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.

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