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

See all 21 photos

See all 21 photos Though it was hardly necessary at the low boost level we were running, we upgraded the high-mileage 5.3L LM7 with a set of Fel-Pro MLS head gaskets and ARP head studs.

See all 21 photos The stock 706 heads are often discarded due to the smaller valve sizes and perceived lack of flow, but this test shows they worked well on a turbo 5.3L.

See all 21 photos Used on the 6.0L truck applications, the 317 heads offered more flow and larger valves than the 706 heads, but they also featured larger combustion chambers (by 10 cc). This dropped the static compression ratio by over 1.2 points.

See all 21 photos Prior to dyno testing, both heads were run on the flow bench. The 317 heads flowed as much as 19 cfm more than the 706 heads, but the extra flow was not enough to overcome the difference in compression.

See all 21 photos Rather than waste our time with the least powerful of all the factory cams, we replaced the LM7 grind with a more performance-oriented unit. The hydraulic roller cam offered a 0.614/0.624-inch lift split, a 227/243-degree duration split, and a 113-degree LSA.

See all 21 photos Both heads were topped off with this factory LS6 intake and manual throttle body.

See all 21 photos To ensure adequate fuel delivery under boost, we installed a set of 120-pound Holley injectors. Holley also supplied this 2-bar MAP sensor.

See all 21 photos Controlling the timing and fuel was critical on the turbo application so we stepped up to this Holley HP ECU.

See all 21 photos To work with the cam upgrade, each of the factory heads received a valvespring upgrade from Brian Tooley Racing.

See all 21 photos First up on the dyno in naturally aspirated trim were the 317 heads bolted to the 5.3L. After tuning, the modified 5.3L produced 448 hp at 6,800 rpm and 398 lb-ft of torque at 5,000 rpm.

See all 21 photos After testing, we performed a head swap to replace the 317 heads with the 706 heads.

See all 21 photos After installation of the 706 heads, the power output of the modified 5.3L jumped to 468 hp at 6,800 rpm and 413 lb-ft of torque at 5,300 rpm.

See all 21 photos After the NA testing, it was time to duplicate the comparison under boost. In this case, boost was supplied by a single Precision Turbo 7675 turbocharger.

See all 21 photos Boost was controlled by a pair of Turbosmart Hyper-Gate45 wastegates.

See all 21 photos Though we limited boost pressure to just 7 psi, we ran the boost through this air-to-water intercooler from ProCharger.

See all 21 photos The turbo system consisted of a pair of stainless headers feeding a custom (3-inch) Y-pipe. The crossover pipe featured a trio of flanges, one for the T4 turbo and a pair for the dual wastegates.

See all 21 photos Run on the dyno with the 317 heads and single turbo, the 5.3L produced 691 hp and 612 lb-ft of torque.

See all 21 photos After installation of the 706 heads, the peak numbers under boost jumped to 706 hp and 641 lb-ft of torque. The gains offered in naturally aspirated trim continued and were slightly multiplied under boost.

See all 21 photos 706 vs. 317 Modified (NA) 5.3LRight off the bat, it is obvious that the stock 5.3L 706 heads easily outperformed the 6.0L 317s. This was even more impressive when you consider the fact that the 317 heads offered more flow, larger ports, and larger intake valves. Credit the difference in chamber size for the majority of the gains, as the 706 heads were a full 10 cc smaller than the 317s. Tested on the naturally aspirated 5.3L, the result was an extra 20 hp, and almost as much torque. Equipped with the 706 heads, the 5.3L produced 468 hp and 413 lb-ft of torque. After installation of the 317 heads, the power numbers dropped to 448 hp and 398 lb-ft of torque. The question now, how would they compare under boost?

See all 21 photos 706 vs. 317 Modified (Turbo) 5.3LThough LS enthusiasts seem to think some sort of magic happens when you introduce boost pressure from a turbo, the reality is that things actually remain the same. The only change is usually that the differences experienced naturally aspirated occur at a higher power level. This was the case when we added boost from the same single-turbo system to both sets of cylinder heads. Run with the Precision turbo, ProCharger intercooler, and 317 heads (at 7 psi), the 5.3L produced 691 hp at 6,700 rpm and 612 lb-ft of torque at 5,000 rpm. After running the same setup with the 706 heads, the 5.3L produced 706 hp at 6,500 rpm and 641 lb-ft of torque at 5,000 rpm. The gains offered by the extra compression of the 706 heads remained (and even increased slightly) under boost.

Sources:

ARP

805.339.2200

Brian Tooley Racing

888.959.8865

DNA Motoring

626.965.8898

Holley/Hooker

270.782.2900

Precision Turbo

219.996.7832

ProCharger

913.338.2886

Turbosmart

909.476.2570