In our LS Engine Picker's Guide article (click here) we scavenged backyards and junkyards for LS engines, blocks, cylinder heads, intake manifolds, and other essential parts for your next build. Well, now it's time we took an in-depth look at the differences between Gen III and Gen IV engines. There are some obvious changes like the cylinder head design and displacement, but there are plenty of less obvious variations that can alter your build drastically. Let's start with a few things covered last time.
While some Gen IV engines do use cathedral port cylinder heads, pretty much all are of the later, LS6 design. By 2009, cathedral port use was mostly restricted to just the 5.3L engines while all but the LS7 used the LS3/L92-style rectangular port. It is worth noting that all three style heads require different rocker arms, intake manifolds, pushrod lengths, and even valve covers in some cases. The mammoth intake valves on the LS3/L92 and the LS7 require offset rocker arms. In fact, on the LS3/L92 you can reuse the LS1 style exhaust rocker (just not the intake). LS7 rockers on the other hand are 1.8:1 ratio, and quite a bit different than even the LS3 (1.7:1 ratio). LS7 rockers attach directly to the head without a separate stand commonly found on other LS heads. The LS3 and LS7 rockers have also been known to require more clearance than LS1 valve covers afford. This can sometimes be rectified with a Dremel, but the bolt pattern is a bigger concern. Early LS1 heads (1997-'98) have perimeter bolts rather than center bolts to secure the valve covers.
Moving on to the short-block, there are some notable differences between the Gen III and IV. The block itself has different knock sensor locations. Gen IVs place the knock sensors on the side of the block, while the Gen III's is located in the valley. As the result, both have different valley covers. Ideally you will want to use a Gen III block with a Gen III PCM and vice versa, but the knock sensors can be made to work in a different location after some necessary rewiring of the engine harness and tuning changes. Gen IV engines also place the cam sensor in the timing cover, while the Gen III's goes through the top of the block at the back of the valley cover. The Gen III has a corresponding sensing ring on the cam itself, at the very back just ahead of the final journal. The Gen IV, conversely, requires a different cam sprocket to grab a signal. The Gen IV cam sensor signal is 4x, which matches a 58x crank reluctor wheel. The Gen III has a 1x cam and 24x crankshaft reluctor ring. If you try to pair a Gen IV ECM, such as the E38 or E67, with a 1x cam signal and 24x crankshaft, it will not run. However, you can make the Gen IV signals work with a Gen III using the Lingenfelter TRG-002 conversion module. If you put a 24x crank in a Gen IV block, or happen to have one of the rare 24x Gen IVs (such as the LS2 in the 2005 Corvette, 2005-'06 GTO, or 2005-'06 Trailblazer SS), you can use Casper's LS1 Knock Sensor and Cam Sensor Adapter Harness.
Overall, the different locations of these types of sensors makes the engine harness quite a bit different from Gen III to Gen IV … as does the choice of ECU. While we know not everyone chooses to stick with EFI on LS mills, your method of control should be the first decision you make. In terms of price, going the Gen III route can be more economical, but these days we'd venture to say it is nearly a wash in terms of the engine itself. The Gen IV, though, is typically a more expensive ECM and it requires an electronic throttle body with a corresponding gas pedal. For this reason, many swappers prefer the Gen III style with a cable throttle body. But the E38 and E67 controller, as well as the drive-by-wire throttle bodies, do offer a level of refinement and control not found in earlier models. They are faster computers with more features and tuning parameters, such as the ability to control the 6L80E and 6L90E transmissions, Flex Fuel, VVT, and more. Of course, you can always throw an aftermarket ECU into the mix, which alleviates some of these concerns. Though, that is a discussion for another day.
Here are six variations on the LS throttle body. Top left is a traditional 78mm cable-operated F-body-style throttle body while the two below it are electronic truck throttle bodies. The right side is all Corvette pieces. The top right is a 78mm C5 unit, and then there is the 90mm LS3 with the gold blade and the 90mm LS2 with the silver blade. The LS2 is compatible with E40 and earlier PCMs, but the LS3 is not and vice versa since the blade opens in the opposite direction.
The various truck versions may look similar, but the older versions have an 8-prong plug while the newer versions have a smaller 6-prong plug. Conversion harnesses are readily available.
Another feature of older throttle bodies like this F-body version is the coolant passage. Unplugging and blocking off this coolant passage was part of the infamous "free mods" that nearly any hot rodder was accustomed to doing on 1998-'02 F-bodies to pick up a few hp. Note: this one has been ported, another "free mod" to pick up power.
While the LS3 and LS7 use a straight exhaust rocker like the cathedral ports, the intake rocker is offset to accommodate the massive valves. The LS7 (seen here) is also a 1.8:1 ratio, unlike every other factory LS rocker (that is 1.7:1), and bolts directly to the head.
Moving on to the block, all Gen IVs have bosses for Displacement On Demand (aka Active Fuel Management) in the valley.
DOD uses these spring-loaded lifters on half the cylinders that disengage under light load and idle, so that the engine can run on four cylinders and save gas. There are some serious durability concerns using these in performance applications (especially with boost).
The valley cover for DOD engines houses the brains of the systems, so the underside looks quite a bit different than a standard LS valley cover. On the left is an LS3 cover, which still has gaskets that seal to the bosses on the block. Make sure you use a Gen IV valley cover with a Gen IV engine.
Gen IV engines also place the knock sensors on each side of the block. It is possible to use these knock sensors with a Gen III PCM, but generally the preferred method is to relocate the Gen III knock sensors to the unused set of bosses on the side the block (they won't fit in the Gen IV location). If you want to use a Gen III block with a Gen IV computer, you can use these same bosses for the Gen IV knock sensors.
The Gen III valley cover looks quite a bit different because it houses the knock sensors (removed from this engine already, along with the rubber gaskets that seal it). This is another reason the valley covers aren't interchangeable.
Here is another look at where the knock sensors screw into the Gen III block.
The cam sensor is also located topside on the Gen III, at the back of the valley area and under the intake manifold. A corresponding sensing ring is on the cam below.
All Gen III engines have a plain timing cover that looks like this.
Under the Gen III timing cover you will see a rather unsophisticated timing set that looks like this, though this is actually a Comp Cams piece.
Meanwhile, the Gen IV timing covers have the cam sensor in them.
As the result, Gen IVs require a matching timing set that it can use to produce the 4x signal. This is also an aftermarket double roller, but it has the necessary pattern to grab a signal.
The timing cover on Gen IVs with cam phasing (aka VVT or variable valve timing) is also noticeably different. Only certain Gen IV ECMs—like the E38—can handle VVT (and DOD for that matter), and Mast Motorsports' M-90 and M-120 are two of the only ones in the aftermarket.
In lieu of a regular timing set, the L99/LY6/L92 and other variants have a cam phaser that advances or retards timing.
VVT cams use a special bolt/solenoid that controls the flow of oil and connects it to the phaser instead of the traditional three-bolt cam. Thankfully, swapping back and forth isn't too problematic. In fact, it is quite popular to swap a three-bolt LS9 camshaft into a junkyard L92 with an LS3 timing set, cover, and valvesprings.
The reluctor wheel sits on the back of the crankshaft by the flexplate flange, which is where the crank sensors draw a signal (from each side). This is a 24x for a Gen III that has been tack-welded by Late Model Engines. The factory uses a two-piece, stamped steel design that is riveted together. It has the potential to get loose and separate, which can greatly affect the ignition timing if it gets more than 2 degrees from spec. LME recommends switching to its billet one-piece design that maintains the factory out of balance. Thankfully, most Gen IVs use a single-piece 58x design from the factory.
There are quite a few variations on the factory Powertrain Control Module, known as the Engine Control Module once you get to the Gen IV. The most desirable Gen III PCM is the 2003-'07 truck and 2004 Corvette version, which have a faster Motorola 68000 chip. They have a green connector, unlike this blue version, which is a 1999-'02 and very common. The 1997-'98 is the least desirable piece of hardware, and is identifiable by its larger size. All LS computers are flash-based, which means tuning is done through manipulating the calibration with computer software. That calibration is then uploaded to the PCM (no chip burning!).
For the Gen IV crowd, there is the option to buy new. Chevrolet Performance sells the Engine Controller Module and Harness as a kit for various Gen IV crate engines, along with the electronic gas pedal. Whenever you use an electronic throttle body, you need a gas pedal to match. You can see how the ECM looks quite a bit different than the earlier style—the plugs especially.