So you’re thinking about rebuilding your Gen III/IV General Motors small-block? Congratulations—an engine rebuild can be a very rewarding task. Right now, the prospect of enjoying a freshened engine probably excites you most; but we hope that with the help of this book, you’ll enjoy the actual rebuilding process as well!
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Before beginning the planning stages of your project, there are a few questions you’ll need to ponder. Not the least of which is, what is a Gen III/IV? And how do you know that the engine you’re looking to rebuild really is a Gen III or IV GM engine—or even a small-block at all? After all, the pioneering Gen III engine architecture has only been in existence since model year 1997—and its close cousin the Gen IV is even younger. While hundreds of thousands of Gen III and IV engines have been installed into production GM cars and trucks since the late 1990s, production year overlap with other GM V-8 engines means that while it’s likely that your late-model’s mill falls within the family, there is also a chance that it does not.
This chapter shows you what a Gen III/IV is and tells you a little about what makes it such an excellent engine design. We also provide you with information that will help you determine whether you’ve got one. But first, to put things in perspective, let us discuss how this engine architecture evolved from GM engines of years past.
The Small-Block Family Tree
In the decades leading to the 21st century, Detroit produced an incredible number of V-8 engines. While each manufacturer had experimented with different valvetrain configurations on its V-8s from time to time, by far the most common style was the so-called pushrod V-8. This type of engine, whose valves are actuated by a cam encased in the engine block, was originally known simply as an overhead valve (OHV) V-8. The success of this type of engine was such that it became a staple for most American-made automobiles.
Within General Motors, pushrod V-8s were made in a staggering variety. Numerous versions of Pontiac, Oldsmobile, Buick, Chevrolet, GMC, and Cadillac engines were installed in many cars and trucks, with a few engine types even overlapping into GM vehicles outside their respective division. Each engine design had its own particular advantages and quirks, but over time (and in the interest of cost cutting and cross-compatibility) only those V-8s GM considered the best of the best were to continue on in production. In the final decades of the 20th century, many V-8 engine lines were discontinued; and by the early 1990s, this left only small- and big-block Chevrolets as the pushrod gasoline V-8 engines used in all GM cars and trucks.
It was the small-block variant of the Chevrolet engine that would eventually form the inspiration for the Gen III. This, then, begs the question: “What is a small-block Chevy?”
One of the most successful and versatile engines of all time, the original small-block debuted in 1955 as the Turbo-Fire—Chevrolet’s first OHV V-8. Tasked with the development of a powerful upgrade for the “Stovebolt” six-cylinder, a Chevrolet engineering team (led by none other than the legendary Ed Cole) came up with this engine’s compact, easy-to-manufacture design. Features like 4.4-inch bore spacing, a single-piece intake manifold, and an internal lubrication system contributed to a lightweight package, while wedge-shaped combustion chambers and low-mass, stamped-steel rocker arms helped yield a broad performance curve and high RPM potential. Powering everything from Corvettes to pickups, the small-block quickly earned a reputation for power and durability.
While in the years following it would spawn many engine displacements and undergo several design finesses (like changes in bearing sizes and a switch to a one-piece rear main seal), the small-block Chevrolet still kept to the same basic architecture as the original 265-ci version. The versatile small-block saw it all: from the muscle car wars of the late 1960s, to the suffering of the 1970s gas crisis, to the eventual use of electronic fuel injection beginning in the 1980s. The engine was so successful that it eventually was used almost without exception throughout all GM divisions, and since it represented the first generation of what would eventually be the corporate GM pushrod V-8, it is now best to refer to the beloved original small-block Chevy simply as the Gen I.
Confined to only light truck and van use in its last decade or so, the final production-vehicle-bound Gen I came off the assembly line in 2002. This marked an incredible run of nearly 50 years—making this engine one of the most successful of all time for any company. As a testament to its enduring legacy, it continues to be used in some marine applications and heavy-duty trucks—and high-performance, Gen I-based engines are still available to this day from GM Performance Parts.
Despite several changes to the original small-block Chevrolet engine design over the years—some fairly substantial—GM never saw fit to come up with a new “generation” designation for its popular pushrod V-8. This all changed when the Gen II version was released in the early 1990s. This revised small-block is perhaps better known by the name of its most popular variant, the LT1.
Introduced in the 1992 Corvette, this short-lived engine family’s most defining feature was its reverse-flow cooling system. Sending coolant to the cylinder heads before the engine block was said to enable a higher compression ratio and improved efficiency. Also, unlike the rear-mounted distributor of the original small-block, the Gen II used a front-mounted unit known as the Opti-Spark (a misnomer if there ever was one—this design has proven itself to be exceptionally unreliable). Other slight changes to the engine block, cylinder heads, and other components meant that very few engine parts were actually shared with the Gen I.
The LT1’s name was a throwback to a high-power variant of the original small-block used in the early 1970s called the LT-1 (note the hyphenation difference). Sporting the same 350 ci as its namesake, the LT1’s high-performance car use was confined to the 1992–1996 Chevrolet Corvette and V-8 versions of the 1993–1997 F-body (i.e., Chevrolet Camaro and Pontiac Fire-bird). Aside from the L99, the only other engine in the Gen II family was the LT4. Simply a higher-output version of the LT1, it was produced primarily in 1996 for manual-transmission-equipped Corvettes, with a few more being thrown into SLP-modified F-bodies available through dealerships in 1997.
Gen II small-blocks were never used in any GM trucks, and further indicative of this engine family’s unique nature, few of the design changes it incorporated would be used subsequently in future small-block generations. It also fell out of production at a time when versions of the Gen I continued to be built. In short, the Gen II is a unique animal, and since it was not a ground-up redesign, it’s perhaps best thought of as a special version of the original small-block and not as a completely separate engine. Yet, we’ll call it a Gen II—if only because GM did.
The Gen III Era Begins
By the mid 1990s, 40 years of small-blocks had earned Chevrolet and GM a reputation for building some of the most versatile and durable V-8s ever made. But for all the variations within the first small-block generation (and even with a few Gen II motors thrown in), the same 1950s-era architecture had largely been retained. New corporate and federal guidelines for performance, manufacturability, durability, and emissions were looming on the horizon—it was time for a change.
A clean-slate design, the Gen III engine family would be the first truly new small-block since the original 1955 version, and its introduction marks the one true delineation between small-blocks of years past and those of today. The first Gen III engine was a 5.7L version introduced in the newly redesigned 1997 Corvette. Referred to as RPO LS1, this 346-ci wonder mill gained an enormous high-performance following almost immediately—leading enthusiasts to refer to all Gen III (and later, Gen IV) engines as simply “LS1s” or the “LS family.” Though confined initially to use in sports cars, the new engine architecture quickly spread across the full gamut of GM cars, light trucks, and SUVs sold in the U.S., completely replacing older small-block V-8s within only a few years of its introduction. This is to say nothing of the extensive use of these engines in GM vehicles sold abroad (Australia’s Holden nameplate and the U.K.’s Vaux-hall brand are just two of many), proving that this engine family’s appeal and effectiveness is definitely not limited to North America!
A detailed analysis of the ground-up design of the Gen III would fill volumes, and even a brief dabble in the literature on this engine inspires awe. We’re not here to duplicate that information, but a complete gloss-over of the glory of Gen III engineering would be an injustice. That said, below are a few of the more major design advances GM incorporated into its new-generation small-block (we’ll also go into more detail on many of them later in the book where they become relevant).
Aluminum engine block
The Gen III marked the first time an aluminum block was used by GM for a mass production pushrod V-8. Aluminum’s main advantage is that it allows an engine block to weigh up to 50% less than a similar block cast from iron. But not all Gen III engine blocks were made of aluminum; while it’s GM’s material of choice for engines destined for passenger cars, trucks sometimes ended up with iron blocks (see our Gen III/IV Engine RPO Table for this information).
Cross-bolted main bearing caps
Most Gen I and II engines had only two bolts securing each main bearing cap to the block, while some higher-performance engines had four. By virtue of using a deep-skirt design for the Gen III engine block, GM was able to not only use 4 bolts holding each of the 5 caps to the block, but incorporate an additional 2 holding each from either side.
Revised firing order
The Gen III did away with the familiar 1-8-4-3-6-5-7-2 firing order of previous small-block engines. The new firing order (1-8-7-2-6-5-4-3) reduced crank arm stresses, quelled vibration, and improved main bearing performance.
Cathedral-port cylinder head
The radically different-looking cylinder head that debuted on the Gen III LS1 was designed in the interest of meeting many needs. The most striking feature is that its ports are tall and narrow. Shaped partially by the desire to target the fuel injectors at a specific location on the intake valve, this profile was also simply mandated by the narrow spacing afforded between the head bolts and pushrods. The ports are also replicated, so they’re identical for each cylinder.
Revised valve angle
In the interest of engine durability, ease of manufacture, and other considerations, the Gen III retained the inline valve setup of previous small-blocks. However, its valve angle was changed from the traditional 23 degrees to 15 degrees from vertical. Among other things, this specification streamlines the transition from the exhaust port floor to the valve seat and creates a shallower combustion chamber. All of this translates to improved engine efficiency and output. This valve angle would be reduced even further on some Gen IV engines.
Even though the Gen III retains a pushrod-actuated OHV design, all components were maximized for high stiffness and low moving mass. The Gen III also incorporates a so-called net build scenario for valve lash—this pretty much means that the valvetrain is nonadjustable. GM saw this approach as more robust, in part because with rocker assemblies rigidly bolted to the cylinder heads, valve lash would be less likely to change. Also, the new rocker arm design is significantly different from the traditional stud-mount units used in previous small-blocks. Their cast, rollerized design allows for better stiffness and less rotational inertia.
Composite intake manifold
Gen I small-blocks used various types of intake materials, and Gen II engines used aluminum exclusively. But for the Gen III project, GM invested in the development of thermoplastic intake manifolds. This was thought worthwhile thanks to lighter weight (the LS1 intake was less than half the weight of the LT1, a savings of over nine pounds), not to mention the new material’s heat insulating characteristics helping to yield a cooler intake charge and reduced temperatures of fuel running through the fuel rails. In addition to the new material composition, the manifold no longer sealed the lifter area—a significant source of leaks over the years. Instead, a separate valley cover resulted in the manifold being totally isolated from the inside of the engine.
In order to deliver spark ignition, the Gen I used a rear-mounted distributor driven by a gear on the back of the cam, while the Gen II had the aforementioned front-mounted distributor, which was driven off a pin on the front of the cam. The Gen III has absolutely no distributor provisions, instead relying on eight individual computer-controlled coils to deliver spark. A crankshaft position sensor mounted at the rear of the block reads an encoded sensing ring (a.k.a. reluctor ring) located at the rear of the number eight counterweight—this is near the lowest deflection point in the crank. A separate camshaft position sensor serves to indicate which half of the firing sequence the engine is in. Simply put, this coil-near-plug system reduces losses inherent in a mechanical switching distributor and through long secondary leads. It’s far more precise, and GM claims it resulted in a net ignition energy increase of 50 percent.
New sealing technology
Past small-blocks relied on a fairly wide variety of gasket material and RTV silicone to seal metal parts of the engine together, and they often had to adhere to curved surfaces and join parts that met at an angle. To help reduce the risk of leaks, the Gen III was designed with extensive use of single-plane sealing surfaces, and utilizes so-called controlled compression aluminum carrier gaskets that are a hybrid of silicone and aluminum.
Revised oil pump location
Gen I and II oil pumps hung on the rear main bearing cap and were driven off of the back of the camshaft. But for the Gen III, the oil pump is a gerotor design that sits on and drives off of the front of the crankshaft. This change alone reduced the block length by well over an inch compared to the previous-generation small-block, and it also allowed for the use of a shallower oil pan, resulting in more favorable engine packaging options.
With these and other improvements all working in conjunction, the Gen III took the automotive scene by storm. After winning accolade upon accolade from the press and industry alike, it quickly became the standard pushrod V- 8 engine by which all others would be judged. The 5.7L LS1 spread to the Camaro and Firebird in 1998, and in 1999, GM began introducing truck variants in 4.8L, 5.3L, and 6.0L displacements. And as if the muscle provided by the LS1 weren’t enough, GM was quick to up the ante with its 2001 model year release of the LS6, an engine whose output greatly eclipsed that of the LS1 thanks to improvements like a higher-lift cam and revised cylinder heads.
Yes, the Gen III family was an official success—but never one to rest on its laurels, GM almost immediately began to finesse its already-stellar design, and soon enough the Gen III itself was to be superseded.
The Gen IV: Improvements Abound
Beginning in model year 2005, GM introduced the first of a slew of revised versions of its enormously successful new small-block, and these new additions to the product line carried the designation of Gen IV. Taken as a whole, the mechanical differences carried in this new generation of engines were minor—enough that parts compatibility between Gen III and Gen IV engines is more than substantial. At the same time, though, the Gen IV began incorporating many impressive new technologies rarely or never before seen on cam-in-block engines.
One of the Gen IV engine program’s main revisions included moving the camshaft position sensor from the upper rear of the block (where it read off the back of the camshaft) to the front cover. Additionally, the location of the knock sensors was changed from the lifter valley area to the exterior lower sides of the block. The primary reason for the relocation of these items was to accommodate GM’s exciting new Displacement-On-Demand (DOD) technology, also known as Active Fuel Management (AFM). By deactivating half of the engine’s cylinders under certain light load conditions, AFM provides significant fuel savings. This is all accomplished via special switching valve lifters and a so-called Lifter Oil Manifold Assembly (LOMA) located in the lifter valley. But, for reasons mainly involving the particular vehicle application of each engine, not all Gen IV V-8s had the AFM system. You can see in the accompanying Gen III/IV Engine RPO Table a rundown of which engines featured AFM, and surely more such variants are being released as you read this. Also debuting on some members of the Gen IV line were features like E85 Flex-Fuel capability, variable valve timing (VVT), late intake valve closure (LIVC), and even hybrid gasoline/electric drive systems. Though these and other impressive technological features were unique to the Gen IV, some Gen IV engines failed to incorporate a single one of these advances—so judging the break between the Gen III/IV is not as simple as it may seem.
Other Gen IV features were simply carryovers of late-Gen III improvements or extensions of them over a broader range of engine offerings. These included items like an improved timing chain, now incorporating a vibration dampener (a feature originally developed for the LS1 but that hadn’t made the production cut) or even a tensioner on some applications. Also taking a cue from some later-production and high-performance Gen III variants, all engines now featured floating piston pins and coated piston skirts for reduced noise and increased durability. Other mild upgrades like revised coil packs separate a Gen IV from its immediate predecessor, but just as one example of the two’s similarity, Gen III LS6 and Gen IV LS2 engines utilized the exact same cylinder head casting!
In sum, these differences taken as a whole are enough to distinguish Gen III and Gen IV block castings and make them technically incompatible with one another, even though the majority of engine parts are interchangeable. (We should also note that, with some help from the automotive aftermarket, it’s usually a straightforward task to convert from one style of engine to the other—see our “Block-Swapping Points of Interest” Workbench Tip in Chapter 4.) Again, we’ll cover the Gen III/IV distinctions in detail where they become important in later chapters—but as far as we’re concerned, they are two similar versions of the same spectacular LS engine!
Determining Whether Your Engine is a Gen III/IV
We’ve created a couple of helpful tables in this chapter to help you figure out exactly which LS engine you’ve got in your hands. One lists all variants of the Gen III/IV, along with their RPO designations and some important specifications. The other lists all GM passenger cars equipped with Gen III or IV engines.(Because there are so many more versions of trucks and truck engines offered by GM, and because of the aforementioned Gen I and III overlap, it’s best to simply refer to the RPO table for exactly what engine your truck has.) Combine these tables with the other information in this chapter, and you shouldn’t have a problem determining whether your engine is an LS.
The Small-Block Lives!
We hope that even seasoned LS fans have learned something from this chapter about the Gen III/IV small-block and its roots. But before ever opening this book, any reader probably knew that the GM small-block has always been about producing gobs of power and torque out of a sensible package: whether it be for speed, towing capacity, or just that extra oomph to make driving more pleasurable. Perhaps you’d like to rebuild your old Gen III or IV so that it will perform like new, or maybe you’re here to unlock additional power out of your LS (we’ll begin our next chapter with a discussion of these goals). Whether you’re here for one reason or the other—or are even building an LS from scratch—we’ve got you covered. Welcome!
Written by Chris Werner and Posted with Permission of CarTechBooks
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