Even a stock cathedral-port head like this LQ4 317 cylinder head flows much better than th
L92 LS Head Swap
Josh Meyer; Sacramento, CA: I have a question regarding an article I found online about testing the new GM L92 cylinder heads called "550 hp for Under $4,900." If I want to reproduce the numbers posted in the article, all I need are L92 heads, a Comp Cams XR281HR cam, an 850-cfm carburetor and manifold, a stock bottom end, and the same headers? I'm very interested in the bottom end of the motor. Did you do anything to it to make this power?
Jeff Smith: The story you are referring to ran in the May '07 issue where we used a stock, 6.0L short-block to make this power. The engine did make 551 hp at 6,800 rpm using the more aggressive Comp Cams XR281 you mentioned. The baseline test for the story included a GM Performance Parts Hot cam with specs of 219/228 degrees of duration at 0.050 tappet lift and 0.525-inch valve lift for both the intake and exhaust using the stock 1.7:1 rocker ratio. There was also a slight error in the story when it originally ran that a couple of readers noticed. In the story, we claimed that the engine was a base, iron-block LQ4, because that's what we thought we had. But in the photo in which we show checking valve-to-piston clearance, a reader caught flat-top pistons that are actually used on the higher-compression LQ9 engine. The LQ4 engines are rated at 9.4:1 with a dished piston, while the LQ9 engines enjoy 10.0:1 compression using a flat-top piston, which means we actually had the better LQ9. We completely missed that little clue because we were focused on making sure the valves didn't hit the pistons. The difference in compression is roughly a half point, which is worth less than 2 percent power. Even at 550 hp, that's worth 5 to 7 hp. The original baseline with the stock LQ9 cam certainly benefited from this additional compression, making 439 lb-ft of torque at 4,400 rpm and 425 hp at 5,600 rpm.
In the story on Thomas Moore's shop, one of the more interesting backstories
While the long-block was stock LQ9, it also enjoyed a large, single-plane, GM Performance Parts intake, a Holley 750-cfm carburetor, and a pair of Kooks 1 3/4-inch headers feeding a pair of 2 1/2-inch Flowmaster mufflers. Next, we added the GMPP Hot cam and the engine improved to 450 lb-ft of torque at 5,000 rpm and 483 hp at 6,000 rpm. That is incredibly impressive given the stock LQ9 cathedral-port heads. The test also included a set of LS6 valvesprings that came with the Hot cam kit. These initial dyno tests were outlined in the Apr. '07 issue of Car Craft. To run these LS engines with a carburetor, you will also need an MSD PN 6010 LS6 spark box that allows you to create your own spark curve to operate the factory distributorless ignition system (DIS).
The second installment in the May issue pushed the power from 483 hp at 6,000 rpm to 551 hp at 6,800 rpm with the addition of both the larger, rectangle-port L92 castings, a Comp XR281 cam (228/230 degrees of duration at 0.050 with 0.571/0.573-inch lift and a 112 degree LSA), and an 850-cfm Holley carb. While this is impressive horsepower, you are smart to ask about the short-block because at 6,800 rpm, those cast-aluminum pistons quickly become an endangered species. Higher rpm imparts more stress on cast pistons and those stock rod bolts. The question is not whether a piston will crack, but rather how much damage will occur when it does. For durability with this larger camshaft, you will need to add a set of forged pistons, aftermarket rods, and a good torque-plate-honing job. The cast crank should be fine for street use even up to 6,800 rpm.