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.
Thomson's hybrid LS combination starts with GM Performance Parts' standard-deck LSX Bow Ti
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
Machine work prior to assembly included deck-plate honing of the cylinders and line boring
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.