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_01_z Twin_turbo_lsx_engine Turbocharged 2/24

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.

Sucp_0909_03_z Twin_turbo_lsx_engine Iron_casting 3/24

Underpinning the twin-turbo project is GM Performance Parts' LSX cylinder block. Its iron casting makes it very durable for forced induction combinations, while delivering the added benefit of great affordability-it retails for less than $2,000. It also is designed for six-bolt-per-cylinder head clamping, which tremendously reduces the chance for head gasket problems under high boost.

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.

Sucp_0909_04_z Twin_turbo_lsx_engine Internally_balanced 4/24

A Callies internally balanced, 4340-forged steel crankshaft complements the LSX block's 4.125-inch bores with a 3.750-inch stroke, for a grand total of 401ci.

"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.




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