Back in 1990, a Corvette cost $31,979 and the ZR-1 option added another $27,016. What that bought you, mostly, was the most fearsome engine ever bolted into an American car. Instead of the pushrod, two-valve heads used in the standard Corvette engine (and every other Corvette engine before and since), the ZR-1 had a 5.7-liter V-8 with quad overhead cams and four valves per cylinder. This allowed it to rev high and breathe deeply, spinning out 375 horsepower back when a Ferrari 348 TS made only 300. A friend who was reviewing cars at the time managed to wreck a ZR-1 so violently that the car broke in half. “When the GM people showed up,” he said, “they only cared about the half with the engine.”

That engine, dubbed LT5, wasn’t built by GM. It was built by Mercury Marine. And when the ZR-1 went out of production in 1995, Mercury went back to making boat engines and GM went back to using pushrods. Ah, but what if? What if GM kept developing the LT5, eschewing the current Corvette Z06’s supercharger for high-revving, naturally aspirated power? Well, we have an answer. It’s called the Mercury Racing SB4 7.0, and you can buy it for $29,000. The key design element is the heads: four-cam, 32-valve, just like the original ZR-1. By combining airflow with big-bore displacement, the SB4 achieves impressive numbers: 750 horsepower at 7,500 rpm. That’s 100 more horsepower than the current Z06, without a supercharger. With the SB4’s four-valve-per-cylinder heads, the engine flows a lot of air and thus can be tailored for a sort of split personality—livable at idle and still explosive at high rpm.

I test-drove the engine in an Ultima GTR, a midengine, rear-wheel-drive British kit car. Ricky Rhodes

To find out just how explosive, I strapped into Mercury Racing’s test mule, a ratty Ultima GTR, on a runway at Fond du Lac airport in Wisconsin, down the road from Mercury’s headquarters. Mercury chose the Ultima, a midengine kit car from England, as a testbed because it’s lightweight and uses a steel tube frame designed to accommodate engines making as much as 2,000 horsepower. The SB4 fires up with a lumpy idle, the staccato lope of a race engine awaiting its orders. But it’s not belching flames or threatening to stall or displaying the sort of untoward behavior you might expect to accompany 750 horsepower. Indeed, the 2017 460-hp Corvette Grand Sport that I drove the same day sounded more ragged at idle.

The engine’s specialness, though, is evident in motion. At 6,000 rpm, the Corvette is ready to upshift but the SB4 is just getting started on a manic surge toward its 8,000-rpm redline. You’ve got to will yourself to hang in there for the last couple thousand rpm, as your ears tell you to shift but your eyes on the tach tell you you’re not there yet. I’ve never driven an original Corvette ZR-1, but I’d imagine it’s more like this, frenetic and linear, than the current Z06’s sledgehammer delivery of supercharged torque. This is the alternate reality where GM and Mercury Marine kept collaborating on spin-dizzy V-8s that dare you to keep the throttle down.

Alas, they didn’t, and probably won’t. Chevrolet’s ZR-1, for all its greatness, cost twice as much as a regular Corvette—the new Z06 is $24,000 on top of the $55,450 for a stock Corvette. And with few exceptions, all the modern big-horsepower engines are going to forced induction, turbos and superchargers that make more power with smaller, comparatively fuel-efficient engines.

These days, natural aspiration is an indulgence, and the SB4 represents a chance to experience it in its highest form—American flavor, bolted into whatever hot rod or track car you care to put it in. A new Corvette would be appropriate, but I’d like to drop an SB4 in a 1969 Chevy C10 short-bed. Or better yet, an old Donzi 22. I don’t know if it’s occurred to Mercury, but one of these could make for a pretty fast boat.

Pushrods vs. Overhead Cams

Popular Mechanics

A camshaft is a cylinder with little protrusions that, as the cam rotates, press on levers that open valves to let air in or out of the combustion chamber. In a pushrod engine, a single cam pushes rods to activate the valves—hence, you know, pushrods.

In overhead-cam engines, the cams are above the cylinders, often acting directly on the valves. This setup makes it much easier to have more than two valves per cylinder, which means more airflow and thus more power and tuning flexibility.

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