It’s hard to believe that GM’s LS platform has been around for more than 25 years now. And while this engine has found a new home in just about everything, we’re just now starting to see more and more boats with an LS swap. However, we know that the powerplant is great in classic cars and restomods, but will it also be more advantageous in boats?
This concept led us down an exploratory rabbit hole when we decided to remove a 489 cubic-inch big-block Chevy from a 1996 Baja and LS swap it with an LM4 Gen III engine. But, how would it perform compared to the cast iron counterpart when it was all said and done? Of course, we had to find out the answer.

Our test subject is a 1996 Baja inboard/outboard boat that was equipped with a big-block Chevrolet. Out with the old and in with the new, all-aluminum small-block engine.
LS Swap A Boat
To give you a quick recap, we removed the big-block, which we have done numerous times for different reasons over the years. You can read more about the engine installation here in a previous article that we did. The installation of the LS mock-up engine wasn’t overly complicated. All you need is a flywheel, which we sourced from SPEC clutch and ICT Billet motor mount adapters. Other than that, we had to modify the factory Mercury OEM motor mounts, and the engine dropped right in and lined up perfectly. With the mock-up in place, we turned our attention to building the engine for final installation.
The Engine
The LM4 started life as a 5.3-liter aluminum block, but we had Boostline Performance that the engine and punched it out to a 5.7-liter with all of the right goodies, including ICON pistons, Gen IV rods, Brian Tooley Racing (BTR) Stage I LS3 camshaft, ARP bolts, along with Holley‘s LS oil pan, mid-mount accessory drive, dual plane intake, and a 750 cfm Quick Fuel carburator. If you want more information on the engine combination, we have a story on it as well, named Going Overboard: Building An LS Engine For A Boat.

Our Boostline Performance engine started life as a 5.3-liter LS but was punched out to a 5.7-liters. We opted for a Quick Fuel carburetor and Holley intake manifold. An MSD ignition box handled the spark and timing curve duties.
With the new Boostline Performance engine set in the boat, it was time to figure out the last few remaining challenges. Exhaust, ignition, and plumbing were all at the top of the list.
Exhaust-ing
If you plan to LS swap your boat, aftermarket exhaust systems are available. However, they are super expensive. If you are a boat enthusiast, this will come as no surprise. The acronym B.O.A.T (Break Out Another Thousand) certainly rings true. High-performance LS water jacketed headers from companies like Hardin Marine are commanding mega dollars, ranging up to $5,500, which is hard to swallow when doing, or trying to do, an affordable LS swap.

Finding marine exhaust for the engine was not a simple task. We started with these units that were several hundred dollars and extremely heavy. Later, we found a more accommodating set that worked much better.
Several headers would work if your local lake permits non-water-injected over-the-transom headers. Most jet boats and V-drives are accustomed to these types of exhaust. However, since our Baja is equipped with a sun deck and through-the-transom exhaust ports, we were in a pickle. We had to have water-jacketed manifolds to keep the heat out of the engine bay, and wanted to retain our factory sun deck (engine cover).

We lucked out and found a set of water-jacketed headers off of a Malibu ski boat. With everything buttoned up, it was time to hit the water.
With exhaust on my mind, I turned to Facebook for help with an affordable solution. And wouldn’t you know it, a good friend of mine happened to have a set of headers that would work that came off of an LS engine in a Malibu ski boat. The units are water-jacketed and include bungs for O2 sensors, should we decide to go fuel-injected. The cost, $800, is thousands of dollars less than that of the new counterparts. The only other thing we needed to complete the exhaust was two three-inch pieces of stainless steel tubing and some silicone couplers.
The Final Stretch
The final stage of the build is always the toughest, not because of a difficulty level but because you can see the finish line. We wired in the MSD LS box and connected it to the Performance Distributor Sultans of Spark ignition coils, which is a super simple task. Next, we reached out to Snow Performance for the company’s PTFE hoses and ends to connect the Holley power steering pump to the hydraulic steering cylinder on the outdrive. Finally, we plumbed the water lines and hooked them to the crank-driven sea pump for cooling. With the steering system bled and water lines attached, it was time to fire the boat up for the first time since we removed the big-block.

Performance distributors Sultans of Spark ignition coils can be seen here mounted to a set of ICT Billet brackets.
Firing up a new engine is always a bit stressful because there’s so much that can go wrong. The beauty of a boat is that you can have the engine out in an hour or less if needed. But at this point, we want to go to the lake and not continue working on the Baja. We checked the fluids, connected a water hose to the lower unit, and then hit the ignition switch. The LS roared to life with zero effort and solid oil pressure. The BTR camshaft was chopping harder than a roundhouse kick from Chuck Norris. Finally, it was time to hit the water.
Boating has always been a family affair, and today was no different. My Dad (Bob Havins), son (Chase Havins), cousin (Brent Havins), and I all headed to Lake Arrowhead to put the Baja through its paces. Dad would pilot the Baja with me aboard, while Chase and Brent would act as the support boat and get some footage as well. I backed the boat into the water while Dad hit the ignition switch, and the little LS roared to life. He backed the boat off the trailer, and I parked the truck. It was at this point that I noticed the engine sounded great. I love big-blocks and the sounds they make with big camshafts, but the LS was holding its own. We were getting thumbs up from people on the bank, which made me smile.

The old Baja fired right up and backed off the trailer with ease. The only question now is how it would run across the lake.
As we were idling across the bay, Dad looked at me and asked how we needed to break it in. One of the benefits of an LS is that, unlike conventional flat tappet small- or big-blocks with flat-tappet cams, you don’t need to break it in. We had already fired it up on the water, and everything was good, so I said, “Hammer it.” Dad looked at me and, without hesitation, directed his attention to the boat’s bow while simultaneously hammering down on the throttle.

We purposely left some of the interior out of the boat, making it much easier to work on if we needed to. Fortunately, things went well, and we only had to make a few minor adjustments to the Baja.
The LS swap did not disappoint and jumped right up on the plane effortlessly, much to my surprise. We made several trips up and down the lake, with the only issue being the lack of a tachometer. The boat felt nimble with the new powerplant and didn’t feel down on power, which was a surprise to both Dad and me. With a 21-pitch prop, the BAJA hit 70 mph trimmed out according to GPS. The big-block would push 78 mph with a much more aggressive 26-pitch prop. But, we couldn’t see our RPM since the tachometer decided not to work. Plus, Dad left the other prop at the house; otherwise, we could have made a change just to see what happened.
To LS Swap A Boat Or Not, That Is The Question
Attempting projects that others have not undertaken is often a double-edged sword. Sometimes it works, and sometimes you come up short. We had a lot of people telling us the LS would not be as good as the old Chevy engine, which brings up a couple of questions.
Will the LS get closer to the big-block speed with a prop change?
Absolutely…
Will it be as fast as the 489?
Probably not, but the trade-offs are worth it.

The little LS ripped across the lake with a nice balance of acceleration and speed. It even throws a nice rooster tail when we trimmed out the Mercruiser Bravo One outdrive. This image should be enough for you to want to LS swap a boat. It certainly won’t be the last time we will.
All in all, we’re impressed with the performance of the LS in the Baja. This is not a full-blown race boat and was never meant to be. The boat now has a lighter, more modern powerplant with ridiculous reliability and performance. It gets on the plane super fast thanks to a 220-pound weight reduction in the stern and will likely use less fuel than the 489 cubic-inch engine with its 960 cfm Holley double pumper. Plus, we can now add fuel injection with ease. Who knows, we might even add a supercharger at some point to see what kind of speeds we can get. I can guarantee that the little 5.7 cubic-inch LS with a ProCharger will hurt a lot of big-block owners’ feelings on any lake day. And in our opinion, the only thing better than an LS swapped car is to LS swap a boat.