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Unconventional Boost

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by Barry Kluczyk  More from Author

Installing an STS rear-mount turbo system on a C5 Z06

Adding a forced-induction system to your Corvette is a big step. If you’re aiming to simply pump up the power of your street driver, there are endless options in bolt-on superchargers and, increasingly, turbocharging systems.

But don’t let the term “bolt-on” lull you into a false sense of security about how a blower or turbo kit can be installed. Most kits require at least some fabrication or chassis modification for a proper fit. On a theoretical scale of installation simplicity, Roots-style blowers are the easiest, followed by centrifugal superchargers, and finally, turbo systems.

Turbo systems are the most complex mostly because of the additional system components that must be routed in and around the engine compartment, as well as the specific mounting requirements for the turbochargers themselves. And while seasoned installers and, of course, the various manufacturers will argue the merits of their respective and preferred technologies, when it comes to stuffing a power adder into a C5 or C6 Corvette, a conventional single- or twin-turbo system is the most difficult project. Extra room under the hood is not easy to come by, and that makes snaking the necessary inlet/outlet tubes a painstaking endeavor.

Defying convention, however, is Utah-based Squires Turbo Systems (STS) and its twin turbo kit, which mounts the turbochargers nowhere near the engine compartment. In fact, they’re located way down behind the rear axle. Seriously.

STS has carved out its niche in the power adder world with rear-mounted turbo systems, citing advantages such as lower underhood, intake air, oil, and turbo temperatures, all of which are undeniable advantages for making more power – and making it safely. Rather than mounting the turbos directly at the exhaust manifold as traditional turbo systems do, an STS system sends the exhaust gas all the way to the rear of the car, where the isolated mounting position offers the advantages we just mentioned. It also saves the cost and required installation of turbo-specific exhaust manifolds.

Despite the unconventional design, it operates just like any other turbo system: The exhaust gas is sent to the turbochargers’ turbines, which spin impellers on the compressor side of the units. That compressed air is sent back up to the engine and fed to it as boost. Skeptics have questioned the STS design’s affect on boost, suggesting it falls off because of the longer route the air follows. Our experience with the project outlined here showed achieving the advertised boost level wasn’t a problem, but some turbo lag was present. More on that a little later.

In most other vehicles, an STS kit is much easier to install than other turbo kits, too. That’s because there’s no fighting to fit a maze of tubing; it’s more like installing a more complicated exhaust system. We recently followed the installation of an STS system on a C5 Z06 (the process is largely the same for a C6 Corvette) and discovered it had its own challenges. Sure, the installer – Stenod Performance – was relieved not to have to fight the fabrication battle under the hood, but routing tubing through the rocker panels added more than a few hours to the labor estimate.

The Corvette’s low-slung bodywork and mostly flat chassis bottom are the primary reasons for the more involved installation procedure. On a Silverado pickup, for example, the inlet and outlet tubes have plenty of room. Not so much beneath the Corvette.

Nevertheless, the STS design’s inherent advantages still apply, and adding the heavy turbos essentially at the rear bumper was much better for weight balance than mounting them on or near the engine. It was also much easier to connect all the necessary oil lines and other paraphernalia that went with the kit.

The basic turbo kit includes a pair of Garrett T3/T04B Stage III turbochargers that are designed to produce about 6 pounds of boost with the included air-to-air intercooling system. That boost level is comparable with other bolt-on supercharger and turbo systems, which aim on the conservative side to work safely with otherwise stock engines. Detonation under boost can kill an engine, so every precaution must be taken to avoid it.

As we mentioned, Stenod Performance performed the installation, as well as the tuning and dyno testing, at their Detroit-area facility. The car’s owner, Marcus Leff, had already upgraded the C5 Z06’s LS6 engine with a higher-lift camshaft and complementing valve springs, as well as a higher-flow BBK intake manifold and larger-diameter throttle body. In other words, it was set up perfectly to receive a forced induction system.

On Stenod’s chassis dyno, the turbo system pushed rear-wheel output to a maximum of 494 horsepower and 472 lb-ft of torque, or the equivalent of 593 horsepower and 566 lb-ft of torque at the flywheel. That’s a very strong performance, considering the car was one of the earlier, 385-horsepower Z06s. The “before” dyno test, which featured the LS6 engine running a mild cam and aftermarket intake system, produced 358 rear-wheel horsepower, or 429 horses at the flywheel. So, with 6 pounds of boost from the rear-mounted turbos, the LS6’s horsepower output jumped by more than 38 percent – that’s impressive.

While the big dyno numbers didn’t lie, they didn’t tell the whole story, either. To Joe Borschke, Stenod’s founder and chief dyno operator, the graphs depicting the car’s performance on the rollers showed the boost coming in comparatively late in the rpm band, at around 4,500 rpm. To him, that meant one thing: Turbo lag.

“Ideally, you want the boost at lower rpm, to feel the power more immediately on the street,” said Borschke. “With a Roots blower and some quick-spooling turbo systems, the power comes on right now. It seemed to take this system a while to really spool up.”

Then again, turbo lag may all be a matter of perspective. Stenod’s assessment of the system’s power delivery was not shared by owner Leff, who was thrilled with the Z06’s performance.

“He couldn’t have been happier,” said Borschke. “He thought the power came on just fine and that the car pulled strong and hard all through the rpm range. He loves driving the car – and if the customer’s happy, I’m ecstatic!”

That driving joy comes at a price, of course. The retail cost for the kit is around $8,000, with professional installation and dyno tuning adding another $2,000 on the bottom line, for about $10,000. Truth be told, some supercharger systems are less expensive and deliver comparable power levels, but for those convinced turbocharging is for them, the cost is in the ballpark of what a forced-induction power adder costs to buy and install these days. The turbo system offers another advantage, too – adjustability. Adding more boost, or larger turbos and different wastegates, can deliver significantly greater power on an almost unlimited scale. That’s not the case with a blower.

Because there simply isn’t room to show every facet of the STS kit’s installation, the photos accompanying this story provide an overview of the components and major steps in the project. It’s a job for professionals or DIYers with a lot of wrench-turning time under their belt. Study them before deciding whether this is the forced-induction option that’s right for your Corvette.

 

 

STS has turbo systems for C5/C6 Corvettes. And oh, we forgot to mention…they mount in the rear!

 

The guinea pig for our installation story was a C5 Z06 that was already prepped with a boost-compatible camshaft and higher-flow induction system. A lift is all but mandatory for this project, as almost all of the work goes on under the car. And while this installation takes place on a C5, the layout of the STS kit and the procedures for installing it are largely the same for C6 models.

 

Laid out on the workbench, the scope of the project becomes apparent. The kit includes a pair of Garrett T3/T04B Stage III turbochargers, complementing 38mm TiAL wastegates, an oil pump system (to feed engine oil to the turbos), and a large collection of clamps, silicone hoses and other fasteners. A Predator hand-held tuner system to upload the requisite programming to the engine controller is also included.

 

The remainder of the kit includes the myriad of tubing for the exhaust-to-turbo lines, turbo-to-intercooler lines and the intercooling system’s heat exchanger. It’s at this point we should remind the DIYers to study the instruction manual to identify and account for every last piece. That’s important to not only make sure all the components are accounted for, but to confirm the basic layout of the tubing. The tubing included with the kit is HPC-coated.

 

Attaching the exhaust outlet section and over-the-axle tube to each turbocharger was the first step. The trick here was orienting the turbine and compressor side of the turbocharger so that their outlets faced the correct positions. Known as “clocking,” this steps ensures the inlet and outlet tubing will meet the turbo’s inlet/outlet ports precisely.

 

Trial-and-fit adjustments are made with the turbochargers temporarily hung in their approximate positions. At this stage, the installer can check whether the clocking adjustments were correct prior to tightening the fasteners to their final torque specs. In this shot, the air inlet tubes for the compressor side of the turbos have been test-fitted, too. They are seen at the opposite side of the turbos’ exhaust outlets.

 

The air inlets for the turbochargers draw fresh air from behind the car’s rear fascia – an area definitely unaffected by hot underhood air. The intake tubes wrap up around the taillights and are capped by reusable-style filters. Fastening them requires the removal of the taillamps.

 

The rear sway bar required disconnection. This was to make room for a bracket that will hold a pair of oil pumps to circulate engine oil from the engine to the turbos, cooling and lubricating them. Here, the sway bar has been loosened and moved out of the way, while the approximate positions for the pump fittings are laid out.

 

With the placement of the turbochargers confirmed, more of the tubing is routed to and from them. Here, one of the tubes from the exhaust system is connected to the over-the-axle tube that mounts directly to the turbine side of the turbo.

 

On the other side of the turbochargers are the tubes that carry the boosted air charge back up to the engine. Because there’s no real room under the car to run the tubes, STS routes them through the rocker panels. That requires removing the rear wheels and fender liners to feed the tubes through the rockers and toward the engine compartment.

 

Prior to entering the rocker panels, the air tubes wrap over the rear wheel houses. It’s an unconventional solution for an unconventional turbo kit. When the tubes are properly installed, they’re hidden and protected from the elements by the stock inner fender liners.

 

This photo shows why it was necessary to route the air tubes through the rockers: The flat bottom of the Corvette leaves no room for low-hanging tubing. The center tunnel, seen here, is filled with the exhaust tubes that feed the turbochargers.

 

Remaining at the rear of the vehicle, the oil pump mounting bracket’s mounting holes are drilled into the aluminum crossmember.

 

Here’s the installed bracket. It keeps the pumps tucked up under the car, shielding them as much as possible from the elements and road debris. As is clearly visible, there are a number of fittings associated with the pumps, as well as complementing fittings at the engine and the turbochargers. Extreme care must be taken to ensure all are fastened correctly to prevent leaks.

 

Work moves to the front of the vehicle, where the air inlet tubes exit the rocker panels and run up beneath the front fascia. A matching tube is mounted on the opposite side of the chassis, too.

 

The inlet tubes feed the air-to-air intercooling system’s heat exchanger, which mounts in front of the radiator and air conditioning system’s condenser. Disconnection of the radiator from the chassis is necessary to provide the “wiggle room” necessary to slip in the heat exchanger.

 

Here’s a shot of the front chassis, showing both of the air inlet tubes as they meet the bottom of the heat exchanger, sending the boosted air charge through the intercooler before it reaches the engine. The process reduces the inlet temperature of air, which makes it denser. That promotes greater power and, more importantly, helps stave off detonation.

 

A connector between the heat exchanger and the intake manifold is the last coupling that completes the turbocharger system’s circuit with the engine. This step often requires some fine adjustments of the intake tube to ensure it doesn’t interfere with the hood when it’s closed.

 

With the intake system in place, the mass air meter is inserted. Its relocation may require a wire harness extension on some vehicles. With some kits, a clip-in extension jumper harness may be included.

 

Here’s a look at the completed installation, showing the polished exhaust tips that come with the kit. According to installer Steve Harmon, the quality of the materials included with the kit and the engineering of them – including how well they fit together – was first-rate. Some may question whether the turbochargers are too exposed; they’re not any more so than those in systems that mount the turbos essentially at the bottom of the engine compartment. The air inlets are protected, too, under the rear fascia.

 

After the engine was started, all of the fittings for the oil system were checked for leaks before the installation was deemed complete. If higher-rate fuel injectors are used to support any forced induction system, the engine MUST NOT be started before the engine controller is reprogrammed – or “tuned” – to accommodate them. The results could be catastrophic if the flow rate of the injectors doesn’t match what the controller believes they are, resulting in engine damage.

 

Here is one of the system’s 38mm TiAL-supplied wastegates – there is one for each turbo. It is used to regulate the system’s pressure, holding it to the desired boost level. They’re adjustable to enable greater boost, if desired. The kit also included a 50mm blow-off valve, which vents excess boost to the atmosphere, such as when the driver backs off the throttle.

 

Because the turbo system supplies greater airflow, it must be matched with an equally fortified fuel system, including the fuel pump(s), injectors and regulator. For this project, a set of 42-lb/hr fuel injectors was installed, along with a Lingenfelter Performance Engineering fuel pump.

 

A DiabloSport Predator hand-held tuner is included with the kit and is used to upload a pre-programmed “tune” to the engine controller, but because the Z06’s engine had modifications that weren’t part of STS’ program, a custom tune had to be created.

 

On Stenod’s Mustang chassis dyno, the twice-turbo’d Z06 put down 494 horsepower and 472 lb-ft of torque, or the equivalent of 593 horses and 566 lb-ft at the flywheel. Installer/tuner Borschke felt the boost came on late in the rpm band, but performance on the street is exceptional. The car feels great and when the revs climb, the boost kicks in satisfyingly.

 

 

SOURCES

 

Stenod Performance

248-307-0056

www.stenodperformance.com

 

Squires Turbo Systems

801-224-3477

www.ststurbo.com

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