What happens when you make a post in any of the many LS performance forums and ask the following question? What is the best factory LS head to use on a turbo engine? The obvious answer for any question regarding factory cathedral port heads is never the stock 706 heads, and always the legendary LS6 heads, right? I mean, the 243 (or later 799) is the go-to head for any application, but the 317 usually isn’t far behind. After all, the 317 truck heads were blessed with the same ports as the 243 and 799 heads, but were saddled with much larger combustion chambers. For turbo (and blower) guys, the drop in compression is often seen as a positive, as boost brings all that back, right? In truth, the drop in compression can be beneficial for keeping detonation at bay, assuming the chamber design of the 317 heads is less prone to detonation than the 706 heads we tested them against. That is a different debate for a different day, but right now we plan on finding out just how much power the drop in compression is worth when you replace the stock 706 heads with a set of 317s on a turbo 5.3L.
To fully demonstrate the change in power offered by the head swap, we ran the 5.3L test engine both naturally aspirated and turbocharged with each pair of cylinder heads. This way we could demonstrate that the power gains offered in naturally aspirated trim carried over under boost. The 5.3L test engine used to illustrate this was a high-mileage LM7 yanked from a local LKQ Pick Your Part. In preparation for the test, the 5.3L received a cam upgrade, a fresh set of Fel-Pro MLS head gaskets, and ARP head studs. Rather than rely on the stock truck intake, we installed an LS6 intake and manual throttle body. Both pairs of heads were run with stock rockers, hardened pushrods (of the same length), and long-tube headers. The fuel supplied by the Holley 120-pound injectors and the timing values were controlled by a Holley ECU. Run with the 317 heads, the modified 5.3L produced 448 hp at 6,800 rpm and 398 lb-ft of torque at 5,000 rpm. After installation of the 706 heads, the power jumped to 468 hp at 6,800 rpm and 413 lb-ft of torque at 5,300 rpm. The 706 heads consistently improved the power output of the naturally aspirated 5.3L by 20 hp. But how would they compare under boost?
To illustrate the gains offered under boost, we set up the 5.3L to accept a single turbo system. The kit featured a custom Y-pipe designed to work with a pair of stainless steel turbo headers. The headers featured V-band clamps to connect to the Y-pipe crossover tube. The crossover tube included not only the necessary T4 turbo flange but also a pair of wastegate flanges designed to accept Turbosmart Hyper-Gate45 wastegates. The use of two wastegates ensured proper boost control, as our Precision Turbo 7675 turbocharger was capable of supporting over 1,100 hp if left unchecked. Boost from the Precision turbo was channeled through an air-to-water intercooler supplied by ProCharger. Like the turbo, the intercooler was designed for 1,000+ hp applications so it had no trouble on our 5.3L running just 7 psi. The intercooler was fed 85-degree dyno water. Obviously, ice water would work best, but this test wasn’t about maximizing the combination. Also part of the turbo system was a 4.0-inch exhaust system and a Race Port blow-off valve.
As with the naturally aspirated testing, we ran the turbocharged 5.3L with both the 317 and 706 heads. The discharge tube from the intercooler was plumbed to the throttle body. Run with 7-psi wastegate springs, the 317 heads produced peak numbers of 691 hp at 6,700 rpm and 612 lb-ft of torque at 5,000 rpm. Running just 7 psi, the turbo system improved the power output by 243 hp. But the real question is, would the 706 heads still make more power under boost? After installation of the 706 heads, we got our answer.
Run with the same air/fuel ratio, timing, and boost, the turbo 5.3L produced (ironically enough) 706 hp and 641 lb-ft of torque. Despite running just on the wastegate springs, the boost with the 706 heads was down slightly (by 0.5 psi) at the power peak, but even with that difference in boost, the 706 head still offered almost 20 hp and nearly 30 lb-ft of torque where the boost differential was only 0.2 psi. Given that every pound of boost was worth 32.66 hp, the extra 0.5 psi might be worth as much as 16 hp. Either way, the extra power offered in naturally aspirated trim translated directly into power under boost. After this test, it looks like LS guys might be digging out all those old 5.3L heads! CHP
Photos by Richard Holdener