Cat's Cradle

If you’ve taken a peek under the hood of any car built in the past decade, odds are you were greeted by a great big expanse of plastic bearing the manufacturer’s logo smack-dab in its middle. These covers serve a variety of purposes—hiding the potentially dirty bits of the drivetrain from the blissfully ignorant masses, for example—but one of their primary functions is packaging.

If nothing else, a big plastic cover gives the impression that it’s shielding an equally large complex of performance equipment, privately chugging away. If you can’t see the pavement when the hood is up, then certainly it’s because every square inch of space in the engine bay is being used to generate power, right?

A few quick turns of a wrench, however, can remove these plastic covers and peel back the facade, and doing so often invites disappointment. If the plastic veil is stripped away and air is found underneath, our natural response is to want to do something about it. Ideally, we’d prefer to pop the hood and see a massive lump that threatens to dent the engine compartment from the inside thanks to its sheer imposing volume.

Aaron Couper isn’t the first person to put a bigger engine in his car, but his story is a twist on the standard monster V8 shoehorn story. After all, big is relative when your starting point is a diminutive 1961 Austin-Healey Sprite. For Aaron, there was no better choice for his Sprite than the robust 3.8-liter Jaguar straight-six.

Alley Cat

Jaws drop when Aaron pops the hood of this Sprite to reveal a massive Jaguar straight-six.

Aaron, who’s originally from New Zealand and now makes his home in West Pawlet, Vermont, first encountered the starting point for this project some 15 years ago. “I found the Sprite in an alley in San Diego,” he reports. “The price was right at $200.”

As the all-in-one owner, chief restorer and self-described floor sweeper of Couper’s Classic Cars, Aaron spends most of his time working on customers’ cars. As a result, his Sprite sat idle for the better part of those 15 years. The eureka moment came, as such moments do, unexpectedly one day.

“I had sort of started on the Sprite and built up a 1275 turbocharged engine to put into it,” Aaron continued. “I was finishing a customer’s Jaguar XK120, and as I went past the Jag engine on the stand, I said, I wonder.”

Just like that, the turbocharged 1275cc engine was pushed to the side. Aaron started measuring up a big Jaguar six he had at the shop and compared the numbers to the available space in the Sprite’s engine bay.

“I love Jags,” he confesses. “I just said, Hey, why not?” Then he answers his own question. “Mostly because there’s no way a Jaguar engine would fit into an Austin-Healey Sprite,” he laughs, echoing the sentiment of those who caught wind of his plan in the early stages.

Once he started work on the car, Aaron learned that there was a semi-famous 1959 Sprite Mk I that had received a bored-out 4.2-liter Jaguar six. The car was built in England in the 1960s by Alan Brooks. Dubbed The Fright, the creation currently lives in Australia. If nothing else, word of The Fright was proof of the concept’s viability, and by all accounts the car was a missile. Aaron continued with his plan.

Fitting the Feline

Aaron Couper did all the fabrication and detail work at his shop, Couper’s Classic Cars, in Vermont.

After taking a few basic measurements, Couper was a bit surprised to discover that the big Jaguar six would fit in the Sprite’s engine bay without any dramatic surgery. The transmission, however, would not be so accommodating.

“The Jag transmission is kind of wide—once I got it in there wasn’t much room for the seats,” Aaron recalls. “Everything else was pretty straightforward. The engine mounts bolted right up to the original Austin-Healey mount position.”

There was one casualty, however. “I had to cut the heater out,” he explains, but it didn’t seem to concern him too much. “No need for a heater when you’ve got a British piece of iron!”

To make it all fit, Aaron also had to fabricate a removable transmission tunnel. In the end he sacrificed about three inches of leg room on each side of the foot well in the firewall area. It’s cozy, but not uncomfortable for the average driver.

A pair of 2-inch SU carburetors feed the hungry block. The exhaust is a mix of Jaguar cast-iron headers, some custom front pipes and the stainless steel dual-pipe exhaust from an XK120.

The torquey 3.8-liter Jag block was far too lusty for the little Sprite differential, so Aaron needed to find a suitable upgrade for the stock pumpkin. A trip to the junkyard bore fruit in the form of a Toyota pickup truck rear end. “The price was right, something like $75 for the rear end,” he recalls. “I ended up making a four-link rear end with a Panhard rod and coil-over shocks.”

The shocks came from eBay, and the generic springs were soon found to be too soft. Aaron cut some XKE springs and was happy with the end result.

Up front, Aaron mated the Sprite’s stock lower A-arms to custom upper A-arms and once again turned to the Internet for the finishing touches. “The front springs came from eBay. They’re 800 lbs./in. springs—the biggest you can buy for a Sprite off the shelf is a 500-pound spring. This guy paid big money to have them made in some race shop and no one else bid on them. He basically gave them away.”

Since the Jag block was quite a bit longer and taller than the original Austin-Healey unit, the hood was in need of some alteration. Aaron initially broke out the hammer and English wheel to modify the stock piece, adding a 21/2-inch bump to clear the cam covers. It was an attractive solution that did the trick, but he wasn’t in love with the end result.

“I was also thinking about the heat issues,” he explains, “It’s one thing getting air into the engine bay, but it’s another to get it out. I happened to have an old E-type hood that was all ugly around the edges. I cut about 13 inches or so out of the middle of it, wheeled it and got it all nice and smooth. I cut the old skin off the Austin-Healey frame and it just kind of fit on the frame.” The E-type hood provides functionality thanks to the large stock vents near the windshield, and it also hints at the nature of the beast lurking beneath.

Kitty’s Got Claws

The hardest part of the swap was getting the transmission to fit. Aaron had to widen the transmission tunnel by about three inches on each side at the firewall.

The hardest part of the swap was getting the transmission to fit. Aaron had to widen the transmission tunnel by about three inches on each side at the firewall.

Thanks to years of experience and more natural talent than Aaron would ever admit to having, little guesswork was involved in the build. As a result, the Sprite came together as precisely as if it had begun life with the big Jag block in the nose.

“The first time I drove the car, it was about three o’clock in the morning. Everything worked well together, I was very surprised. I ran it with no doors. It would spin the wheels in first and second—way too much torque for the car,” he recalls.

Traction is tricky when you’re dealing with big power in a small chassis, so an upgrade to the wheel and tire combo was necessary. He chose 15x6-inch Superlite wheels fitted with 165mm wide tires up front and 175s in the rear.

The larger wheels also allow for bigger brakes, so he opted for Spitfire rotors and MGB calipers up front, a common big-brake setup among Spridget owners. A vacuum booster from a Mini keeps the pedal effort reasonable. “If I stand on it, it pulls up pretty quick,” he reports.

Most people expect that the addition of such a massive engine into such a tiny car would send the handling to the dumps, but that’s not the case. Aaron says that despite its length, the engine is still mounted behind the front wheels. As a result, only 55 percent of the car’s 1820 pounds rests on the front end. He assures us that with two people and a full tank of gas onboard, the balance is just about neutral.

Aaron admits that the great looks of the Jaguar engine were as big a factor in his decision as the power. In contrast to the ambiguous response that a plastic cover would generate, the shapely and massive straight-six in his Sprite has an almost physical impact, even when it’s not running.

“The other day, one of my daughters said, ‘Dad, your car makes people swear!’ when I open the hood,” Aaron laughed. Good thing his daughter can’t hear what they must say when he hits the gas.

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