Pontiac Engine 505ci - Twin-Turbo Tail Twister

On The Dyno
Since C16 race fuel was in the tank for the previous dyno session, it was used for this one as well, as it would be necessary to make the 13psi and 21psi pulls as well. All of the dyno pulls were made in Third gear because that is the 1:1 ratio.

What we quickly found was 13 psi of boost on the engine dyno dropped to less than 6 psi in the car, even with a full 3-inch exhaust system. A switch from a 4psi to a 10psi spring in the wastegates restored the boost to approximately 8.5 psi. After we plumbed some boost to the topside of the wastegates' diaphragms, we could dial in all the boost we wanted.

The first pull netted 628 hp and 682 lb-ft of torque at 8 psi boost. Before we started cranking up the boost, we removed the pair of K&N air filters that shipped with the turbos just to check their efficiency. We saw a modest gain of 12 hp at two-hundred rpm higher in the next pull. Torque increased by 24 lb-ft at the same rpm. At 13 psi of boost, the horsepower jumped to 742 at the wheels, while the torque climbed to 788 lb-ft.

About the only changes Brian felt were necessary as we increased the boost was to add about 1 point of fuel (12.3 to 11.2) at the higher boost level, as he was concerned that things might be getting a bit too lean as we increased power levels. Since we're simply using the fuel to cool the cylinder via a richer A/F ratio, we saw negligible power differences, but we now had a bigger margin of safety built in.

Upping the boost to 21 psi, horsepower jumped to 771 at 6,100, but the torque actually dropped a little to 781 lb-ft, perhaps indicating that we are past the optimum boost level for this combo with the exhaust system in place.

But even at only about 8 pounds of boost when running pump gas, Jeff's 12-inch tires will not be able to handle the nearly 700 pounds of torque that this engine can dish out, so Brian enabled the Intelligent Traction Control feature in the FAST software, so Jeff can get the most out of all the power he has under his right foot.

The system is amazingly simple. A sensor measures the driveshaft speed and the ECU can then calculate both the driveshaft rpm and the rate at which that rpm changes. If the rate of change of driveshaft speed exceeds a tuner-set rate, a simple 2D graph determines how fast the ECU will pull timing out of the engine to attempt to bring that rate back under control. Jeff can mat the throttle and the computer will add power only to the point that the wheels start spinning. Of course, Brian saved two sets of tunes in the FAST ECU, so with the flick of a switch in his glovebox, Jeff can turn off the ITC and boil the hides at will.

Conclusion
You may ask why this engine isn't putting 900-plus hp to the wheels, after all it's a twin turbo 505 Pontiac. The answer is that it was built to be driven primarily on the street with some track time, for an owner who desired not only plenty of useable power, but also plenty of eye pop under the hood, and the idle quality of a stock 455. Though the boost pressure readings are high, it doesn't mean that all of it is actually reaching the cylinders.

Nevertheless, the potential to make hundreds more horsepower is built into the combo already. Should Jeff decide to add power in the future, a more turbo-friendly camshaft will provide a substantial leap forward-even a rocker ratio increase could show more power. But for now, 640 hp and 706 lb-ft of torque with 8 psi boost, 742 hp and 788 lb-ft of torque with 13 psi boost, and 771 hp and 781 lb-ft of torque with 21 psi boost should satisfy his need for speed.