By Richard Holdener/Photos By Author
Why do you need to upgrade your cam when you have boost?
When it comes to improving power on almost any motor, few things can rival boost. This is especially true of the LS platform, as they seem to take to boost like a proverbial fish to water. The great thing about boost is how easy it is to get extra power. Whether it comes from a supercharger, or (like in our test) a turbo, adding more to power to the combination is a simple matter of cranking up the boost. In the boost world, there is an old saying, if 5 psi is good, then 10 psi must be even better, right? While there is no denying the power potential of a boosted LS, what happens when you toss in the most popular LS upgrade, the cam swap, to the mix? We know from testing that replacing the stock cam on a typical (normally aspirated) 5.3L with almost anything else can result in big power gains. It stands to reason that any upgrade that makes more power on a normally aspirated motor should work equally well on a boosted one, right?
To find out if this was indeed true, we set up a simple test. We took our junkyard 5.3L motor and first subjected it to a cam swap, by replacing the stock LM7 cam (weakest of all the factory offerings) with a healthy cam from COMP Cams. The 273LR HR12 grind offered a .610/.617 lift split, a 223/231-degree duration split and 112-degree lobe separation angle. Note that this cam was designed for a normally aspirated LS application, and was sized perfectly for the 5.3L. The cam upgrade was combined with a set of 26918 valve springs to ensure both adequate rpm potential and coil-bind clearance. The LS motor was run with a set of 1 7/8-inch long-tube headers, a Meziere electric water pump and FAST XFI ECU. Because we would later be adding a turbo system, we also replaced the factory injectors with a set of 85-pounders from FAST. Run first with the stock cam, the 5.3L produced 355 hp at 5,200 rpm and 381 lb-ft of torque at 4,100 rpm. After installation of the COMP cam upgrade, the power output of the 5.3L increased to 443 hp at 6,300 rpm and 408 lb-ft of torque at 5,000 rpm.
The results of the cam upgrade were hardly surprising, as we have come to expect big gains from a cam swap on an LS. The cam improved the peak power output by 88 hp and 27 lb-ft of torque. Measured out past 6,000 rpm, the power gains exceeded 100 hp, but what would happen if we added boost to the mix? To find out, we installed a custom single turbo system that featured DNA stainless headers feeding a custom Y-pipe equipped to accept a T4 turbo and a pair of wastegates. For this test, we chose a single Precision 7675 turbo capable of supporting well north of 1,000 hp, or more than we planned to run on this 5.3L. The Precision Turbo was teamed with an air-to-water intercooler from Procharger and a pair Turbosmart wastegates. Turbosmart also supplied a Race Port blow-off valve to eliminate lift-throttle pressure spikes. The 85-pound injectors supplied plenty of fuel, and both the stock and COMP Cams were run at the same boost, air/fuel and timing values. With our turbo installed, it was time to see how much power the cam was worth under boost.
To test the merits of the cam swap, we once again started with the stock LM7 cam. The turbo 5.3L was run under boost and produced peak numbers of 546 hp at 5,200 rpm and 589 lb-ft of torque at 4,500 rpm at a peak boost of 8 psi. Even at the low boost level, the turbo improved the power output of the stock-cammed 5.3L by 191 hp. After running the stock cam, it was replaced by the COMP 273 version. Equipped with the new cam, the turbo motor produced 692 hp at 6,100 rpm (actually the same from 6,100-6,300) and 643 lb-ft of torque at 5,000 rpm. You will remember that the cam swap was worth 88 hp on the normally aspirated motor, but the gains increased to 146 hp under boost. This is a clear example of how important the camshaft is, even on a turbo application. Turbos and cams are both good individually, but together they are unbeatable.
Graph 1: NA 5.3L LM7 Cam Test – Stock vs COMP 273
We first tested the cam swap on the normally aspirated 5.3L. The motor was equipped with long-tube headers, Fast 85-pound injectors and XFI management system. Run first with the stock cam, the 5.3L produced 355 hp at 5,200 rpm and 381 lb-ft of torque at 4,100 rpm. After installation of the COMP 273 cam, the peak numbers jumped to 443 hp at 6,100 rpm and 408 lb-ft of torque at 5,000 rpm.
Graph 2: Turbo 5.3L LM7 Cam Test – Stock vs COMP 273
After completing the test on the normally aspirated 5.3L, we decided to see how much the same cam was worth on a turbocharged combination. The motor was first run with the stock LM7 cam and a single turbo system that included a Precision 7675 turbo, Procharger ATW intercooler and Turbosmart wastegates.
Run first with the stock cam at roughly 8 psi, the turbo 5.3L produced 546 hp at 5,200 rpm and 589 lb-ft of torque at 4,500 rpm. After installation of the COMP 273 cam, the power output (at the same boost level) jumped to 692 hp at 6,100 rpm and 643 lb-ft of torque at 5,000 rpm.