In the end, with 21 degrees of total timing, the turbocharged Silverado laid down 506 horsepower and 535 lb-ft of torque to the rear tires at the same 12-pound boost level as the original tests. The 40 additional horsepower represented an increase of more than 8 percent, which ain't bad at all. And although the 2 lb-ft of increased peak torque was essentially a flat comparison with the stock heads and cam, it doesn't tell the whole story. In this case the optimized camshaft really helped fatten up the torque curve in the heart of the RPM band. Making identical torque at 4,500 rpm – which was peak for the stock engine – the heads and cam produced an additional 40 lb-ft of torque at 5,300 rpm, even after the engine had already peaked at 4,700 rpm. The combination showed its benefit the higher the RPM climbed. The peak power was achieved at just a tick under 5,400 rpm, but held that peak to 5,700 rpm, whereas the stock heads/cam had totally nosed over after 5,000 rpm – where the peak horsepower was achieved.
An argument could be made that while significant, the 8-percent greater horsepower didn't exactly set the chassis dyno on fire. It also seemed obvious that the higher-flowing LQ4, with its more voluminous cylinder heads and boost-loving cam, might benefit even more significantly with more pressure. And since the truck wasn't spitting out fluids or hard parts after the 506-rwhp pull, Schoen decided to pull out the stops – and the plug on the boost controller – to see what a few extra pounds of air pressure would do. It was clear the engine hadn't peaked at the RPM levels used for pulls at 12 pounds of boost. Schoen eagerly wanted to add about 6 pounds of boost, but the light wastegate spring, and added exhaust pressure, only allotted for about 15 psi. Nevertheless, several runs were made up to 5,500 rpm with 15 pounds of boost and the three extra pounds made a tremendous difference: 559 rwhp and 590 rwtq for peak numbers – 93 horsepower over stock. Keep in mind: that's through the comparatively restrictive stock intake manifold and exhaust manifolds.
That was a very exciting discovery because, as expected, the peak numbers would be reflected in the added boost, but what was not expected was the largely broadened torque curve over the 12-psi tests. When compared against the stock engine's performance with 12 pounds of boost at 4,900 rpm and 15 psi at the same RPM, it jumped 88 horsepower and 93 lb-ft, while also adding 89 lb-ft and 90 horsepower at 5,300 rpm. Again, all of this while still maintaining the same low-end horsepower and torque numbers as of the stock cam. As long as a safe tune is maintained, the 15-psi threshold for the engine seems like a winner, although the 40-horsepower jump in power at the 12-psi level was certainly admirable.
With a larger intake and optimized runners, the benefit of the new heads and cam would likely have been greater, but that's all part of the discovery process when it comes to building power. And at the end of the day, Schoen added 88hp without being on the ragged edge like he had with the stock parts. Sure people have made 800-rwhp with a stock 4.8- or 5.3-liter, but for how long? Thankfully Aaron will never have to wonder about his Silverado.
Elements of an Optimal turbo Camshaft
Generally speaking, a turbocharged engine benefits from a milder cam – one with less duration than other applications – in order to make the most of the exhaust gas pressure that drives the turbocharger's turbine. In a nutshell, the camshaft should be used to help move heat and exhaust out of the cylinders.The turbocharger itself is an exhaust restriction that increases exhaust gas pressure – and it's the exhaust gas pressure that spins the turbo. A milder cam with lower duration will help exploit boost-enhancing exhaust gas pressure.
Ideally, the boost pressure of a turbo engine should be greater than the exhaust pressure at the low end of the powerband, as the engine nears its peak torque. The boost pressure and exhaust gas pressure are nearly equal when the engine approaches its peak horsepower; however at peak horsepower there is typically greater exhaust backpressure than boost (with a street turbo system).
This is the reason Billy Godbold, Valvetrain Engineering Group Manager at Comp Cams, says: "With turbo systems, you want to make sure the air is going the correct direction at intake opening. Your goal is to get as much air/fuel charge as possible into the chamber by the time the intake valve closes and not build up huge exhaust pumping losses." When selecting a camshaft Billy says, "you just need to consider what the forced induction system does very well and not-so-well. Then you can choose the cam to help accentuate the positive and (maybe more-so) to hide any negatives." With a street turbo engine then, you can design the cam "to be responsive at low to mid RPM as if it were N/A, then you can use the boost at higher RPM to extend the power range."