that would still maintain the daily driving civility of the car. We ended up with a car that puts 476 hp to the wheels at 7.5 lbs of boost with 93 octane gas. That power is achieved without internal engine modifications, on a car that retains its stock catalytic converters, and thus complete emissions legality.

"This GTO is an important car for Hahn Racecraft because we've been very impressed with the LS series GM engine, and see its massive potential in the years to come. We wanted to demonstrate what it can achieve with our unique flavor. While we have done custom V-8 turbocharging technology since the early 1990's, only as of late are we bringing V-8 turbo systems to the general public. This GTO had to show what we are capable of giving the customer in terms of design excellence and results. It's been a fabulous prototype for the next steps. We'll start with turnkey installations at our facility, and if demand permits, we'll then package the turbosystem to be sold and installed in the field."

The system's design criteria are evident from the first view. It has an OEM-yet-trick flavor, with every part purposeful. Extensive heat shielding is used to show off fabrication techniques and also provide a finished look. Note the lack of spark plug wires-they've been cleverly re-routed using ACCEL leads to clean up the appearance and protect them from heat.

Hahn RaceCraft chose ultra-modern Garrett GT 2871R ball bearing turbochargers for "Street Cleaner." With proper internal engine modifications, this particular iteration of the turbos is capable of handling up to 800 hp. But that's not the limit, as if 800 hp weren't enough! These units were chosen by Bill for a number of reasons, and their ability to be upgraded is one. They can be fitted with upgrades on both the turbine and compressor sides. This allows not only higher-horsepower choices up to 1,000 hp, but also tailoring of specific rpm response characteristics, as well as favoring specific portions of the available rpm band.

As an example, should an engine with modified cylinder heads and extra rpm capacity be used, the turbos can be sized to take advantage of this high-end breathing advantage. If, on the other hand, a wicked midrange is desired for fantastic drivability, this can also be enhanced via the correct turbocharger trim.

An additional advantage is the ability to upgrade the turbos as the desire for more power unfolds. For instance, one might choose to start with a stock engine, and later advance to a built engine. At that time, the turbochargers could be upgraded to accommodate the new power capability.

The choice for the twin-turbo configuration, as opposed to a single unit, was based on a number of factors. With the GTO, packaging and plumbing had to be minimally intrusive with the other underhood components. Consideration had to be given to the possibility of larger turbos in the future, so thoughtful design was a must. Using a single larger turbo on a V-8 engine typically involves a bit of crossing over with the exhaust plumbing to accommodate both cylinder banks, leading to complex piping that can be hard on neighboring components due to heat.

The twin turbo, one unit per side approach might seem more complex at the outset, but it can actually simplify things. In addition to the packaging benefits, the performance gain of using the two smaller turbos creates a lower rotational mass to accelerate, in that the smaller wheels will spin up to speed more rapidly. Thus, properly sized twin turbos will respond, or get up on boost more quickly, than one large single turbo. With the twin turbo setup, the turbos are also closer to the source of heat, the exhaust port. This further improves the response of the turbos. Here is where Bill Hahn, renowned professor of turbocharging, steps in for an explanation:

"Turbochargers are not so much processing exhaust gas flow as they are processing exhaust gas heat energy. If we go back to our high school physics classes, we recall that energy can never be destroyed, it can only be converted. When we monitor exhaust gas temperatures at the inlet and outlet side of the turbocharger, which is a space of a mere several inches, the exhaust gas temperature at full load will drop 200 degrees from the inlet to the outlet side of the turbo. What does that tell us? Where did that energy go? Simple ... it's been converted to the motive energy that spins the turbine shaft to create the rotation of the compressor side of the turbo. That's what this type of turbine does: it uses heated gas energy to create rotational energy. Therefore, the more heat energy we can deliver to the turbocharger, the quicker it will come on boost, and the greatest amount of recovered heat efficiency will occur. We've also learned that today's massive power capabilities can be detrimental to catalytic converter life, actually overheating and destroying the internal catalyst at these high power levels. Turbochargers located close to the engine, and before the catalytic converters, reduce heat stress on the converters by acting as a 'heat sink.' GM chose to equip its new 260hp, 2.0 liter Solstice GXP with turbocharging instead of supercharging for this reason, and that little monster is the highest hp-per-cubic-inch engine that GM's ever put in a passenger car!"

Commenting on the killer Dual-V intercooler hidden under the RK Sport bumper cover, Hahn concedes, "The intercooler is so huge simply because of the 'wretched overkill' personality we wanted this system to exude. Even though we are currently making only about 550 flywheel hp on the stock LS1, this wicked dual-core intercooler will support 1,000-plus hp, and it will not become parasitic in terms of pressure drop, even at those levels. So, while this GTO is putting out 'only' 476 rwhp, the turbosystem's capabilities can be radically upgraded with improvements in the engine's bottom end. Combined with the larger turbos and bigger injectors, this system can be a 1,000-hp-plus package."

The intercooler and induction duct combination is an outstanding accomplishment by itself, combining two gigantic 6-inch tall, 24-inch wide, and 4.5-inch deep bar-and-plate cores into a V-shaped sheetmetal and TIG-welded work of art. Its unique configuration takes the boosted air from each turbocharger and merges it together in the center of the car, where it is then ducted past the MAF meter on its way to the throttle body. Its configuration also greatly reduces the amount of charge air piping normally required to support a twin-turbo design. Bill estimates that well over 100 hours of design and fab time went into this wild prototype intercooler/ducting, and it shows!

Enthusiasts speak of turbocharging and "turbo lag" from the context of 1960s and '70s cars, when engine management and turbocharger design was relatively primitive. Modern turbochargers and improved engine management, coupled with modern engines themselves, produce the ability to build turbo systems with virtually immediate throttle response. The turbochargers on the GTO begin making boost at 2,000 rpm, not very far off idle. With that kind of throttle response, "turbo lag" is a non-issue, even a misnomer.

A well-executed turbo system, with the turbos close to the engine where they can react to exhaust gas temperature rises immediately, is the formula for what Hahn calls "Power on Demand." Bill says, "The power level achieved then becomes simply dependent on how hard one pushes the accelerator, like an infinitely variable supercharger you control with your right foot. No need to wait for higher rpm to get more power, just push the pedal harder!"

In a street turbocharging application, the tightrope walk is a balance between power and excellent response. For a car to be completely tractable, while possessing good throttle response, it all comes down to engine management. Engine management today is a more complex picture, not simply spark and fuel. Current engineering in engine management now involves variable camshaft timing, torque management working with electronic throttle controls, catalytic converter heat management, and a myriad other factors that increase performance, while at the same time improving emissions and gas mileage. Fortunately, the same tools used by OEM engineers to optimize performance, emissions control, and fuel mileage can also be used to optimize a turbocharger system.

At Hahn Racecraft, optimum engine management is the highest priority. Hahn explains, "With the turbos installed, we then need to do the calibration of the computer. Even though this turbo system was designed, fabricated, and installed in a marathon session of only six days prior to the 2004 SEMA show, the actual calibration and validation of the engine management system took a period of months, and is always subject to further improvement should we discover an area that can benefit. The engine management is progressively calibrated with data collected, and then the system is validated in testing and real world driving."

Is Bill Hahn a true fan of turbocharging? In his words, "No engine is complete without a turbocharger (or two!)". Because turbochargers use a soft coupling between the engine and the turbocharger (exhaust gases), the power onset is always a linear increase of power that isn't sudden or abrupt, and doesn't tend to shock driveline components.

Durability and drivability, with phenomenal horsepower potential, are certainly the clear advantages of turbocharging. Again Hahn adds, "With turbos, we can take near stock engines and in some cases get two times the stock power out of them, while still achieving reasonable durability. How is this possible? A turbo is able to provide a denser air/fuel charge into the cylinder. With a turbo system, we don't cause a more violent explosion in the cylinder; rather, we create additional power via a longer power cycle due to this denser charge. A normally aspirated motor's combustion event ends with the piston still on its way down. With turbocharging, there is a longer combustion event that is continuing to push the piston down for a longer period of the power stroke and crankshaft rotation. As a result, there is more power without dramatically compromising the durability of the engine through violent pressure rises. In today's times, better engines, better engine management systems, and better turbos have combined to produce the golden age of turbocharging."

This twin-turbo system is undoubtedly outstanding design and craftsmanship coupled with world class daily drivability and extreme horsepower. This twin-turbo GTO delivers stock-like mileage and 550 hp at the flywheel. That type of capability was not available until the emergence of the LS1, and now the LS2s have appeared on the market.

Referencing the twin-turbo GTO's ability to create this exceptional performance package, Rick Bottom sums it up nicely, "The car has the stock cats with the Corsa noise cancellation technology. It's whisper quiet. When you stand on it, there's some sound, but it's really about power and refinement. If you're rolling along in Second, it breaks the tire loose. In Third gear the motor is really working strong ... each gear pulls harder and harder. The old GTO felt bad but it worked. The new GTO is really fast, but it doesn't feel as fast because it's more effortless. The new cars hook up and go. There is no comparison in the performance."

Hahn's turbo kit allows the LS1 to produce 476 rwhp. Editor Jensen spent some time behind the wheel of this beast on the backroads of Yorkville and found it to be a refined rocket with excellent spool-up and enough power and torque to deceptively move the big GTO. Not so much like a rip-snortin' F-body--more like an M5 BMW on steroids!


Hahn Racecraft
Oswego, IL 60543
(630) 801-1417



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