The LS4 is very unique in the LS family of engines. It was only used in front wheel drive (FWD) cars. On a stock engine, the following upgrades can improve performance and fuel economy:
Engineers had to make a lot of compromises to get an LS engine to fit sideways in a FWD engine bay. Due to those compromises, the LS4 has limited horsepower potential. It is also not a good candidate for an engine swap, unless you are willing to do a lot of custom fabrication.
Intake Manifold and Throttle Body
The LS4 has a small 76mm throttle body. It also has a smaller intake manifold than other LS engines. The smaller units are needed to clear everything else under the hood. However, they restrict the engine's ability to breathe.
It is possible to upgrade to a LS6-style intake with a larger 3-bolt throttle body. However, you will need to also modify the following components to make it fit:
- PCV,
- EGR,
- Valley cover,
- MAP sensor,
- Oil pressure sender,
- Injector pigtail adapters,
- Alternator bracket,
- The underside of the intake manifold, and
- The throttle body adapter.
Camshaft and Valvetrain
You will need to drop the entire engine cradle to do a Cam Swap in one of these cars. Keep in mind that going bigger than about 220° of intake duration (at 0.050 in.) is not practical with the stock intake and torque converter. The 2005-2006 models came with the 3 bolt cams with a 1X single pole to match the 24X reluctor. Sometime in 2007, they went to a single bolt 4X 4 pole gear to match the 58X reluctor.
Don't forget to pick up a Spring Kit to match your new camshaft. Titanium retainers are also available and will reduce
The trunnion bearings in the stock rocker arms are a known weak point. A Trunnion Upgrade Kit should be installed whenever you upgrade the valvetrain.
The LS4 was equipped with Active Fuel Management (AFM). It's good for gas mileage, but not for performance. If the valvetrain is in good condition, an AFM Disabler plugs into the OBD port and will turn it off.
When upgrading the camshaft, an AFM Delete Kit is strongly recommended. These kits replace all the AFM components with standard parts. It also becomes easier to fit an LS6 intake manifold. The smooth, non-AFM valley cover won't interfere with underside of the manifold.
Cylinder Heads
The cathedral port cylinder heads are based on the LS6 heads and make good power. You can mill them about 0.030 in. for more compression. They can also be CNC ported for better airflow.
There are a few Aftermarket Heads available. The heads will flow better than stock and have thicker decks to maintain head gasket seal.
Other Upgrades
Other, more involved upgrades to the LS4 are really not practical. The intake manifold limits the engine's ability to breathe. The FWD driveline can really only handle about 400 hp. Even upgrading those components to make more power will result in issues with the steering and handling of the car.