It’s been just about 10 years since the C6 Corvette Z06 roared into the hearts and onto the chassis dynos of enthusiasts around the world, with its 7.0-liter LS7 engine pushing LS performance to an unprecedented level.
The LS7’s 427-cubic-inch displacement and deep-breathing, racing-derived cylinder heads combined to produce 505 horsepower—an output that would have generated cover stories for a modified car only a few years earlier. Better still, the LS7 proved to be an adaptable foundation that tuners exploited for tremendous gains, ushering in the modern era of LS high performance.
Detroit-area Thomson Automotive has been at the forefront of max-performance LS combinations, which have been renowned as much for their durability and driveability as their big power, and one of the company’s most successful has been a hybrid of the LS7 and supercharged LS9. By adapting a faster-spinning LS9 blower to the 13 percent larger displacement of the LS7 block, the results typically deliver 800-850 horses and 800 lb-ft on pump gas.
Recently, Thomson has experimented with an E85 version of the combination, which pushed output past the 1,000-horsepower threshold while retaining the characteristic docility that makes these engines suitable for street cars and track stars, such as the all-conquering Pro Touring ’69 Camaros of Mark Stielow.
“More power is the obvious advantage of E85, but we were also looking for a cooler, denser air charge with the supercharger,” said the shop’s founder and president, Brian Thomson. “This engine is going in a C6 Corvette, which doesn’t offer a lot of cooling through the grille for an engine making this kind of power. The cooler-burning E85 will help keep down the air inlet temp, while enabling big power.”
That’s all with a safe air/fuel ratio of about 11.4:1 under full boost, too. Thomson admits there are a handful of horses that could still be had from the engine, but the slightly conservative tune ensures a grenade-free driving experience.
“It’s a sweetheart of a street engine, but we build these engines mostly for owners who will use them primarily for the street and maybe a few trips to the dragstrip,” says Thomson. “It starts, idles, and has the low-speed driveability of a stock engine. And when you tap into it, it’s amazing. But sure, there’s probably another 20-25 horsepower or so that could be had if we got more aggressive with the tune.”
Then again, who’s going to notice 20 horsepower when the engine is already pushing more than 1,000? That’s like complaining about a couple of oat flakes slipping into a special, all-marshmallow box of Lucky Charms.
A strong foundation
While the LS7 is the source of the engine’s short-block, Thomson has evolved his unique combination so that only the LS7 block is used in the bottom end. All components of the reciprocating assembly are replaced with heavier-duty parts, including swapping the original cast crank for a Callies forged steel piece and swapping out the LS7’s exotic titanium connecting rods. For one thing, the featherweight titanium connecting rods of the LS7 simply aren’t designed for the load, stress, and power range of the supercharged engine. They’re strong, but comparatively brittle. Instead, a set of Oliver forged I-beam rods are used.
As for the stock LS7 pistons, the hypereutectic (cast) material of them causes concern in a supercharged application, but they also deliver too much squeeze in the cylinders. The LS7 has a high, 11.8:1 compression ratio—way too much for street-driven blower engine. In their place, a set of Diamond forged aluminum pistons is used, each with a sizable dish to help lower the compression to a more blower-friendly 9.0:1.
Thomson uses CNC-ported LS9 heads on the engine rather than the high-flow LS7 heads because the intake ports on the LS9 supercharger don’t match the LS7 ports. That’s not a detriment; however, 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 good-breathing L92 design, but feature swirl-inducing wings cast into the intake ports for better mixture motion.
Another thing going for the LS9 heads is their superior construction. They’re manufactured with a roto-cast method that spins the mold while the aluminum is poured, resulting in more even material flow and almost no chance of porosity. They’re also made from a stronger, A356 T6 alloy than other LS heads—including the LS7 heads—that should help them withstand warping and other heat-related issues.
Straddling the heads, of course, is the ubiquitous Eaton TVS blower from the LS9, with four-lobe rotors. Because it was developed for the factory LS9’s 6.2-liter displacement, it wouldn’t provide the same maximum boost on the 13 percent larger 7.0-liter engine. So, Thomson uses a Lingenfelter blower snout with a smaller drive pulley, matched with a custom ATI balancer/pulley. This allows the blower to spin faster and deliver more boost—about 16 psi or so in the E85 version of the engine.
In a nutshell, ethanol-based E85 fuel delivers the gasoline equivalent of 105-octane and burns cooler than straight pump gas, allowing for more aggressive tuning—as well as the cooler air intake charge—Thomson was seeking for his supercharged engine.
The fuel system in Thomson’s engine also incorporates the same FlexFuel sensor used in production GM vehicles, which allows the engine to run on E85 or 92-octane gasoline. In fact, the controller carries distinctive calibrations for ethanol and gasoline, automatically switching between them based on the input from the fuel sensor.
“The E85 tune is really set on kill for track use and is pretty aggressive,” says Thomson. “The gasoline calibration is friendlier for street driving, but still delivers about 900 horsepower.”
There’s another reason to run gas primarily on the street: fuel consumption. E85 systems at this level demand a lot of fuel and it’s thrown into the engine via fire hose-like 130-pound injectors. In fact, supplying sufficient fuel in the Corvette is an issue Thomson is still working on.
“We’ve already replaced the fuel line with a 1/2-inch stainless line, but we’re still working out how to get enough fuel pump in the Corvette’s tank for sufficient supply at WOT,” he says.
01. The standard cylinder block for the 7.0-liter LS7 is the foundation for Thomson’s LS7/LS9 hybrid engine and is available through Chevrolet Performance under part number 19213580. It’s aluminum with pressed-in steel bore liners and forged steel main bearing caps; the bores measure 4.125 inches in diameter.
02. Machine work prior to assembly includes deck-plate honing of the cylinders and line-boring of the crankshafts mains. These are common high-performance engine-building procedures, with the line-boring supporting the addition of ARP main studs.
03. To reduce piston temperatures—especially under boost—Thomson Automotive machined the cylinder block to accept oil squirters, similar to what’s standard in the production LS9 engine. It is a precise, multi-step process that taps into one of the block’s primary oil passages. The squirters themselves are carburetor air bleeds.
04. The camshaft is one of the key components in making the hybrid LS7/LS9 engine deliver big power with surprising street civility. We were sworn to secrecy on it, so we can’t reveal its specs. It works well at giving the engine a docile idle and excellent low-speed tractability.
05. A Callies forged steel crankshaft is used, delivering a 4.000-inch stroke that combines with the 4.125-inch bores to give the engine its 427-inch displacement. It’s used with an LSA oil pump, which is a high-volume unit for a conventional wet-sump oiling system, but doesn’t require the longer-snout crankshaft of the dry-sump LS9 or LS7.
06. Stock LS engines (except LS9 and LSA) have press-fit dampers and consequently no keyway on the crankshaft. To prevent slippage at the boost and power levels this engine will be capable of, Thomson insists the crank is fitted with one.
07. Forged I-beam rods (in the stock 6.067-inch length) from Oliver are used in place of the LS7’s titanium rods. The lightweight titanium rods simply aren’t designed for the load and power levels of this engine. The effect of greater mass on the rotating assembly with heavier I-beams will be quickly offset with the rise in boost pressure under load.
08. The LS7’s cast-aluminum pistons were replaced to enhance strength under high cylinder pressure and reduce compression. Forged aluminum, coated pistons from Diamond are used. They are dished to lower the compression ratio to a detonation-avoiding 9.0:1. The blower-specific pistons also have a thick crown and reinforced pin bosses.
09. The pistons’ gold-colored top coating is a ceramic material that reduces heat absorbed by the pistons—a notable concern with a high-horsepower, supercharged engine. Also, the skirts of the pistons are Teflon-coated to reduce friction.
10. The rod/piston assemblies are fastened to the crankshaft with ARP bolts via the stretch method, which is a more accurate way of ensuring the proper pre-load and clamping force on them. Essentially, the bolts are stretched until they are within the sweet spot of elasticity. That’s usually between 0.005 and 0.006 inch, which is right where the bolt shown here registered on the gauge.
11. Interestingly, the larger-capacity LS7 cylinder heads aren’t used, but only because their intake ports didn’t match those of the supercharger manifold. That made the factory LS9 heads the only real choice, which is fine, because they’re based on the high-flow L92 heads and manufactured with a stronger alloy that helps them withstand the extra heat that comes with forced induction.
12. Stock 2.16-inch titanium intake valves and aftermarket (but stock-size) 1.59-inch Inconel exhaust valves are used. The thermal properties of the Inconel valves better suit the higher combustion temperatures of a forced-induction engine.
13. Valvetrain details read essentially like those for a stock LS9, including the stamped, non-roller 1.7-ratio rocker arms. The valvesprings are matched to Thomson’s proprietary camshaft specs.
14. To align with the LS9 supercharger pulley, Thomson uses a complete LS9 front dress assembly, including the water pump, brackets, accessories (alternator, power steering), and pulleys. Even the LS9 valve covers must be used because the alternator and power steering pumps won’t fit otherwise.
15. The LS9 supercharger assembly delivered from Chevrolet Performance includes both the compressor and an integrated liquid-to-air charge cooler mounted on top, along with the fuel rails, injectors, and throttle body already installed. The sixth-generation, “TVS” positive-displacement blower comes from Eaton and is their largest-ever Roots-type compressor, displacing a significant 2.3 liters per revolution.
16. On the 6.2-liter LS9, the blower generates about 10.5 psi of boost, but only about half that with the 7.0-liter LS7, so Thomson uses a higher-flowing “snout” and smaller-diameter drive pulley from Lingenfelter Performance Engineering to help spin the blower faster, bringing the boost up to about 16 psi.
17. Thomson uses GM’s 90mm throttle body, although it has proved a restriction on the larger-displacement, force-fed engine. There’s probably 20 hp or more to be realized with a larger throttle body.
18. The engine uses Injector Dynamics-supplied ID1300 fuel injectors, which are manufactured by Bosch. Featuring stainless steel guts, they’re designed to stand up to the corrosive attributes of alcohol-based fuels, such as ethanol-based E85. They flow about 130 pounds per hour.
19. Interestingly, Thomson has used production-based ACDelco ignition coils rather than aftermarket parts, noting they (PN 12611424) perform well up to the limit of what an LS engine can handle with a coil-on-plug ignition. In the case of this engine, they handled 1,000+ horsepower without a hitch.
20. Here, the supercharger assembly is bolted in place. The factory 2.5-bar MAP sensor (seen at the rear of the intercooler cover) is used to ensure accurate air pressure readings when the blower is making more than 10 psi of boost. Engine mapping is controlled by a GM E67 controller from Chevrolet Performance (PN 19201861).
21. Another pulley change involves the damper, which is sourced from ATI. Thomson machines a custom hub for it to mount on the crankshaft. Look closely and you’ll see the pulley is divided; the wider section is connected to the blower and power steering pump, while the narrower section is for a separate belt that drives the alternator and air conditioning system.
22. With base timing of 27 degrees and 18 degrees under boost, the engine produced a best of 1,010 horsepower and 1,018 lb-ft of torque with 16 psi of boost. Under maximum boost, the air/fuel ratio goes to about 11.4:1—plenty safe without sacrificing performance.
23. On the dyno, the engine was tested with factory LS7 exhaust manifolds. They make more horsepower, but give up a few lb-ft to headers, but with more than 1,000 of ’em on tap, they’re not missed.
24. The finishing detail for Thomson’s LS7/LS9 combos is a customized intercooler lid on the engine. The original “6.2L” lettering is cut out and either “7.0L” or “427” inserts are carefully inserted and metal-finished for a seamless appearance, while the lid itself is painted typically to complement the color of the intended vehicle. It’s a great touch.
On the Dyno
You can see the performance of the engine on the accompanying dyno chart, but as we mentioned above, the engine produced over 1,000 horsepower and 1,000 lb-ft of torque. Those numbers were achieved with the engine blowing through stock LS7 exhaust manifolds, which Thomson prefers to headers.
“They have great flow attributes,” he says. “GM did their homework designing them.” A broader look at the numbers reveals an almost impossibly strong, broad torque curve. The engine made 950 lb-ft by 2,500 rpm and held above that mark through 5,100 rpm. Similarly, the horsepower came on strong at lower rpm and carried through the entire rpm band. It made 500 horsepower before 2,800 rpm, 600 by 3,200 rpm, and zoomed past 800 by 4,200 rpm—and kept rising from there all the way through 6,600 rpm.
Amazingly, there’s more horsepower on the table, says Thomson, because of the restrictive electronic throttle body on this engine combination. Using a GM 90mm unit, the air pressure drop after the throttle body is about 12 kPa (about 1.7 psi). That indicates the engine isn’t drawing enough air for maximum power.
“There’s probably another 25 horsepower in it with a larger, 100mm throttle body and the same tune,” he says.
Of course, we can’t discuss a 1,000hp/1,000-lb-ft, LS9-blown engine without bringing up cost. Thomson doesn’t dance around the cost so neither will we. It’s $40,000. It is what it is.
Then again, 1,000 horsepower used to be the sort of output reserved for all-out racing engines. This one is built for the street, and it’s hard to put a price on that sort of exclusivity.