LS4 420PS Tune

Building a 350whp Naturally Aspirated LS4

LS4 NA Power!

An LS4 Motor

The GM LS4 is a unique member of the LS family, designed for front-wheel-drive applications in cars like the Impala SS and Pontiac Grand Prix GXP. From the factory, it produces around 303 horsepower, but when measured at the wheels, this translates to only about 240–250 whp. For enthusiasts seeking to maximize performance without resorting to forced induction, building a naturally aspirated LS4 capable of producing 350 whp is a challenging but attainable goal. Achieving this level of output requires a thoughtful combination of airflow upgrades, valvetrain improvements, and tuning to overcome the platform’s factory limitations.

The foundation of the build begins with improving how the engine breathes. A high-flow cold air intake paired with long-tube headers and a performance exhaust system dramatically improves airflow in and out of the motor. Since the LS4 is restricted by emissions-friendly factory manifolds and intake design, freeing up these bottlenecks can easily add 25–30 horsepower at the crank. While these modifications alone won’t get the engine to 350 whp, they set the stage for more substantial upgrades that will fully utilize the engine’s potential.

The most significant gains on a naturally aspirated LS4 come from a well-chosen camshaft. By moving to a more aggressive cam profile, the engine can take in more air and fuel at higher RPMs, greatly improving volumetric efficiency. Supporting this upgrade with high-performance valve springs, pushrods, and lifters ensures the engine can rev safely and reliably. Deleting the factory Displacement on Demand (DOD) system is mandatory at this stage, not only for reliability but also to prevent interference with performance tuning. With a quality cam package and supporting valvetrain upgrades, it’s realistic to add 60–80 horsepower at the crank.

Cylinder head work further enhances the setup. Ported factory heads or aftermarket options allow the engine to move significantly more air, especially in the higher RPM ranges where the camshaft is most effective. Combining ported heads with a slight bump in compression ratio through custom pistons or thinner head gaskets sharpens throttle response and increases power output. Together, these upgrades can add another 30–40 horsepower, pushing the LS4 into the 400+ crank horsepower range. When measured at the wheels, this puts the build close to the 350 whp target.

Of course, airflow upgrades mean little without proper tuning. A dyno-tuned ECU calibration is essential to optimize fuel and spark maps for premium fuel, maximize the efficiency of the upgraded cam and heads, and remove the factory torque management that restricts power delivery. With professional tuning, drivability can be maintained for street use while still extracting every bit of horsepower from the naturally aspirated setup. At this point, the LS4 can produce roughly 420 crank horsepower, which translates to about 350 whp through the 4T65E-HD transmission.

The final consideration in achieving this setup is drivetrain durability. While the LS4 itself can reliably handle 350 whp in a naturally aspirated build, the stock transmission is a weak link. Reinforcing the 4T65E-HD with upgraded clutches, a stronger input shaft, and improved cooling ensures that the additional power can be reliably delivered to the wheels. Once these supporting modifications are in place, the result is a unique, high-revving FWD LS4 that makes a genuine 350 whp without boost, a rare accomplishment that blends factory drivability with muscle-car-like performance in an unconventional platform.

Every motor and transmission has a weakness. It is very important to address your motor and transmission for weaknesses before modifying them. 

Check with your local laws and regulations before modifying, swiping motors, and transmission..👈