The Thomson Automotive 442ci LS7 Engine Rocks Like A Hurricane

Thomson Automotive's 442ci LS7 is a 740-horse force of nature.

Barry Kluczyk Dec 17, 2013 0 Comment(s)
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As the axiom goes, building horsepower is just a matter of pushing more air through an engine. While true enough on the surface, it doesn’t take into account the different ways in which that power can be delivered, or applied. And because horsepower derives from torque (tq x rpm ÷ 5,252 = hp), the twisting power of the engine is essential.

Besides directly affecting horsepower, torque makes all the difference in the performance characteristics of a car. With low torque, a vehicle can feel heavy or slow at low speeds, while greater torque can effectively “lighten” it. With 505 hp clocking in at a lofty 6,300 rpm, the Corvette Z06’s 427ci LS7 engine is all about high-rpm thrust. Its peak torque of 470 lb-ft at 4,800 rpm is impressive, too, but in an era of truly high-output LS engines, it could be better.

Enter Thomson Automotive’s Hurricane 442. It’s essentially a stroked LS7 that adds significant upper-rpm horsepower, along with a ton of low-end torque. It’s rated at 740 hp and 640 lb-ft on pump gas when equipped with the racy-looking Harrop Hurricane individual-runner intake manifold from which it takes its name.

Compared with a stock LS7, stroke increases from 4.000 to 4.125 inches, and the iron liners are honed 0.005-inch, to 4.130 inches. That combination gives the engine its 442ci (7.2L) displacement. It may seem like a relatively modest bump in size, but it makes all the difference on the dyno, where the extra stroke provides a generous boost in low-rev torque.

“With the aluminum block and iron cylinder liners, you can’t add bore diameter to the engine,” says company owner Brian Thomson. “And more importantly, the longer stroke delivers the low-rpm torque we wanted from the engine, which is great for road racing and autocrossing. You can just leave it in a lower gear during turns and explode out of the corners with instant power.”

At 3,000 rpm, for example, the Hurricane 442 is making 447 lb-ft; by 3,500 rpm, its twisting power has increased to 497 lb-ft. It keeps rising until its 652-lb-ft peak at 5,400 rpm. Likewise, the power crescendo starts low and peaks at 6,500 rpm, where the engine is not only making 235 more horses than a stock LS7, it tops the supercharged LS9 from the ZR1 by more than 100—and it’s all naturally aspirated.

“The linearity of the power progression through the rpm band is strong and tractable,” says Thomson. “Power builds predictably, with an exceptionally strong foundation of grunt to pull you out of a corner very quickly.”

You could also look at it this way: 740 hp is 46 percent more power than the stock LS7 rating, from only 3.5-percent-greater displacement. That’s a serious combustion-efficiency increase, and because an engine’s power is all about how much air can be processed, it’s clear the Hurricane 442’s airflow lives up to its name. Much of that airflow capability comes from a set of ported LS7 cylinder heads matched with the exotic-looking Harrop (harrop.com.au) Hurricane intake system, which Thomson says is worth about 40 hp and 50 lb-ft on the dyno when compared with a stock LS7 intake.

Harrop says the long, straight ports of the intake, coupled with a smooth transition, enable greater than 100 percent volumetric efficiency. We can only take their word on that, but the effectiveness of the system on the LS7 seems to be borne out in Thomson’s evaluation.

“It works, and it fits under the hood of a C6,” says Thomson. “And to be honest, it’s not inexpensive, so we wouldn’t have used it if we didn’t feel there was a strong benefit. From the numbers we’ve recorded on the dyno, the benefit is clear.”

Indeed, Thomson offers a slightly more economical version of the 442 engine equipped with an LS7 intake and rates it at 700 hp/600 lb-ft. Of course, “economical” is a relative term, as the Hurricane 442 runs $31,000 a copy, while the 700hp “plain” 442 combo clocks in at $26,000.

Of course, if a measly 15-inch displacement increase with an eighth-inch-longer stroke is good, an even-larger stroke increase would be better, right? Actually, no.

Chevy Ls7 Engine 2/21

01 Thomson Automotive starts from scratch with a bare LS7 block from Chevrolet Performance (PN 19213580). The iron cylinder liners are honed 0.005-in to bring the bore to 4.130-in.

Ls7 Engine Custom Piston Oil 3/21

02 The block is fitted with custom piston oil jets (arrow)—squirters that drench the bottoms of the slugs with oil to help cool them at high rpm. This enhances the combustion process and cuts the chance of detonation.

Callies Dragonslayer Stroker 4/21

03 The stroker crank is a Callies Dragonslayer that delivers a 4.125-inch stroke. The 4340-steel forging is equipped with a 58-tooth reluctor wheel, so it works with a GM E67 or E38 engine controller.

Oliver Forged I 5/21

04 The rods are Oliver forged I-beams that measure 6.125-in—0.095-in longer than stock. The longer rods reduce the side load and stress on the pistons, which are traveling farther in the cylinder than with a stock-stroke combination.

Teflon Skirted Diamond Forged Piston 6/21

05 Teflon-skirted Diamond forged pistons work with the heads to give the engine a 12:1 compression ratio, a full point higher than the stock LS7. Higher compression definitely delivers more horsepower, but a ratio this high is pushing it on pump gas.

Windage Tray 7/21

06 Here’s an example of why it’s important to take care when building an engine with non-stock parts: After installing the rod/piston assemblies, the windage tray was laid in place, so the clearance of the new rods could be checked. There was minor interference, which would have been bad news had the tray simply been snugged in place. A couple of washers on each stud provided the necessary clearance.

Comp Cam Hydraulic Cam 8/21

07 Next up is the hydraulic roller cam from Comp (251-/262-deg duration, 0.669-/0.685-in lift). The lobe-separation angle is a relatively wide 115 deg, which minimizes overlap for greater idle quality and low-speed driveability.

Chevy Performance Six Bolt Lsx Ls7 Cylidner 9/21

08 Interestingly, Thomson uses Chevrolet Performance’s six-bolt LSX-LS7 cylinder head (PN 19201806) but saws off the extra head-bolt provisions along the perimeter to fit the four-bolt configuration of the factory block. Why? Because they’re stronger castings than stock LS7 heads, according to Brian Thomson.

Chevy Performance Six Bolt Lsx Ls7 Cylidner Head Back 10/21

09 The CNC-machined 356-T6 aluminum heads boast 270cc intake ports and 85cc exhaust ports—the same as the stock LS7 head, but with added materials in the ports to support porting, along with reinforced rocker stands for greater rigidity.

Chevy Performance Six Bolt Lsx Ls7 Cylidner Head Combustion 11/21

10 Combustion chambers displace about 70cc’s and are complemented by the LS7’s stock 2.20-in titanium intake valves. On the exhaust side, 1.61-in Ferrea stainless steel, hollow-stem valves replace the sodium-filled stockers.

“The LS7 block will easily accommodate a 4.125-inch stroke, but you really don’t want the stroke larger than the bore,” says Thomson. “That undersquare condition increases piston speed, increases friction, and generally puts more wear on the reciprocating assembly.

“If we were building a big-torque, low-rpm truck engine, then an undersquare combination might be acceptable, but we’re building an engine that we want to rev high and make big horsepower, so there’s a practical limit to increasing the stroke.”

Journalistic instincts prompted us to explore all sides of the story, so we pondered potential alternatives to this naturally aspirated powerhouse. For about $8,200, Edelbrock’s E-Force supercharger kit for the LS7 is rated at 657 hp and 582 lb-ft of torque, or about 88 percent of the power and 91 percent of the twist for about 70 percent less money (not including installation). That’s a significant price difference, but again, the performance profile of a naturally aspirated engine is much different than that of a forced-induction one. Determining which approach is preferable for you really comes down to the manner in which you intend to use the car.

We’ve sampled the Hurricane 442 in Thomson’s personal ’11 Grand Sport and can attest to its performance. The low-end torque is explosive, and the high-rpm power surge seems to have no limit. The engine just keeps pulling until you run out of nerve, road, or both. Low-speed and part-throttle driveability is excellent, too.

Cost, however, is definitely a factor. If you’re simply looking for a faster weekend cruiser and occasional dragstrip warrior, a bolt-on blower kit probably makes more sense. On the other hand, if your plans include exploring the boundaries of your Corvette’s performance on a road course or deserted highway—and your checkbook can swallow it—this is one of the best naturally aspirated LS7 combinations we’ve ever encountered. Quite simply, it rocks. vette

Pac Racing Dualcoil Valvepsrings And 12/21

11 Atop the heads are PAC Racing dual-coil valvesprings and retainers, which support factory LS7 1.7-ratio rocker arms, Trend pushrods (3⁄8-in diameter, 7.800-in long), and Chevrolet Performance lifters.

Chevy Ls7 Factory Ls9 Oil 13/21

12 Thomson used a factory LS9 oil pump on this assembly, but the 442 can also be had with a more conventional wet-sump oil pump or even a racing-spec Dailey Engineering multistage dry-sump system.

Harrop Hurricane Individual Runner Intake 14/21

13 The Harrop Hurricane individual-runner intake-manifold assembly features long intake runners that help increase airflow velocity for greater high-rpm horsepower. Runner length is 310 mm from the manifold face to the end of each trumpet.

Harrop Hurricane Individual Runner Intake Manifold Progressive Throttle 15/21

14 A look beneath the manifold’s carbon- fiber cover reveals a progressive throttle system that helps maintain part-throttle resolution for easier calibration and greater driveability. The manifold is designed to accept factory sensors and other fittings, including the throttle-position sensor, pressure sensor, and vacuum.

Chevy Ls7 Steel 16/21

15 The electronic actuator from an LS7 throttle body is mounted below the forward section of the manifold; it directs the operation of the eight individual intake-runner butterflies. The manifold can also be fitted with a cable-operated throttle body.

Two Piece Aluminum 17/21

16 The two-piece aluminum trumpets are CNC works of art. This detail shot also shows the relationship of the fuel rail and the mounting position for the injectors. Thomson uses factory LS9 44-lb/hr injectors on the Hurricane 442.

American Racing C6 18/21

17 On the dyno, the engine blew through 2-in American Racing C6 headers, providing a more accurate depiction of its real-world performance. It cranked out 740 hp at 6,500 rpm and 652 lb-ft of torque at 5,400 rpm.

Harrop Hurricane Individual Runner Intake Manifold 19/21

18 Filters and an airbox are must-haves on the street, but at the time we finished out story, Harrop didn’t offer these parts for Corvettes. The one seen here on Thomson’s Grand Sport is a prototype, but a carbon production version is scheduled for release any time now. (Note that it may drive up the cost of the manifold system by a few bucks.)

Harrop Hurricane Individual Runner Intake Manifold 20/21

19 Although the Hurricane intake was designed for universal use, the C6 was a consideration during its development. It’s a tight fit with the prototype airbox, but fit it does—although the extra clearance of the factory CFZ or aftermarket raised hood certainly helps.

Chevy 21/21

20 Powered by the Hurricane 442, Thomson’s GS won the Optima qualifying competition at last fall’s Lingenfelter Performance event at Norwalk, Ohio. Besides outgunning competitors on the autocross, it posted a traction-limited 11.68 e.t. on the dragstrip.


Engine-Dyno Test Results
(STP Correction)

Rpm Lb-Ft Hp
3,000 447 255
3,100 457 270
3,200 469 285
3,300 501 315
3,400 502 325
3,500 497 331
3,600 483 331
3,700 478 336
3,800 484 350
3,900 496 368
4,000 507 386
4,100 525 410
4,200 559 447
4,300 598 490
4,400 617 517
4,500 610 522
4,600 595 521
4,700 594 531
4,800 597 546
4,900 606 566
5,000 619 589
5,100 624 605
5,200 641 635
5,300 648 654
5,400 652 670
5,500 651 682
5,600 640 682
5,700 633 687
5,800 625 690
5,900 623 700
6,000 606 693
6,100 610 708
6,200 612 722
6,300 596 715
6,400 595 725
6,500 598 740
6,600 584 733
6,700 574 732
6,800 558 723
6,900 553 726
Peak numbers in bold

Sources

Thomson Automotive
Wixom, MI
248-349-0044
www.thomsonengines.com
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