Twin Turbo LSX Engine - Baddest Of The Bad

Thomson Automotive's Twin-Turbo Lsx Engine Pushes 2,048 Hp From Only 400 Cubes.

Barry Kluczyk Sep 1, 2009 0 Comment(s)
Sucp_0909_09_z Twin_turbo_lsx_engine Distributor_drive_gear 2/24

Because this engine uses a front-mounted distributor rather than the typical LS crank trigger, the camshaft required the addition of a distributor drive gear and fuel pump lobe. It's available from GM Performance Parts in a kit that also includes the distributor mounting bracket, PN 88958679.

CYLINDER HEADS As mentioned earlier, the heads are prototype LSX racing heads that were ported by Utah-based Chapman Racing Heads. They're filled with 2.200-inch titanium intake valves and 1.600-inch Ferrea Super Alloy exhaust valves. The intake port design is patterned after the high-rpm flow characteristics of the C5R head, but the chamber volume is a tight 45.6cc, hence the need for the deeply dished pistons.

CAMSHAFT AND VALVETRAIN A custom grind from Bullet Racing Cams was used, with comparatively mild 0.714/0.721-inch lift and 266/268-degrees duration specs. Lobe separation is relatively wide 113 degrees, which is appropriate for a forced induction engine. The rest of the valvetrain is pretty standard stuff, including Comp Cams springs, titanium retainers and keepers, along with 1.7-ratio roller rocker arms.

INTAKE SYSTEM During Thomson's experiment with a force-fed 454-inch LS engine, it used a custom, CNC-carved tall-runner intake system with a conventional front-mounted throttle body. They found unequal distribution among the cylinders when the boost was turned up (it was blowing past the front cylinders and getting crammed in the rear cylinders), so they redesigned the intake. It now features a pair of ACCEL DFI 2,100cfm throttle bodies mounted on top of the intake, with a custom, sheetmetal air box on top of it. Three-inch tubing from the turbos feeds the air box.

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The cylinder heads are prototype LSX racing heads, but they're based on the same six-bolt-per-cylinder design as GM Performance Parts' recently introduced LSX street head. We're told development is ongoing for the racing head, so stay tuned.

TURBOCHARGERS Two 88cc Turbonetics compressors are used and blow through a custom-built intercooling system. They're fed exhaust pressure from custom headers designed by GM Performance Parts to fit a specific project vehicle (more on that later in the story). A pair of Turbonetics wastegates also is part of the system. Through the intercooler, the turbos deliver about 25 psi of boost, with a maximum of 27 psi recorded on some dyno pulls.

IGNITION SYSTEM High boost can effectively blow out the spark in the combustion chamber, so a very-high-energy ignition system is a must. And, simply put, the factory-style individual ignition coil system of LS engines doesn't have the energy to ensure the spark consistently jumps the gap on the plugs. So, the coil-near-plug system was ditched in favor of a conventional distributor and Mallory Hyfire VII ignition box that delivers 150 mega-joules of energy.

Fortunately, there's a front-drive distributor kit offered by GM Performance Parts that was developed for circle track racers who use LS engines and are required to use a distributor. Thomson adapted the kit, which includes a distributor drive gear and fuel pump lobe that is mounted to the front of the camshaft.

For the distributor, Thomson used an ACCEL PN 77201 Dual Sync unit that dropped right into the front-drive adapter.

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The additional cylinder head bolts of the LSX head add a tremendous amount of clamping power. This is essential in higher-boost LS engines, where the production-style four-bolt design has traditionally been the weak link when the engine is put under pressure.

Additional details Other engine details include 160-pound Bosch injectors, custom valve covers, a Wagner racing-style water pump and a dry-sump oiling system that uses a Moroso five-stage oil pump. One of the pump's stages is used for draw oil out of the turbos to prevent unnecessary buildup that could lead to blown seals or worse.

Thomson handled the engine parts and assembly, but leaned on ACCEL-DFI's Joe Alameddine to help with the engine control system. It was a coordinated effort that was both challenging and rewarding.

Alameddine wired up ACCEL-DFI's recently introduced Thruster EFI engine management system, a stand-alone control system that enabled him to batch-fire the injectors (it also handles production-style sequential firing). It also designed to support tuning of forced induction engine generating up to 40 psi of boost. Thomson's turbo engine would make about 25 psi, so it was well within the Thruster's limits.

"It's a great tool for completely custom engine combinations like this one," said Alameddine. "We designed it to support the wildest racing engine configuration, but it is also great for street/strip engines with more conventional combinations."




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