Michael Galimi
September 17, 2007
Photos By: Super Street Performance

Boost-We all love it and certainly can't get enough of it. The more the merrier is our motto. Most MM&FF readers know that precious boost can be made through two sources-superchargers and turbochargers. Each setup has its own merits, and this month we'll discuss a new way for the supercharger crowd to take advantage of some turbo technology and in the process get more kick for the street or track.

There's a struggle within the performance aftermarket as positive-displacement blowers battle the centrifugal-style units. Internet battles and bench-racing sessions across the country have pitted blower users against each other as they make an argument for their choice of supercharger system. At times it seems as if the '03-'04 Cobra is ruling the roost when a twin-screw huffer is thrown in the mix.

On the other hand, we know that centrifugal blowers have been a big hit with the EFI Mustang crowd since the early '90s when Paxton burst on the scene. The actual roots of that type of supercharger go back decades. J.R. Granatelli and his crew adapted the company's ball-driven head unit to a 5.0L Mustang, and as they say, the rest is history. Once the 5.0 crowd became accustomed to the modest boost levels they started searching for more power, and the blower companies supplied what their customers demanded.

Each passing year, blower companies continue to improve efficiency and power output. Today there are many supercharger systems on the market that can satisfy a stock engine and go up to the wildest Pro 5.0 entry that makes in excess of 2,100 hp.

The great diversity in the blower market has made them popular on the street, too. In recent years, however, the positive-displacement companies have become increasingly more popular. Some people credit the '03-'04 Cobras for bringing that type of supercharger back into the limelight as the Kenne Bell and Whipple companies quickly came to market with blowers that upped the power tremendously. Some say the Cobras are king of the street due to both Kenne Bell and Whipple.

The seat-of-the-pants feel and low-end torque are the battle cry for twin-screw backers. While centrifugals have sparkling top-end charges, their boost curves are linear. That means boost goes up as rpm increases-and peak boost normally occurs near peak rpm, or at least higher in the rev range than with Roots- or screw-type blowers.

But we want our centrifugal boost and we want it now. So this month we are arming a centrifugally blown Mustang with a component that will take the battle to the twin-screw supporters.

In order to fend off the twin-screw contingent, Job Spetter Jr. of Turbo People turned to a part that is more familiar to the turbocharger crowd than the blown one. Spetter instructed Andrew Grundman, a customer of the famed tuning shop in New York, to add a wastegate and a Turbo People boost controller to his combination. The idea was that he could run high boost on the street and control it with the wastegate rather than always detuning for the street. It was a surprising suggestion, one that would prove quite effective in the end.

But what is a wastegate, and how does it help a blower car? In simplest terms, a wastegate is fastened to the exhaust piping (hot side) of a turbocharger system to control boost level. It regulates boost by bleeding off exhaust gas to slow the turbocharger impeller speed. The wastegate is fed boost via a vacuum hose, the user predetermines the boost level at which he wants to bleed boost, and the turbo will not exceed that number. The uber-cool whooshing sound from a turbo engine is also created by the wastegate when it opens.

Why use it in conjunction with a centrifugal blower when it's not exhaust driven? Because in this application, the wastegate is not installed on the exhaust side but rather on the inlet tube. So why not just use a bypass valve? Bypass valves control boost and are a fixed entity in that they are open or closed. There is no way for the bypass valve to open partially to bleed off boost and achieve a max boost setting. The wastegate is designed to perform this function, however, we are diverting compressed air rather than exhaust gases. It's a different type of relief, but in the end, the wastegate controls the amount of boost in the manifold.

Utilizing the wastegate in this application has a distinct advantage when tooling around the streets. It allows the supercharger to be pulley'd up for maximum efficient impeller speed at the engine's peak rpm. Our test subject (Grundman's '90 Saleen Mustang) is equipped with a D.S.S.-built 331-Dart block, steel crank, TFS heads, and so on-with a Vortech YSi-Trim super-charger. Running near maximum impeller speed means boost will be somewhere in the neighbor-hood of 19-20 psi-far too much for a pump-gas application and to keep the tires under the back of the car on the street. Grundman often prowls the New York roadways and didn't want to keep burning the expensive high-octane fuel every time he ran to the Qwiki Mart. He also didn't want the hassle of doing a lot of work when he hits the strip in race trim. Spetter's solution was to add the wastegate and boost controller to give his customer the best of both worlds.

The "Oh s**t" switch. If the Cobra competition starts to get tough, Grundman just needs to add high-octane race fuel and flip this switch. It changes the engine tune in the DFI (jumps timing to 18 degrees) and closes the wastegate (enables 19 psi).

The wastegate was set up to drop the boost to 14-15 psi when Grundman runs pump gas. Essentially this Saleen will be making greater average boost rather than just a peak number. Because the YSi-Trim unit spins hard, impeller speed at low rpm is greatly increased. The final result is a super-aggressive boost curve up to 15 psi, then a flat line across to the peak rpm.

Hell Bent Race Cars was responsible for mounting a custom air-to-air intercooler in the front bumper and adding the wastegate kit, which Turbo People sells. The chassis shop also upped the safety margin by installing a custom-bent, six-point rollbar and racing harness. Once the hardware was mounted, Super Street Performance (Farmingdale, New York) wired up the boost controller and interfaced it with the Accel DFI Gen 7 computer system. Spetter tended to the tuning and engineering of the combination. Super Street Performance also added a switch in the ashtray to activate the system. A quick flip of the switch enables the wastegate and dials up the pump-gas tune-up in the computer system. In the heat of battle, the switch is thrown (on the fly, if necessary), the wastegate is slammed shut, and the DFI switches over to the race tune-up. The wastegate control solenoid is wired into the Gen 7 thanks to utility ports that are normally used to control nitrous solenoids. The wastegate kit requires the use of an aftermarket fuel-injection system or blow-through MAF sensor setup.

To put Spetter's theory into practice, chassis dyno testing was completed at Crazy Horse Racing (South Amboy, New Jersey). Its Dynojet chassis dyno is capable of data logging boost so we could follow the curve under all conditions. A quick analysis of the graph shows how effective the wastegate was. On pump gas, the wastegate restricted boost to the desired 14.9 psi. It ramped up quickly, and by 5,000 rpm the manifold saw nearly 15 psi of boost, and the wastegate opened to relieve pressure down to the 14-psi range. In this trim, Spetter coaxed 655 rwhp and 655 rwtq from the stout pushrod contender. It was a tune that was regarded as safe, with only 15 degrees of timing and 11.3:1 air/fuel ratio as read by an Accel DFI O2 sensor.

Thanks To the wastegate and boost controller, we could run a small pulley on the YSi-Trim. It will make 19 psi with the wastegate shut and Spetter regulated it to 15 psi, perfect for the street. Spetter did say he could have set up a knob on the controller, giving Grundman a few more boost choices. He said a knob would allow 10 psi, 12 psi, and 15 psi settings.

One observation about the car was that the air inlet temperatures would actually go down as the engine increased in rpm, showing the extremely efficient heat-reducing capabilities of the intercooler. We were also quite amazed at how timid and mild the '90 Saleen sounded, despite throwing out 655 raw horsepower. The boost curve was nothing like any centrifugal blower we have seen, as right at 5,000 rpm the wastegate opened and dumped boost. The curve flat-lined until the peak rpm of 6,400.

"The curve looks like a turbo car," Spetter says. "It may not be as aggressive as a Kenne Bell or Whipple Cobra, but if you compare this graph to another one of my customer's Cobras, you can see that Grundman's car crosses past the Cobra at 4,000 rpm. From a roll, this Saleen would be all over a Cobra." The Whipple Cobra made 614 rwhp at 17.99 psi of boost (on race fuel), while the Saleen knocked down a best of 655 rwhp with a wastegate regulated 15.02 psi. Spetter couldn't find a similarly prepped turbo car that had been saved in the Dynojet program, but he said a turbo street car would have a boost curve that looks similar to Grundman's Saleen.

The battle for blower supremacy is getting tougher, and the centrifugal blower camp now has another weapon to use in fighting the enemy.