A stock LQ4 engine's heads have 71cc combustion chambers, giving a compression ratio of about 9.4:1. The L92 heads have smaller 68cc chambers, which bumps compression with the LQ4's stock dished pistons to about 10:1, making the combination safe for pump gas.
Using a carburetor with the L92 would have been almost impossible a couple of years ago, but GM Performance Parts recently released an aluminum intake manifold that fits the heads' unique angle and rectangular port shape. After that it's simply a matter of picking the right four-barrel; there's even a version with cast-in bosses for nitrous or EFI.
To complete the more traditional, carbureted appearance for the engine, the ignition coil packs, which are usually mounted on the valve covers, were relocated to the rear of the engine with a scratchbuilt bracket with Marsa and Urmanic-made 1-inch stock and 3/16-inch tube spacers. Other assembly details included the fabrication of a replacement valley tray designed to eliminate the need for breathers in the valve covers. The tray was made from aluminum stock, with a breather welded to the front for easy access, along with the PCV valve located at the rear of the tray. These custom touches allowed the great-looking Chevrolet-script valve covers from GM Performance to show without the unsightly coils or breathers.
Hot Cam vs. Showroom Stock Cam
From the outset, the plan was to test the engine's performance on the engine dyno with both GM's Hot Cam (part number 12480033) and Showroom Stock cam (part number 88958606). The Showroom Stock cam delivers more lift-0.570 inch versus 0.525 inch-and duration-239/251 degrees versus 219/228 degrees-than the Hot Cam. With the L92 heads' tremendous airflow capability, we figured the Showroom Stock cam would deliver big dividends. It did.
On the dyno, the Hot Cam-equipped combo had a total timing of 32 degrees and delivered 471 horsepower at 6,600 rpm and 417 lb-ft of torque at 5,000 rpm. The Showroom Stock cam put up the big numbers, posting a best of 506 horsepower at 6,600 rpm and just a hair less than 437 lb-ft of torque at 5,400 rpm. By the way, the Showroom Stock cam requires relief notches in the piston, which we anticipated and had cut into the pistons prior to assembly.
But the peak numbers don't tell the whole story. The Hot Cam is the perfect street cam. It has that "idle lope" that everyone wants at 800 rpm but has broad horsepower and torque bands, making it great for propelling a musclecar or hot rod. The Showroom Stock cam is a beast. It idles at 1,400 rpm and has a definitive race-cam sound. It is a wicked cam for a street engine and requires some more compression, a rear gear, and a higher-stall torque converter to make the most of its potential. But boy, it sounds awesome when it's idling at 1,400 rpm.
In the end, Marsa and Urmanic decided to go with the Hot Cam as the final combination. It should prove to be a great street performer, and more than that, it should give these creative engine builders the satisfaction of building a powerful and affordable new-age small-block.