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Twin Turbo LSX Engine - Baddest Of The Bad

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

By Barry Kluczyk, Photography by Barry Kluczyk

Big turbo engines are nothing new in the world of Outlaw drag racing, but it's a rare engine combination that makes 2,000 hp from only 400 cubic inches. And we're pretty sure the number of LS engines that have achieved that impressive result can be counted on one hand.

This is the story of one of them.

In an experiment backed by GM Performance Parts (GMPP), Detroit-area engine builder Thomson Automotive ( decided to push GMPP's LSX block to see whether it lived up to the advertised claim of supporting 2,000 hp. It did it with some custom parts and a couple of huge turbochargers.

Because of projects like this one and a symbiotic relationship with GMPP, Thomson is rapidly becoming one of the country's foremost experts in high-horsepower LS engine development. The turbo LSX project was launched nearly two years ago, when the LSX program was in its infancy and parts for it were being custom-built.

GM Performance Parts was eager for an independent party to verify its claims for the LSX platform, so it donated the cylinder block, as well as a set of prototype LSX racing heads.

"One of the keys to the success of the LSX block is the additional cylinder head bolt provisions," says Brian Thomson, president of Thomson Automotive. "Production LS blocks have four head bolts per cylinder, but the LSX blocks accommodate six bolts per cylinder. It makes a huge difference in clamping power and, frankly, with the amount of boost this engine makes, it wouldn't survive without that added clamping power."

Those prototype heads were all the more necessary, too, because when the project started, they were just about the only six-bolt heads Thomson could locate. Today, GMPP and a couple of other aftermarket manufacturers offer ready-built six-bolt heads.

So, Thomson had the block and heads secured, but the rest of the assembly was still up for debate-including the displacement. A previous experiment with a boosted, 454ci LS engine brought about concerns of crankshaft flex, so the decision was made to go a little conservative on the stroke.

"We planned to rev the engine pretty high and throw a lot of boost at it, so we felt we could overcome the displacement deficit without too much trouble," says Thomson. "What we gave up in cubic inches, we'd hopefully make up in longevity and durability."

Thomson's caution paid off. The engine has made nearly 2,050 hp on the dyno-more than five horsepower generated for every cubic inch of displacement-and has survived approximately 150 full-load dyno pulls without so much as an oil leak.

"We've never lost a head gasket or had any real issues with it," Thomson says. "We inspected the bottom end numerous times and it all looked great. We replaced some bearings for good measure after so many dyno pulls, but the engine has been very reliable."

THE BASIC COMBINATIONWhen you first look at the twin-turbo LSX engine, it looks as if a couple of Caterpillar loaders were robbed of their turbochargers. A pair of 88mm Turbonetics turbochargers dominates the assembly, along with a custom intake system that looks somewhat like a Mad Max version of a tunnel ram. There's a front-mounted distributor in place of the typical LS-engine coil packs.

Here's an overview of the basic parts and why they were selected:

ROTATING ASSEMBLY A Callies 3.750-inch-stroke forged crankshaft is connected to a set of GRP forged aluminum connecting rods. The pistons are from Diamond and in order to keep the compression ratio at a boost-friendly 9:1, they feature large, 50cc dishes.

The rotating assembly is housed in a tall-deck version of the LSX cylinder block. Its 9.70-inch deck height enabled the racing-style connecting rods to swing freely, without the need for internal block clearancing.

By the way, the bores measure 4.125 inches. With the 3.750-inch stroke, that makes the displacement just a hair less than 401ci.

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.

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.

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."

Capabilities notwithstanding, Alameddine spent many hours sneaking up on the turbo engine's tune. It wasn't a simple plug-and-play operation. The fuel trim was kept safely fat and both rpm and boost were initially limited, as dyno pull after dyno pull revealed what the engine was capable of handling.

Alameddine's attention to detail paid off when Thomson began making full dyno pulls, but there was something clearly holding back the engine. It was making more than 1,500hp, but was stymied by the mufflers on the dyno's exhaust system.

It was a simple matter of too much exhaust backpressure and once Thomson's technician's uncorked the system, the engine blew unrestricted and instantly hit the 2,000hp mark. Keep in mind that the performance-hampering backpressure wasn't anticipated because the dyno's exhaust system already measured a large five inches in diameter.

With the unrestricted engine blowing hard on the dyno, it recorded a peak of 2,048hp at 7,140 rpm and a neck-straining 1,507 lb-ft of torque at the same rpm level. And while the engine makes big power even at lower revs-about 700hp at 4,000 rpm-the power comes on like a sledgehammer from about 5,000 rpm onward. In fact, power jumps from about 980 horses at 4,800 rpm to nearly 1,400hp at 6,400 rpm.

Similarly, torque leaps from the merely super-strong 906 lb-ft at 4,800 rpm to nearly 1,700 lb-ft at 6,400. That's nearly 90 percent more torque in the time it takes the tach to swing only 1,600 rpm higher. Talk about driveline shock.

Thomson says there was even more power to be extracted from the engine, but the dyno sessions ceased when GM Performance Parts asked to put the engine in a '96 Impala SS they'd constructed especially for it. Of course, it's no ordinary Impala SS. It was disassembled and rebuilt was a full-frame race car.

"With more time and more boost, I think we could have seen perhaps 2,500hp," he says. "There's a lot left in it."

As we went to press, the folks at GM Performance Parts were not ready to make any passes in the car, but we've got a detailed look at it and we'll bring it to you in a future issue.

For now, contemplate the implications of a 2,000hp, 400-inch LSX engine and what the trickle-down benefits will be for street/strip engines. It's pretty heady stuff, for sure.

Thomson Automotive
GM Performance Parts
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By Barry Kluczyk
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