If you’re the kind of gearhead who gets fired up by oddball combos that actually work, this one’s for you. Boostline Performance’s latest experiment involves combining a stock Gen V L83 cylinder with a stock Gen IV 5.3 liter aluminum block to make an LS/LT hybrid, and then adding in a big turbo to punch out over a thousand horsepower. It was a science experiment to see just how much a sea of modern LT cylinder heads can benefit a more traditional LS build, and the guys at Boostline made it work even though LT heads were never originally intended to work on an LS block, and we love it.

Let’s dive into how this LS/LT hybrid project came to life, what it took to make it work, and even why it may not catch on even with its thousand-horsepower potential.

Engine builder Tommy Keeter has welded up the direct injection bosses and reworked the combustion chambers to be more friendly for port fuel injection, so this doesn’t look like a fifth-generation (LT1) cylinder head anymore, but this is an OEM L83 cylinder head sourced off a junked truck.

Potential of the Gen V L83

The idea came about when engine builder Tommy Keeter at KPE Racing and Boostline Performance (the two companies work hand-in-hand to machine, assemble, and tune engines) found himself staring at a set of take-off L83 heads. These Gen V heads were originally designed for direct-injected truck engines starting in 2014. Keeter immediately saw the serious flow potential for these heads and wondered how much better it would be than the stock heads available for Gen IV LS engines.

“Especially compared to cathedral port LS heads, the L83 has a much better intake port design,” he explains. “With the canted valve layout, you can fit a bigger valve in a small bore. That means more flow, more efficiency — and potentially, a lot more power.”

The block is an aluminum Gen IV 5.3-liter engine (Vortec 5300) that probably came out of a GMC Envoy or Chevy Trailblazer. The cylinder bores were opened up 0.020 inch from stock to 3.800 inches, but the stroke was kept stock (3.622 inches).

Instead of bolting them onto an LT short-block as GM engineers originally intended, Keeter went in the other direction. He decided to retrofit the Gen V heads onto a Gen IV 5.3L LS block to make an LS/LT hybrid. Specifically, an aluminum OEM block sourced from the junkyard. The aluminum 5.3L blocks are rarer than the iron blocks commonly found in pickup trucks, but can be found in some SUVs and pickups. The idea was to use an OEM block and heads to test the power potential in a budget-conscious platform that’s still plentiful in junkyards.

The team bored the aluminum 5.3L out 0.020 over to get to 3.800 inches and added a stock-stroke crank from Manley Performance. In particlar, a forged Turbo Tuff 4340 steel unit, made from a non-twist forging. Available in both 24X and 58X variants, Boostline opted for the Gen IV 58X variant. Keeter says he’d trust the stock crank up to 800 or so horsepower, but since they were shooting for way beyond that mark, they went for a Manley forged unit.

While the stock crank is strong, there was no reason to take chances. A Manley forged Turbo Tuff 4340 crankshaft with the stock stroke was used. This will alleviate any concerns of making well over 1,000 horsepower to the tire.

“Durability was key,” Keeter says. “We kept the stock stroke, but the aftermarket crank and rods were necessary because we had a power target north of 1,000 horsepower. A factory crank and powdered metal rods just wouldn’t have held up.” Once again, Boostline turned to Manley for a set of their H-Tuff forged H-beam connecting rods. With the standard aftermarket LS dimensions (6.125 inches long, 2.100 big end, .927 wrist pin), the H-Tuff rod was designed with this application in mind.

On the end of those rods are a set of Manley’s Platinum Series 3.800-inch diameter flat top pistons. An off-the-shelf LS part, they are forged from 2618 aluminum, and have 2.0cc valve reliefs. The pistons feature a 1.2mm, 1.2mm, 3.0mm ring pack from Manley as well, with a steel top ring. The wrist pin is a thick-wall pin made from Manley’s Turbo Tuff material, to handle whatever gets thrown at it.

The rotating assembly rides on King Engine Bearings XPC-coated tri-metal bearings. The pMaxKote polymer coating on the XP-series main and rod bearings helps not only reduce friction, but also helps prevent oil cavitation and resist damage and seizing in the case of temporary oil starvation.

For the LS/LT hybrid break-in and baseline testing, the engine was run in naturally aspirated form on the dyno. But, even then, it made an eyebrow-raising 616.9 horsepower at 7,400 rpm with 10.5:1 compression. That’s serious grunt for a naturally aspirated 5.3L, which proved that Keeter was onto something with the flow potential of the L83 castings.

Like the crankshaft, the connecting rods are made from forged H-Tuff 4340 steel. The 6.125-inch H-beam design will handle everything this combo can throw at it. Hanging off the rods are a set of Manley’s flat-top Platinum series 2618 pistons and Turbo Tuff wrist pins.

Making the Heads Fit

Getting Gen V heads to sit happily on an LS block isn’t exactly plug-and-play, but it’s not impossible, either.

First, the heads for the LS/LT hybrid went off to Frankenstein Engine Dynamics in Weatherford, Texas, for CNC porting. Keeter didn’t opt for anything custom or even extreme. “They already had a port program for the L83,” he explains. “We just had them run it.”

The results were very impressive. GM engineers already did a great job with the L83’s intake port design, but after Frankenstein did its thing with the CNC port work, the 260cc intakes came back flowing 300 cfm at 0.700 valve lift. The exhaust ports weren’t too shabby either, at 202 cfm at the same valve lift.

Keeter says one of the big advantages of the Gen V L83 cylinder head is the canted valve angles (2.61 degrees for the intakes and 2.38 degrees for the exhausts) that point the valves toward the center of the bores. This not only improves airflow on its own, but it also allows for larger valves to be installed. For this build Keeter used off-the-shelf Manley stainless valves sized at 1.930 inches for the intakes and 1.560 for the exhausts.

When he got the freshly ported heads back, Keeter welded up the bosses for the direct injection system that provides fuel to the combustion chambers. That was done not just to block them off, but also to allow him to reshape the combustion chambers specifically for old-school port injection. “It let me increase the quench pad and reshape the chamber for better burn,” he says.

With new seats and guides installed, the heads were fitted with Manley stainless steel 1.930-inch intake and 1.580-inch exhaust valves, with plenty of room to increase valve size down the road, since the Gen V head can take advantage of a larger valve than traditional cathedral-port heads can. The ability to accept larger valves than the Gen III/IV LS family with relatively small 3.800-inch bores is made possible by the canted valve layout of the Gen V architecture. “Thanks to the valve angles of the Gen V heads, you can fit larger valves than you can in a cathedral port head with a bore this small. We opted for the off-the-shelf Manley valve, but can go bigger down the road,” Keeter notes.

The stock rocker arms were retained but given a trunion upgrade for better durability.

Physically bolting an LT head to an LS block isn’t too difficult because they share the same 4.400-inch bore spacing so everything basically lines up, but that doesn’t mean there aren’t issues. The most obvious thing is you will need to redrill the deck of the heads for the LS dowel locations. The L83 heads also swap the intake and exhaust valve positions compared to traditional LS heads, so a standard LS cam won’t work. The team needed a cam ground specifically for this LS/LT hybrid reversed valve layout.

Thankfully, Brian Tooley Racing had an off-the-shelf solution. They have swap cams specifically for putting LT heads on an LS block. This one is ground with 230 degrees of duration at 0.050 inch of tappet lift with a 113-degree lobe separation angle and 0.644 inch of valve lift for both the intakes and exhausts. Obviously, this isn’t a full-bore race cam; it’s designed to be a reliable performer for the street and mild drag racing.

“That was the only shelf cam we could find at the time,” Keeter says. “It worked, but there’s so much more potential in this combo with a custom grind. The problem is core availability. Getting the right blanks to grind custom cams is tough.”

The cam is an off-the-shelf unit designed by Brian Tooley Racing to work with LT cylinder heads on LS blocks. This is not an extreme grind by any means, with 230 degrees of duration and 0.644 inch of valve lift.

To keep the heads of the LS/LT hybrid clamped to the block when the boost pressure reaches a ridiculous 30 pounds, the block was drilled and tapped to accept larger half-inch head studs. This is a relatively common — and very effective — upgrade when preparing for big-power LS builds.

For rockers, the team stuck with factory Gen V pieces upgraded with a trunnion kit. Valve covers are LS-style pieces mounted with ICT Billet adapters that allow the center-bolt valve covers to mount to the perimeter-bolt heads. The reason? Keeter says he just likes the look of smooth LS valve covers better. Plus, they were able to use a nice set of coil mounts as well. The coils themselves are super-affordable LS3-style replacements.

And what about headers for the LS/LT hybrid? “You can run off-the-shelf LS headers with this setup,” Keeter says. “No need to reinvent the wheel there.”

One tip worth noting is that if you do swap LT heads on your LS block, there is no guarantee your accessory drive system will bolt up because the Gen V heads didn’t keep the same accessory bolt locations. Also, notice the ICT Billet adaptor used, which will allow a set of center-bolt LS valve covers to bolt up. We agree with Keeter that LT valve covers with their weird bulges just don’t look as good as a pair of sanitary LS covers.

On the Dyno

For dyno testing of this LS/LT hybrid, Boostline used a Holley Hi-Ram intake and 100 lb/hr Holley injectors. That’s larger than necessary but bigger injectors are necessary when burning methanol, and that’s just what they had on hand. But the results in naturally aspirated form were very impressive. They saw peaks of 474.7 lb-ft of torque at 6,300 rpm and a very healthy 616.9 horsepower at 7,400. And remember, even with the extra bore, this engine still hasn’t reached 5.4 liters of displacement!

The turbo setup was done with the engine installed in a car and on the chassis dyno, so these numbers are at the rear wheel, so chassis and driveline horsepower losses must be factored in. The turbo is an S400 frame, 94mm unit from Forced Inductions, and the intake manifold has been replaced with a casting from Brian Tooley Racing.

Still, once boost was added to the mix, things got pretty rowdy. We’re including the LS/LT hybrid dyno graph with this story, and you can see by the wavy torque line starting just before 140 mph, the torque converter was likely running into issues. Still, they saw 1,092 horsepower and 888 lb-ft of torque. And remember that’s now at the rear wheels and not the crankshaft!

Boostline did a few baseline runs on the engine dyno in a naturally aspirated setup using a Holley Hi-Ram intake manifold. Without any help from a turbo this little engine surprisingly pounded out peaks of 474.7 lb-ft of torque at 6,300 rpm and 616.9 horsepower at 7,400 rpm. Talk about punching above your weight class!

Based on the math and Keeter’s conservative estimates, the setup is capable of well over 1,200 horsepower at the wheels. That’s wild for a block you are most likely to find in a 300-horsepower GMC Envoy. Even more impressive is how the engine handled the pressure. Thanks to the smart parts selection, the build survived without drama. “You could absolutely daily drive this motor,” Keeter says. “It’s not some delicate race piece.”

LS/LT Hybrid Lessons Learned

So what’s the takeaway? Keeter believes the Gen V L83 head is worth a solid 125 horsepower over a ported cathedral-port LS head, even on a small-displacement engine.

“I built this as close to an apples-to-apples comparison with a standard Gen IV build as possible,” he says. “Same short block, same displacement, just better heads. The difference was night and day.”

The turbo setup was tested with the engine in a Fox-body Mustang with a 94mm Forced Inductions turbo and a Brian Tooley intake manifold. Burning methanol with 30 pounds of boost, this 5.3L engine managed to put nearly 1,100 horsepower to the rear wheels.

If someone wanted to do this LS/LT hybrid swap at home, Boostline now offers a top-end package that includes the L83 heads, cam, and intake to bolt onto any LS short-block. All the machine work has been completed before it is sent out, so for the customer, it’s a simple bolt-on project. Or, you can buy the entire long-block as a turn-key unit, ready for boost.

While the full potential of the combo wasn’t shown in the below dyno graph, it doesn’t diminish what Boostline accomplished. They proved that with the right cylinder head and a little ingenuity, even a humble 5.3L can make four-digit power. And it might be even more valuable in naturally aspirated racing classes where power is tougher to come by.

“I just saw the head lying on the bench one day and thought, ‘That could be cool,’” Keeter says. “And then we did it.”

The build might have started as a science experiment, but it ended with serious implications for LS tuners everywhere. If you’ve got a 5.3L sitting around, a set of L83 heads might be the secret weapon you didn’t know you needed.

Keeter believes this engine is capable of more, but on the chassis dyno the combo quickly revealed that the Mustang’s torque converter wasn’t up to the task. But the car owner decided to go in a different direction before we were able to go back to the dyno and find out more.