Calling something a boat generally isn’t a compliment in the car world, even if it has a big-block under the hood. The recipients of that verbal barb are usually massively hefty rigs with a pillow-soft suspension and the curb weight of two pony cars combined — everything you don’t want in a high-performance machine.

But that may not be entirely fair. Believe it or not, the boating world can teach performance enthusiasts plenty about potent V8s with everyday-usable punch. Beneath the towering flybridges and polished hardwood decks of large seafaring cruisers often lie familiar Detroit-style V8s fortified with the same tricks and trimmings that have been spicing up Chevelles, Camaros, and other Bow-Tie blasters for decades.

There are some key differences between boat engines and car engines, however. Marine engines spend relatively long periods at constant, fairly high throttle settings. Think of them as the opposite of drag-racing engines, which use their power in short, intense bursts. And while a car cruising on the highway will turn about 1,500-2,500 rpm, a boat engine will generally be doing more like 3,500 to 4,500 rpm at cruise. It’s also not unusual for boats to be pinned at full throttle for extended periods, turning 5,000 to 7,000 rpm for as much as an hour or more.

Because of the way boat propellers are designed, the engines driving them “need a wide, flat powerband as opposed to a narrow peaky one. For that reason, a fat, even torque curve across the entire operating range is ideal.

A Streetable Boat…

Does that kind of power delivery sound familiar? Yep. It’s exactly what a potent street-performance engine needs. “People think that boat engines are so different from car engines,” says Phillip “Junior” Joyner at United Speed World (USW) in Tampa, Florida. “That’s really not the case. If anything, I’d rather have a marine-style engine in a car,  because of the torque they make on the bottom.”

To get a better picture of what he’s talking about, we dug into a big-block Chevy that Junior and his crew recently put together. Although this big block is a relatively inexpensive build that wasn’t the least bit radical, the final dyno results are eye-opening for anyone who understands the wicked punch that torquey, big-inch engines can offer.

Big Block Beginnings

This big block began life as one of a pair of Mercury-branded 502-ci big-block Chevys powering a rather substantial boat — “a 36- to 40-footer,” says Junior. But like many automotive projects, this engine had bounced from one shop to another after suffering subpar workmanship. “The customer initially came here in 2021 to have us build this engine,” says Junior. “But we told him it would be six months because we were slammed when the Covid shutdown happened. He couldn’t wait that long, so he took it somewhere else. They got it all together, put it in the boat and it blew up again. Then he decided to bring it to us so we could do it correctly.”

When the engine finally came to the USW shop, it didn’t look promising. “He had all the parts, but it came to us in a basket,” says Junior. “Also, the water jackets in the cylinder heads were completely rotted out.”

Unlike car engines, which have a closed cooling system filled with corrosion-resistant antifreeze, many boat engines are cooled by external water drawn from whatever the boat is floating in. Of course, seawater is the worst — the salt it contains can quickly corrode cast iron into oblivion, especially if the cooling system isn’t flushed properly after each use. But even freshwater from lakes is hard on engine components because it lacks the corrosion protection of antifreeze.

Headed In A Better Direction

To replace the corroded original heads, Junior ordered a pair of fully-assembled GM cast-iron replacement heads (PN: 12562920), designed to fit Gen-V and Gen-VI big-block engines. These are rectangular-port heads with 2.18-inch intake valves, 1.88-inch exhaust valves, and 118 cc open combustion chambers. Port runner volume is 325 cc for the intake, and 110 cc for the exhaust. Other than double-checking the valve spring pressure as set up by the factory, Junior and his crew left the heads as-is and moved on to the rest of the components.

We were aiming for about 10.0:1 because we were trying to squeeze more power out of it but still be able to run pump gas. – Junior Joyner, USW

Fortunately, the customer’s four-bolt block (GM PN: 19170540) was still useable and didn’t require major work. After the usual cleaning and prep, the USW crew bored the block .030-inch over. The original forged, 4.00-inch crankshaft was in good shape, so it was left alone. The overbore yields an overall displacement of 509 ci.

Junior selected forged JE Pistons (PN: 212140). With these pistons, the compression ratio is bumped up. “It was 8.75:1 from the factory — pretty weak,” says Junior. “We increased it quite a bit. We were aiming for about 10.0:1 because we were trying to squeeze more power out of it but still be able to run pump gas.”

The original forged-steel connecting rods were resized as needed and fortified with ARP fasteners, but otherwise left alone. Clevite bearings (PNs: CB743HN and MS-829HK) were used for the rods and mains.

When the big block came to USW, it had a COMP Cams hydraulic roller cam (PN: 11-999-3) that spec’d out at .598/.610 lift, with a duration of 285/292 and 231/237 at .050-inch. The lobe separation was 114.5 degrees. The cam had provided strong overall performance with the previous setup, and with a bit of rework, it was in good enough condition for this somewhat budget build. “The cam had just a couple of little marks in it that we didn’t approve of,” says Junior. “So we sent it back to COMP and they cleaned it up.”

“It’s a pretty smooth idler,” adds Junior. “That way the engine doesn’t suck water back up the exhaust.”

The rocker arms are COMP’s Ultra Pro Magnum 1.7:1 roller units (PN: 1620-16). Junior notes that there are two types of Ultra Pro Magnum rockers, but he doesn’t feel it’s worth spending more for the pricier version in this case. “One has a larger trunnion and bearing,” he says. “I generally don’t go that route just because they’re so much more money.” The lifters are stock GM hydraulic rollers (PN: 17120061), teamed with .080-inch-wall pushrods.

Once all the components were gathered up and the machine work was done, the engine was put together using standard assembly methods, with no significant surprises along the way.

Getting The Big Block All Fueled Up

The engine originally came with a proprietary Mercury fuel-injection system, but it was clear that it had to go. “The customer was starting to have issues with the MerCruiser fuel injection,” says Junior. ” Nobody really works on that stuff. You have to take it back to MerCruiser. ” The owner decided to eliminate anything where we have to rely on Mercury and go with Holley components so we can do whatever we need to.’

“The factory MerCruiser stuff had an intake manifold that was somewhere between an old-school tunnel ram and a modern Holley EFI Hi-Ram,” continues Junior. “It was a pretty good-looking manifold, but it just wasn’t cost-effective to break up the Holley stuff and get all the parts individually. So we just got their entire kit.”

The kit they chose was Holley’s HP Multi-Port system (PN: 550-838) with a single-plane manifold and a 2,000 cfm throttle body. Sold as a complete kit, this self-learning system is designed to be a simple bolt-on that requires no initial setup or any additional components other than a fuel pump and injectors. For injectors, Junior chose Holley 42 lb/hr units, (PN: 522-428).

Once the induction system was in place, all that was left was the ignition. For easy setup with the Holley EFI system, Junior chose an MSD distributor (PN: 8366) which is based on a GM HEI unit. “Holley gives you several different options of how you can run your ignition,” he says. “We chose the early 1990s GM distributor. It’s an external-coil HEI, as they like to call it. Because that distributor is already essentially locked out — there’s no advance, and it just has that small module that goes in the bottom. The beauty of doing that is it plugs directly into the EFI and you don’t have to chop or hack anything.”

Proven Power

When it was all together, the moment of truth arrived — dyno time. With the Holley EFI system, there was very little tuning or prep required before making the first dyno run. “I can’t say enough about the Holley self-learning feature,” says Junior. “It just takes off and does its own thing. It’s awesome. I ran it a while to get a temp up and let the EFI start learning. I think we limited the timing to 30 or 32 degrees fully advanced and that was about it — nothing crazy. I set the rev limiter at 6,200 rpm so we don’t have to worry about over-revving if the boat ever comes out of the water.”

Although Junior had a general idea of what this refreshed big block would put out, it’s his strict policy to never predict what an engine will do in testing. “I avoid guessing at all costs,” he says. “Even when people are doing it just for fun. I don’t want to let myself or the customer down. I let the dyno tell the story.”

When the dyno pull was done and the noise settled, the results were pleasing to everyone — 533 horsepower at a relatively low 5,200 rpm. But even more impressive was the torque curve. It was tall, fat, and flat, starting with a potent 520 lb-ft at just 2,900 rpm and steadily rising to a peak of 565 lb-ft at 4,600 rpm.

That kind of output will certainly make for lively performance on the water — especially when you consider that this engine is just one of a matched pair powering the customer’s boat.

And, as we mentioned before, it would make for a whole lot of fun in a street car. The secret is this engine’s mountain of torque that kicks in just off idle, as Junior explains. “People get confused on horsepower versus torque,” he says. “They think, ‘Wow, this thing’s making 600 horsepower.’ Well yeah, but if you have to rev it to 7,000 rpm to get there, how long are you going to do that on the street? You’re not. Almost all of your street driving is from idle to 4,500 rpm.”

“This engine would kill some tires,” he adds.

Best of all, it would be as un-fussy as you can get. With an iron four-bolt-main block, forged internals, and a mild hydraulic-roller valvetrain, this engine would be hard to kill at the relatively low RPM it will run. And the self-learning Holley EFI system does the hard work of extracting good performance from it. “It’s not like a carburetor where you’re changing jets and stuff,” says Junior. “It does its own thing based on the mass airflow sensor and the O2 sensor.”

With the big block ready to go back into the boat and its identical twin on the way in for the same treatment, Junior and the customer are both happy with the results. The only thing he says he’d do differently would be maybe some lighter components, depending on the application. “I’m a huge fan of aluminum heads just to get rid of some weight off the frontend if it was going in a car,” he says. “But for this application, the second engine is going to be exactly the same.”