Our test vehicle has a stock '04 Cobra 4.6 engine (281 ci). That size engine reacts nicely to the 76mm turbo supplied in the kit. It makes great peak horsepower, but still builds boost in the lower rpm ranges. One look at the power curves in this story shows how broad the torque and horsepower curves are. "It took some time to work with Turbonetics to find the right turbine/ exhaust side of the turbo to eliminate lag and maximize peak horsepower," Urist says.
Turbos are essentially two different halves--a compressor side and a turbine side. Often, people look only at the compressor side of things, i.e. 76mm turbo, 91mm turbo, or 101mm turbo. It is the exhaust/turbine side that drives the compressor wheel. Think of the turbine side as the pulley in a blower setup. The hot exhaust gases expand in the turbine housing to help spin the wheel. That drives a shaft that spins the impeller on the compressor side. It is not entirely exhaust flow that spins the wheel but rather expanding exhaust gases. This is known as thermal expansion.
When selecting a turbocharger for any engine, being able to turn the exhaust wheel efficiently is just as important as what the compressor produces at peak operating impeller speeds. The bigger the exhaust housing, the faster it spins at higher rpm, but if matched improperly to the engine, low-end compressor speed will suffer. Too small and peak horsepower suffers miserably. It is a balancing act between the turbine and compressor wheels for a given combination.
While the car was on the dyno, we tested a few different compressors and exhaust housings just to see the difference in power. A more efficient exhaust housing netted the same horsepower results but at a lower boost level and less fuel consumption. Going up one more size resulted in excessive boost lag--remember our test subject had a 281ci engine. It goes to show how matching the compressor and turbine is critical for different applications. Our test car made 847 hp to the tires at 24 psi of boost with the 76mm turbo and 16 degrees of timing. This was also with stock exhaust manifolds. Urist says the exhaust manifolds have not been a restriction in power with the 76mm turbo. He has taken the turbo unit to its maximum output, and it was the compressor side that maxed out before the exhaust system was a problem.
A little curiosity never killed anyone, and we asked what it would take to crank out 900 or even 1,000 hp at the tires. Urist got a look in his eye and asked if we really wanted to make 1,000 to the tire. He did remind us that was eight-second power, and was that necessary for the street? We're horsepower junkies--of course we need it. We have a feeling Hellion has something brewing to satisfy our desires.
Nate Phillips' '04 SVT Cobra At A Glance
Surprisingly, the car remains mostly stock, and Phillips has upgraded parts only as needed. We want to point out that the stock exhaust manifolds have been left in place, but the Hellion kit will work with aftermarket shorty headers. The IRS remains stock and hasn't caused any problems, despite rolling around on Mickey Thompson Drag Radials. The long life is attributed to no drag racing-like launches at the track or on the street.
The fuel system was in dire need of attention thanks to the planned turbo installation. A pair of Lincoln Aviator pumps, rated at 270 lph, was installed as well as a -8 feed line and prototype Hellion fuel rails. DiabloSport tuning was utilized to tune the beast, and Phillips chose to use a chip instead of reflashing the computer. The stock MAF sensor was used, but is aided by a MAFia. The DiabloSport MAFia recalibrates the MAF sensor signal enabling use in high-horsepower applications. It also recalibrates for larger sized injectors--in this case we were using 60-pound high-impedance fuel injectors.
The transmission is holding up just fine, but the clutch is a different story. The stock unit is long gone. Urist joked that Hellion was also a McLeod dealer. A McLeod clutch, pressure plate, and lightweight flywheel transfer the power into the six-speed transmission.