Homegrown: This Porsche Boxster Hides a Bowtie-Branded V-8 Secret

At first blush, the notion of a Chevrolet-Porsche pairing seems like an unholy matrimony sure to bring out purist pitchforks from both camps. But Hagerty member Mark Mrocca subscribes to no social norms. This 70-year-old retired engineer is not only addicted to speed, he also has a lavish shop—equipped with a Bridgeport 3-axis CNC milling machine—to construct anything that strikes his velocity fancy. So, Mrocca purchased a 2007 Porsche Boxster S with the intention of ditching the stock 3.4-liter flat-six in favor of 5.3 liters of well-tuned good old American V-8 horsepressure.

[Welcome to Homegrown—a limited series about homebuilt cars and the ingenuity of their visionary creators. Do you know a car and builder that might fit the bill? Send us an email to tips@hagerty.com with the subject line HOMEGROWN. Read about more Homegrown creations here. —Ed.]

Porsche endowed the Boxster S with 295 horsepower produced by a mostly aluminum engine with four camshafts, 24 valves, and electronically controlled fuel injection. Mrocca’s new Chevy LS3 small block is a traditional V-8 with pushrods, roller lifters, and roller rocker arms opening two valves per cylinder. But that’s before a lengthy list of aftermarket parts and the constructor’s resourcefulness are factored in. The final answer: 425 horses worth of throaty V-8 to thrust this Porsche roadster down the road with utmost alacrity. That’s 44 percent more power energizing an end product that only gained 60 pounds.

Mrocca notes, “Porsche engines aren’t very durable. Most don’t last 100,000 miles so I didn’t hesitate yanking the broken German six, selling it to a Porsche rebuilder, and moving up in terms of displacement and cylinder count.”

While he kept his new V-8’s iron block, Mrocca didn’t hesitate hiking power and cutting weight with upgrades. LS1 aluminum cylinder heads, providing an 11:1 compression ratio top this engine. An Elgin Industries Sloppy Stage II roller cam activates larger valves. A molded polymer LS1 intake manifold assures excellent breathing with cathedral-port fuel injection.

Since his new V-8 is physically larger than the flat six he yanked, Mrocca did everything possible to trim his engine’s external dimensions. A shallow Camaro oil pan reduced overall height. At the front of the block, a more compact Dorman harmonic balancer was installed and several belt-driven accessories were eliminated. A new Meziere electric pump circulates coolant through the engine and radiator. The original Porsche alternator and AC compressor are bolted to the block as low as possible. Fitting a contemporary Porsche electric power steering system eliminated the Boxster’s original hydraulic assist pump. The new LS1 intake manifold has been reversed so its throttle body is now aimed toward the rear of the Boxster. Longer shift cables from an earlier Boxster S allowed that linkage to wind around the new V-8.

SPEC Clutch in Alabama sold Mrocca the flywheel, pressure plate, and clutch parts needed for this mad alliance. Chavis Performance Engineering out of Maryland supplied the critical V-8-to-Porsche transaxle adapter plate. Pound Design Automotive sold Mrocca a new subframe that supports the engine via Corvette engine mounts, carries the Porsche transaxle, and adds structural stiffness across the bottom of the driveline bay. This steel-tubing weldment is adjustable to suit specific installation needs and is cadmium-plated for corrosion resistance. Unfortunately, commonly available headers don’t fit this application, so Mrocca custom-built his own exhaust pipes out of stainless steel tubing.

Mrocca shifted his entire powertrain an inch rearward to make the most of the existing space, causing the Boxster’s half shafts to operate at a 10-degree angle. Factory anti-roll bar connections also required a bit of rework. To lower the car 60 mm (2.4 inches), Mrocca fitted Godspeed coil-over dampers and adjustable toe-control links. Fortunately, this Boxster S came equipped with Brembo vented brake rotors and four-piston calipers.  For the time being, the original five-spoke aluminum wheels and Continental 18-inch radials will suffice for both track and road duty.

Even with the aforementioned list of mods, the factory engine bay was still too small for a V-8. Mrocca wasted no time wielding his power saw: After a 16×27-in chunk of Porsche unibody metal was chopped out of the firewall, the new V-8 fits nicely in the Boxster. A second opening in the rear trunk wall provides clearance for the V-8’s intake air equipment.

To lift his new powertrain off the floor, Mrocca used a $250 HaulMaster hydraulic table he purchased at Harbor Freight. This device has casters to precisely position the powertrain and 40 inches of lifting range.

Complex fabrication is Mrocca’s joy.  Where his expertise loses momentum is programming the GM electronic control module that operates the ignition, fuel injection, throttle, and anti-theft equipment. After suffering some frustration learning GM’s complex coding, Mrocca discovered a more straightforward aftermarket diagnostics tool called OBDX Pro that works well with LS Droid software. This new equipment adeptly convinces all the electronic systems to play nicely together.

In total, this Homegrown is an impressive piece of work. Like today’s mid-engine Corvettes, there’s a cargo hold at both ends of the car. There’s minimal weight gain over the original Boxster S, so Stingrays of every stripe will be high on Mrocca’s shopping (read racing) list. One negative is that, like today’s Corvette convertibles, there’s no ready means of actually seeing the new V-8. The builder’s planned solution to that shortcoming is converting the removable factory engine compartment access panel to a see-through piece of plexiglass to provide his admirers an excellent look at his handiwork through the rear window when the top is up. This V-8 Boxster is among the more elaborate engineering projects we’ve seen in our entire Homegrown series. At this juncture, the task of adapting a Porsche body, chassis, and transaxle to V-8 duty is about 60 percent complete, by Mrocca’s estimate. That means we’ll be back with a second installment once all the parts are bolted in place and the on-the-road fine-tuning has begun in earnest.