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!"