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.
“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
Engine-Dyno Test Results
|Peak numbers in bold|