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Battle of the Sixes: LQ4 vs LY6

Words and Photos: Richard Holdener 

Time, they say, marches on and that is good news for LS enthusiasts. The reason time is important for LS enthusiasts is that as each year goes by we get more and more engines in the wrecking yard to choose from. Not only do we get a fresh supply of motors for the wrecked vehicles, but we get newer models. The local LKQ Pick Your Parts are full of the smaller 4.8L LR4s and 5.3L LM7s, but as the years go by, we start to see the more modern (Gen IV) variants. The same goes for the larger 6.0 liters. Though more expensive, upgrading to the 6.0L brings with it plenty of extra performance to offset the initial cost difference. The workhorse LQ4 motors are the most prevalent, followed by the high-compression LQ9s. Available since 1999 (with iron LS heads no less), these 6.0Ls were eventually replaced by their Gen IV counterparts. Lucky for enthusiasts, these Gen IV versions are now becoming more commonplace. Not surprisingly, they are more expensive than the usual 5.3L, or even LQ4 6.0L, but let’s take a look at the difference between the typical Gen 3 LQ4 and the newer Gen IV LY6. Compared in stock trim, the new versions have a lot going for them, and even more once modified.

To get things started, the original LQ4 started in 1999 with iron heads. These were replaced sometime in 2000 with aluminum versions. There is little, if any, difference in power between the iron and aluminum versions, but the aluminum headed motors are definitely the ones to go for. The LQ4 offered a static compression ratio of 9.4:1 thanks to a combination of the 317 heads and dished factory pistons. The 317 heads featured 71-72 cc chambers and intake ports said to be modeled after the LS2/LS6 243 heads. Flow numbers confirm they are at least close in design, as the 317 heads flow every bit as well as the famous 243 heads (meaning around 245 cfm). From a power standpoint, the early LQ4s shared the same cam profile as the wimpy 5.3L LM7 and 4.8L LR4. Later versions were upgraded to the more powerful LQ9 cam. The LQ4 and LQ9 versions of the 6.0L did share the same truck intake manifold that featured a 78-mm throttle opening. The LQ4 was rated between 300-325 hp and 360-380 lb-ft of torque depending on the actual vehicle application. The LQ9 shared all the components with the LQ4, but featured flat-top pistons. The slightly higher compression LQ9 was factory rated at 345 hp and the same 380 lb-ft of torque.

Fast forward to 2007 and the LY6 6.0L. The Gen IV motors received a different block and connecting rods that improved both head gasket sealing and the strength of the reciprocating assembly. On the power side, the LY6 got a boost from technology in the form of VVT, or Variable Valve Timing. This fancy label described a mechanism that advanced and retarded the cam timing. This was done under a variety of different conditions, ranging from idle to cruise, and even through WOT. This was done to improve both idle quality, emissions and overall power production. In addition to the VVT, the 6.0L LY6 also benefited from the use of rectangular-port (LS3) cylinder heads and matching intake manifold. The rec-port heads featured peak flow numbers of 315 cfm, a good 65-70 cfm over the 317s used on the LQ4. The flow rate of the intake manifold was improved partially thanks to a 90-mm (technically 87mm) throttle opening and matching DBW throttle body. The improvements to the LY6 increased the power output of the workhorse 6.0L up to maximum of 352 hp (or 364 hp depending on what source you read).

To find out the differences between the two 6.0L LS motor, we ran both on the engine dyno in the same configuration. First up was the Gen 3 LQ4. This example was equipped with the later LQ9 cam upgrade and was rebuilt to factory specs. The rebuild entailed a ball hone, surface, and valve job for the stock 317 heads. The truck intake received a manual 78-mm throttle body to replace the factory (past 2002) DBW unit. Each test motor was also run with long-tube headers, an aftermarket FAST XFI management system and tuned for maximum power (which they were not from the factory). The test motors were also run without any accessories, air intake or full exhaust. Run in this manner and tuned to perfection, the stock LQ4 produced peak numbers of 404 hp at 5,200 rpm and 439 lb-ft of torque at 4,300 rpm. It is obvious from the power curve that this motor was built with low-speed in mind. Torque production from the 6.0L exceeded 400 lb-ft for a 2000-rpm spread. This low-compression LS was designed for heavy hauling, and performed well, if customer satisfaction is any indication.

Up next was the 2008 LY6 6.0L. Initially, some may question the use of the larger, high-performance, rectangular-port heads on a low-rpm truck application, but the LY6 performed admirably. It should be pointed out that this test motor received a cam change to eliminate the VVT, as we had no way to control the system without switching to the factory ECU. According to our information, the VVT cam specs were nearly identical to the LQ9 cam, so we installed it in place of the VVT cam system. Run with the fixed cam profile (retarding the cam would improve peak power slightly), the rec-port LY6 produced peak numbers of 443 hp at 5,400 rpm and 467 lb-ft of torque at 4,500 rpm. Note that peak power and torque occurred slightly higher than in the LQ4 (by 200 rpm), but the power output of the LY6 was greater through the entire rev range. As expected, the gains were greatest higher in the rev range, but the test shows that time and technology certainly march on, and grabbing an LY6 is certainly worth extra power over the LQ4. Better yet, the rec-port heads and matching intake might just offer even greater power gains once you apply a cam upgrade!

6.0L Shoot Out-LQ4 vs LY6

The original LQ4 6.0L was plenty powerful, offering over 400 hp and almost 440 lb-ft of torque (the way we test it), but the Gen IV LY6 is even better. Though we saddled the LY6 with fixed cam timing, the extra compression, head and intake flow all combined to improve the power output through the entire curve. Where the LQ4 (a rebuilt one at that) produced 404 hp and 439 lb-ft of torque, the (well-worn) LY6 upped the ante to 443 hp and 467 lb-ft of torque. Despite the high-performance, rec-port heads, the LY6 offered more power through the entire rev range.