Building A GM Performance LSX Engine- The New Rodent

The terms Rat and Mouse have been used by enthusiasts to refer to Chevy's big- and small-block engines for decades. But, there is growing use of GM's LS engine family in the rodding world, and that brings with it a conundrum: What's the appropriate rodent reference?

As we'll illustrate in this story, the LS engine is capable of producing big-block-style displacement and power in a small-block-sized package. It seems like something larger than a mouse yet smaller than a rat fits the bill, so how about "squirrel" or "marmot"? Nah, doesn't fit. "Nutria?" No, that's no good, either.

Nickname notwithstanding, the LS platform has much to offer for street rod builders. It's a compact, minimalist design that is relatively easy to package, and, with the variety of factory-style aluminum heads, composite intakes, and the like, it's not an anchor over the front wheels, either. Oh, and it has tremendous performance potential.

Last year, General Motors' Performance Parts introduced the LSX cylinder block, which offered a cake-and-eat-it-too proposition-entry-level pricing with competition-level durability. Pro Stock drag racing legend Warren Johnson had a hand in its design, which is targeted at supporting as much as 2,000 hp and very high boost in racing applications. So, a 500hp street engine ought to be a cinch, right?

That's what we hoped to prove with this story. We turned to the LS experts at Thomson Automotive, near Detroit in Redford, Michigan, and GM Performance Parts (GMPP), who were all too eager to participate in a buildup with the new LSX block. Truth be known, GMPP has turned to Thomson Automotive on occasion to help with development work on high-performance and racing variants of the LS engine family, and Performance Parts sent its own Thomas Bates to help with the buildup. We were in good hands with the project.

LSX FoundationThe LSX cylinder block's sub-$2,000 retail price makes it a very affordable foundation for a high-performance engine, and the LSX is made of cast iron rather than aluminum, unlike many other LS production cylinder blocks. Sure, that's inherently heavier than aluminum, but that shouldn't be of great concern, as other components, such as the heads and intake manifold, are lightweight aluminum parts, themselves. Bonus points for the iron casting include exceptional strength and affordability.

Our project engine combined the LSX block-machined to 427 ci-and a raft of LS7 parts from the GMPP catalog. Indeed, almost every component used on our project engine is available from GMPP or GM Parts, including the racing-derived, high-flow aluminum heads, intake manifold, camshaft, and more. The 427-inch displacement enabled the use of GM's newly available LS7 controller, which allowed the engine to start up on the dyno with no separate tuning. The controller was simply plugged into the engine's wiring harness, and the engine was idling smoothly from the first push of the starter button.

While we were hoping for a nice, round 500hp result, the engine combination produced 550 hp and 511 lb-ft of torque on Thomson's engine dyno. This engine combination responds well and is easily converted to carburetion (see sidebar for more information). Most surprising, however, was the ease at which the engine got up and running. By using the GMPP controller, which was pre-programmed with the maps to sustain the 427-inch engine, there was little to do between start-up and the dyno pulls besides let the engine oil come up to temperature, and no timing adjustments, fuel adjustments, or other tuning were required. It was almost too easy.

Driveability with the production-style EFI system is as smooth and linear as you'd expect from a combination using mostly regular-production components. The idle quality and throttle response are excellent thanks to the LS7 controller, and there are no "dead" spots in the power curve, either; the engine simply keeps building power past 6,000 rpm.

Installation And Transmission ConsiderationsWith 550 hp on tap and the torque output of a healthy big-block from the bygone musclecar days, there are numerous implications for installing this engine in a street rod, be it a lightweight, fenderless Deuce or a full-fendered sedan from the late 1940s.

Fitting the relatively compact engine on a chassis isn't so hard, but some adaptation will be required to use an electronic throttle. The alternative is replacing the electronic version with a conventional, cable-operated throttle.

Also, it will be necessary to beef up the drivetrain in order to efficiently absorb all that torque, so that means backing the engine with a stout transmission. A properly built Turbo 400 will do the trick, but we like the idea of GM's late-model 4L60 and 4L80 electronically controlled overdrive transmissions. The 4L80 is the reincarnation of the original Turbo 400, while the 4L60 is based on the lighter-duty 700-R4.

The advantages of the late-model transmissions include the fuel economy benefits of an overdrive gear and the modulated performance of electronically controlled torque converter lockup. Like the engine, these transmissions require a controller, and, as luck would have it, they're also available from GM Performance Parts. An adapter will be needed to convert the transmission's electronic signal to be used by a mechanical speedometer, and TCI Automotive and other aftermarket manufacturers offer such speedo control units.

The engine's fuel-injected driveability and the overdrive transmission's economy make for a great long-haul, worry-free combination. There are no timing or carb-jetting adjustments to worry about, especially on long tours and varied altitudes, so the crispness of the performance is outstanding.

Ballpark Cost If you're wondering about a ballpark price for duplicating this LSX engine, it is roughly $12,000 in the form you see on our pages. And, unless you have a machine shop in your backyard, you'll need to tack on a few hundred bucks for the finish work on the block, since GMPP delivers the LSX block rough-bored.

We'd likely spend a few more bucks dressing up the engine, compared to the as-built condition seen in this story. First, we'd find some snazzier valve covers and a coil pack relocation kit. The valve covers are available from a variety of manufacturers, but we're keen on MSD's coil-relocation brackets, which move the coils to the rear of the engine for a more traditional look. We've also seen the plastic, factory intake manifold painted, and that would be next on our to-do list; painting it to match the car's body color or the same Chevy Orange as the cylinder block are a couple of easy ideas.

Personalization and dress-up costs notwithstanding, the word "value" is not out of place when describing it-especially when considering its performance. And, speaking of descriptions, we've gotten to the end of the story without a suitable rodent reference. Chipmunk? Guinea Pig? No ... too cute. We can't envision a dark shades-wearing rodder rumbling up at cruise night and answering, "It's a blown, alcohol-swilling Hamster motor" when asked what's under the hood.

Maybe we should just call it the LSX and leave it at that.

Dyno Details
Here are the results of the LSX 427 engine on the SuperFlow dynamometer (STP correction) performed at Thomson Automotive. Peak numbers are in bold.

Converting To CarburetionOur project engine used the fuel-injection setup from the LS7 engine, but it is very easy to build the engine with a carburetor. This allows for a more traditional underhood appearance, while also easing the installation procedure in a street rod. This is because a traditional throttle cable and accelerator pedal can be used, and no provisions for the LS7's high-pressure fuel system are required-although a remote-mounted fuel pump is needed, as there is no provision for a block-mounted pump.

Intake manifolds to fit the different styles of LS cylinder heads, round port and rectangular port, are readily available in the aftermarket and from GM Performance Parts. In fact, GMPP offers intakes for the LS7 heads in both a conventional single-plane style and a version with cast-in bosses for throttle-body-type EFI or nitrous.

In addition to the intake and carb, converting an LS engine to carburetion also requires an ignition control box, because there is no distributor used with the engine. The good thing about that is timing is instantly set when the engine is turned on once the ignition coils are mated with the spark box, so your timing light is not needed.

The final consideration for carbing an LS engine is performance. Careful tuning with the correctly selected intake and carb can return greater horsepower than a comparable fuel-injection system.

The starting point for the project was the LSX iron cylinder block (PN 19166454), which can be had in most places for less than $2,000. It is based on the Gen IV LS engine design but with provisions for six-bolt cylinder heads, and the Chevy Orange paint is standard, too.	The LSX block is delivered in semi-finished form, meaning it must visit the local machine shop for finish work before the engine buildup starts. Ours was taken to Thomson Automotive, where the bores were finished to 4.125 inches with torque plates clamped to the decks.	When it came time to start assembling the engine, the wrench-turning was handled by GM Performance Parts' Thomas Bates, who started by inserting the camshaft. We used an LS7 cam (PN 12571251), which has 0.591-inch lift on both sides and 211/230 duration specs.
An LS7 forged steel crankshaft (PN 12568820) came next. It delivers a 4.000-inch stroke, which combined with the 4.125-inch bores, gives the engine its 427ci displacement.	One of the assembly details new LS engine-builders will have to get used to is torque-to-yield fasteners. These are one-time-use bolts that, in addition to a traditional torque wrench, also require a torque-angle meter to achieve the correct final-torque spec.	Look closely at the digital readout on the Snap-on torque wrench and you'll see the 18-degree setting that denotes the torque yield used to securely fasten the gerotor-type oil pump (PN 17801830) to the front of the cylinder block.
One of the toughest jobs of an LS engine assembly is installing the damper (PN 12576652), which is a super-tight fit. We used a tool designed by Thomson Automotive. GM also has a tool for the procedure.	Next up was the assembly of the rods and pistons. We used the cast LS7 pistons (PN 89018171) and the exotic, super-lightweight forged titanium LS7 rods (PN 12586258).	With the rods and pistons assembled, they were ceremoniously dropped into the cylinder block.
ike so many other fasteners on the engine, the rod bolts are torque-to-yield. Bates snugged them down and checked the clearance between them all.	With the rods secured, the windage tray was set in place, but we didn't simply torque it down and move on to the next assembly step.	The rod ends will likely interfere with the windage tray on LS engines with more than a 4.000-inch-or-greater stroke. Bates checked this by carefully rotating the crankshaft and noting whether the rods hit the tray, which they did.
The solution to the rod/windage tray interference problem was adding washers to the attachment studs to serve as spacers. It was all that was necessary to provide adequate clearance.	The oil pump pick-up was next. It is important to get some oil into the pick-up at this stage of the build, because the engine can't be primed via a distributor hole like classic small- and big-block engines.	We installed an F-car oil pan (PN 12558762), since it is the most "universal" of the various LS-engine production pans. In other words, it's the one that will retro-fit into an older car with the least amount of hassle.
With the short-block assembled, the heads came next. Here, Bates dropped in one of the LS7 aluminum heads.	The LS7 heads are masters of airflow, CNC-ported at the factory and flow approximately 350 cfm through 270cc intake ports. The intake valves are titanium and are 2.20 inches in diameter, while the 1.61-inch exhaust valves are sodium-filled. They're all held at a 12-degree angle.	We used GM's LS-series lifter kit (PN 12499225), which includes 16 hydraulic roller lifters.
The lifters on all other LS engines can be dropped in lifter guides that install two at a time. The provisions for extra head bolts on the LSX block, however, make that impossible; so, they're carefully inserted one at a time and held in place with LSX-specific guides that come with the cylinder block.	Offset intake valves are one of the unique features of the LS7 heads, necessitating correspondingly offset rocker arms, one of which is shown being installed here. Both the intake and exhaust ratios are 1.8:1.	Bates took credit for pushing through the LS7 intake assembly (PN 12568976), coming pre-installed with the throttle body, fuel injectors, and fuel rails. It makes topping off the engine a much easier, quicker job.
Surprisingly, the LS7 coil packs don't come as an assembly, even though that's how they installed on the engine. The individual coils and mounting bracket must be purchased separately, and they attach to factory-style valve covers, but aftermarket kits permit remote mounting for a cleaner, more traditional look.	Assembly of the engine was wrapped up with the installation of the water pump (PN 89018053).	One of the key enablers to making our engine project a turnkey affair was the use of GMPP's new, stand-alone LS7 controller (PN 19166567). It allowed the engine's wiring harness to simply plug into this ECM, and-bingo-the engine was timed and tuned from the get-go. It could not have been easier to get the engine up and running.
Here's our finished LSX 427 engine. The Chevy Orange block color is how it comes from the factory, which looks great with the contrast from the aluminum heads. We'd paint that plain-black intake quickly, though.	Our LSX 427 kicked out an impressive 550 hp and 511 lb-ft of torque (STP correction) on the SuperFlow dyno at Thomson Automotive. This was with no tuning or external adjustment; the LS7 controller was simply plugged in, and the engine was run on its pre-programmed tune. We were thrilled with the results, as the performance was achieved with off-the-shelf catalog parts and a by-the-numbers assembly.	This single-plane manifold (PN 25534416) from GM Performance Parts fits LS7-type heads, and note the bosses cast in at the base of the intake runners, which can be plumbed for EFI or nitrous.
With careful tuning, a carbureted LS engine can make more power than a fuel-injected version. The LS7 heads, for example, offer exceptional flow, so a high-cfm carb is a must.	Carbureted combinations require an ignition control box to direct the ignition coils, because LS engines don't use a distributor.