We likely don't have to tell you the details of the Corvette ZR1's LS9 engine, but just in case, it's a 6.2-liter (376ci) mill that's force-fed by a 2.3-liter Roots-type supercharger using Eaton's twin-vortex, four-lobe rotor design. It's a relatively simple recipe that delivers 638 hp and 604 lb-ft of torque, making for one of the sweetest combinations in the history of internal combustion.
Of course, there's also a 7.0-liter (427ci) engine in the Corvette arsenal, in the form of the Z06's LS7. It's only logical to wonder what would happen if you married the big blower from the LS9 with the larger displacement of the LS7.
The Detroit-area LS-engine gurus at Thomson Automotive did more than bench-race about that "what if"-they acted on it, building a 7.0L engine with that factory supercharger setup. It was commissioned by Germany-based American auto enthusiast Ron Flatt, who wanted the engine for a '67 Corvette project destined for eyebrow-raising-and Porsche-stomping-runs on the Autobahn.
For a number of reasons, Thomson didn't simply bolt the LS9 blower onto an LS7 crate engine. Instead, the company built a custom bottom end that would support the projected 700-plus horsepower that the additional 51 cubic inches of displacement would provide.
"The LS7 is an excellent engine, but it has a very high compression ratio that's not compatible with a higher-boost supercharger setup," says Brian Thomson, president of Thomson Automotive. "There were other things we wanted to do to ensure an optimal balance of performance and safe, pump-gas driveability without the fear of detonation-and because it was ultimately being shipped to Germany, we wanted to make sure it was as durable as possible."
The details of the buildup are found below, but highlights include the following:
An iron LSX cylinder block from GM Performance Parts
An all-forged rotating assembly with low-compression 9.0:1 pistons
Custom piston oil squirters in the cylinders
GMPP's new LSX-LS9 cylinder heads
A custom, smaller-diameter blower pulley to spin the blower faster and generate boost comparable to factory LS9 levels on the larger-capacity 7.0L engine
On Thomson's engine dyno, the combination's best performance was 710 horses and 780 lb-ft of torque, with about 12 pounds of boost. We witnessed the dyno test, and what we found just as impressive as the engine's peak numbers was its overall tractability.
Thomson picked a sweetheart of a camshaft that delivers the idle characteristics of a stock engine, but spits out ever-bigger numbers as the revs climb. It's only been within the last five years or so that such performance has been attainable with daily-driveable traits. This engine doesn't stutter or stumble at idle, it doesn't "lay down" at certain rpm levels, and it's not tuned on the ragged edge to make its impressive numbers.
"Germany isn't exactly across town or even in the next state, so we won't be able to service the engine easily," says Thomson. "We went probably a little more conservative than normal on the tuning to make sure the customer wouldn't have any worries. But we think the numbers are still pretty good."
So do we. Here's a closer look at how they were achieved:
LSX Block And Blower Compatibility
Although it would seem logical to start with the LS7 engine itself for a supercharged 7.0-liter buildup, its hypereutectic (cast) 11:1 pistons would have to be replaced-as would its featherweight titanium connecting rods, which aren't designed to support the expected output of the combination.
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.
Thomson's Flexplate Solution
GM Performance Parts' LSX454 crate engine and the supercharged LSA crate engine each use a crankshaft with an eight-bolt rear hub that doesn't match the standard six-bolt pattern of conventional GM automatic transmission flexplates-including the popular Hydra-Matic 4L80-E. To address this issue, Thomson Automotive designed a flexplate kit that features a factory GM eight-bolt flexplate and a custom-machined adapter hub. The flexplate is also modified to accommodate the bolt patterns of popular aftermarket torque converters.