On a stock engine, the following upgrades can improve performance and fuel economy:
The iron block 4.8L and 5.3L engines are very strong. With upgraded internals, they can handle over 1,000 hp. They are also easier to find and cost less than the bigger engines.
Below are some upgrades that will improve the performance of the LY2, L20, LY5, LMG, and LMF engines.
Engine Block
The 4.8L and 5.3L share a small 3.780 in. bore diameter. The extra material between bores helps the head gasket seal under boost. If needed, the block can be bored to the stock LS1 size (3.898 in.). When bored and stroked, displacement can be as high as 383 c.i.d. (6.3L).
Sometimes, the iron blocks fall short of the standard 9.240 in. deck height. It’s recommended to measure the deck before ordering rods and pistons.
The cylinders are also shorter than the aluminum blocks. When stroking, choose a piston with minimal skirt taper. This prevents the piston from rocking at BDC and scraping up the skirts.
Rotating Assembly
The stock pistons are a known weak point. They will crack in high horsepower engines. A set of Forged Pistons should be high on your priority list.
The stock rods will handle about 700 hp. and 6,500 rpm in boosted applications. If you’re getting forged pistons, upgrade to Forged Connecting Rods at the same time.
Even though it’s cast, the crankshaft can handle about 900 hp. and 7,000 rpm (for a limited time). A broken crankshaft is bad news. Upgrading to a Forged Crankshaft early in your build will save you the headache later. If you upgrade, a 4 in. stroke crankshaft costs about the same and increases displacement.
The chart below lists standard specs compared to common performance Rotating Assemblies.
Gen. 4 Standard Spec. | Stroke | Rod Length / Wristpin | Bore Size / Compression Distance |
LY2, L20 (4.8L) | 3.267 in. | 6.275 in. / 0.9431 in. | 3.780 in. / 1.338 in. |
LY5, LMG, LMF (5.3L) | 3.622 in. | 6.098 in. / 0.9431 in. | 3.780 in. / 1.338 in. |
Common Stroker Combinations | Stroke | Rod Length / Wristpin | Bore Size / Compression Distance |
4.8L to 5.3L | 3.622 in. | 6.098 in. / 0.9431 in. | 3.780 in. / 1.338 in. |
4.8 / 5.3L to 5.7L | 3.622 in. | 6.098 in. / 0.945 in. | 3.903 in. / 1.338 in. |
4.8 / 5.3L to 5.7L | 3.622 in. | 6.125 in. / 0.927 in. | 3.903 in. / 1.304 in. |
4.8 / 5.3L to 6.3L | 4.000 in. | 6.098 in. / 0.927 in. | 3.903 in. / 1.115 in. |
4.8 / 5.3L to 6.3L | 4.000 in. | 6.125 in. / 0.927 in. | 3.903 in. / 1.110 in. |
Cylinder Heads
The cylinder heads are based on the Corvette LS2 heads and make good power. Milling up to 0.030 in. is an easy way to increase compression. CNC porting is another popular option for more airflow. For high rpm engines (7,000+), LS3 valves are lighter and can be cut to fit the valve seat.
There are also many Aftermarket Heads available. These heads flow better and have thicker decks to maintain head gasket seal. A wide range of runner and chamber volumes are available.
A 4-Corner Steam Kit is another smart upgrade. It reduces hot spots in cylinder #7 that cause the piston rings to butt and crack the piston.
Camshaft and Valvetrain
LS engines respond well to Cam Swaps. In addition to valve springs and rockers, the cam needs to match the compression, torque converter, rear-end gears, etc.
Spring Kits are available for typical 0.600 in. lift cam upgrades. Titanium retainers are another option to reduce
The trunnion bearings in the stock rocker arms are another known weak point. A Trunnion Upgrade Kit should be installed when you upgrade the valvetrain. Upgrading to Full Roller Rockers is another option.
The stock rockers are pedestal mounted. High spring pressure (over 475 lbs.) can pull the bolts out of the cylinder head. Converting to stud or shaft mounted roller rockers is recommended for cams with over 0.600 in. of lift.
The Active Fuel Management (AFM) system is good for gas mileage, but not for performance. If the valvetrain is in good condition, an AFM Disabler can turn the AFM off. When upgrading to a performance cam, an AFM Delete Kit is strongly recommended. These kits replace all the AFM components with standard parts.
Piston-to-Valve clearance and high spring pressure (over 400 lbs.) can be a problem on engines with Variable Valve Timing (VVT). Some aftermarket street cams require the use of a Phaser Limiter. A VVT Delete Kit is another option. Completely removing the VVT components is more common with larger street/strip and race cams.
Intake Manifold and Throttle Body
The highly sought-after Trailblazer SS (TBSS) intake makes more power than car intakes. By 2007, all trucks used the TBSS manifold design. Porting the manifold is a popular option, but Aftermarket Intakes make even more power.
At 87mm, the factory 4-bolt Throttle Body is fairly large and isn’t a restriction in most applications.
Fuel System and Tuning
Upgrading to larger Fuel Injectors is often needed to meet the demand of increased power. The Flex Fuel injectors flow 25% more than standard and replace the stock injectors without any other modifications. The factory fuel pump will become a limitation around 400 hp. So, plan on upgrading the Fuel Pump as well.
Truck engines have a conservative tune from the factory. Tuning the ECM changes the fuel and ignition curves to increase performance. Plug-in Programmers are easy to use, but they do have limits. Custom tuning requires more knowledge, but will provide even better performance.