So, rather than tear down an expensive LS7 and discard most of its rotating parts, Thomson started with GMPP's LSX iron block and machined it for 4.125-inch bores. The LS7 does come with an admirably strong forged crankshaft, but again, because Thomson was starting from scratch, he used a forged crank from Callies. As for the rods and pistons, they were sourced from Oliver and Diamond, respectively, the latter featuring a friction-reducing skirt coating and a heat-resistant top coating.
Another reason Thomson didn't use the factory LS7 to start with was the heads. Although they're respected for their cavernous ports and straight pathways to the combustion chambers, they aren't compatible with the intake ports of the LS9 supercharger manifold. Consequently, there was no choice but to use LS9 heads. Thomson did, however, manage to up the ante by procuring an early set of new LSX-LS9 heads from GMPP (PN 19213963). They're based on the stock LS9 piece, but feature a six-bolt-per-cylinder design that provides significantly greater clamping strength than the stockers' four bolts per cylinder. That's important for preventing head gasket failure under higher boost.
It's true that LS9 heads don't flow as much as LS7 units, but that's not a detriment in this project, because when the boost level is up, the blower is supplying all the airflow the heads can manage. And fortunately, the LS9 heads are based on the excellent L92 design, with the addition of swirl-inducing wings cast into the intake ports for better mixture motion. The valvetrain consists of 2.16-inch titanium intake valves, and Thomson inserted 1.59-inch Inconel exhaust valves (in place of the sodium-filled stockers). Standard springs, retainers, and pushrods were used.
Capping the 7.0-liter long block, of course, is that Eaton twin-vortex blower. Its four-lobe rotor design delivers a quasi-screw-type profile, helping the blower deliver better low-end torque and high-end horsepower-the areas were Roots blowers are typically the weakest.
Dyno Testing And Conclusion
Testing was conducted using an E67 controller and blowing through stock LS7 exhaust manifolds. It was immediately clear the extra volume of the 7.0-liter engine was a factor to contend with. With the stock supercharger drive pulley, the blower could only deliver about 5 pounds of boost to the larger-diameter cylinders.
It was a baseline, to be sure, but even with that, the engine spit out about the same power as a stock LS9-at only half the boost. A new, smaller pulley was quickly milled, and the testing resumed. The results were more impressive: 710 hp and 780 lb-ft of torque. That's more than 70 additional horses and a whopping 176 more lb-ft from an engine that's only about 13.5 percent larger in displacement. In fact, that 780 lb-ft figure represents nearly 30 percent more torque than the already robust stock LS9-and that's what you really feel with this engine.
Think about all the superlatives and hyperbole used in magazine stories over the years to describe a combination with "earth moving" torque, and they still come up short next to this engine. Without a doubt, a super-strong transmission and rear axle would be must-haves with this monster.
We're looking forward to getting a look at the owner's Corvette project when it's completed. It's bound to discourage more than a few Porsche pilots on the Autobahn.
View a video of this engine on the dyno at www.thomsonautomotive.com.