On a stock engine, the following upgrades can improve performance and fuel economy:
The LQ4 and LQ9 have very strong blocks. They can handle over 1,000 hp. using nitrous, a blower, or a turbo. However, you will need to upgrade the internals to handle that kind of power.
Changing pistons and/or cylinder heads will allow you to change the compression ratio. (You may need to lower the compression to run with boost.) Upgraded components will also perform better and last longer.
Below are some other upgrades that will improve the performance of the LQ4 and LQ9 engines.
Engine Block
There is no replacement for displacement. The cylinders can be safely over-bored 0.030 in. When combined with a 4.000 in. stroke, this will increase c.i.d. to 408 c.i. or 6.7L.
Rotating Assembly
The stock pistons are a known weak point of these engines. They have a history of cracking around the wrist pin and the top ring land. A set of Forged Pistons should be high on your priority list.
The stock rods will handle about 700 hp. and 6,500 rpm. This is plenty of power for most people. However, if you plan on using boost or nitrous, get Forged Connecting Rods.
Even though it is cast, the crankshaft can handle about 900 hp. and 7,000 rpm (for a limited time). A broken crankshaft is very bad news. Upgrading to a Forged Crankshaft early in your build will save you the headache later.
Performance Rotating Assemblies are also available.
Cylinder Heads
Having the stock heads CNC machined will improve air flow and power. Lightweight LS3 valves can be cut to fit the seats and performance valve springs can be installed to decrease valve float. There are also many Aftermarket Heads available. Aftermarket heads offer a range of intake and exhaust runner volumes. Various combustion chamber sizes are also available.
A popular upgrade to the LQ4 and LQ9 is to install a set of LS3 heads and intake. The LS3 heads will fit on any LS block with a minimum 4.000 in. bore. However, the LS3 had a 4.065 in. bore. This means that the valves will be closer to the cylinder walls. Less space around the valve can limit the air flow.
A 4-Corner Steam Kit can reduce hot spots that can cause #7 ring end gaps to butt.
Camshaft and Valvetrain
LS engines respond extremely well to Cam Swaps. Just match the other parts you choose to support it. This is not just Valve Springs and rockers. It includes intake, heads, exhaust, torque converter, rear-end gears, etc.
The trunnion bearings in the stock rocker arms are another known weak point. Retrofit Trunnion Kits are available if you reuse the stock arms. Upgrading to Full Roller Rockers is another popular upgrade when lift approaches 0.600 in.
Stock rocker arms are pedestal mounted. High spring open pressures (475+) can pull the bolts out of the cylinder head. Converting to stud or shaft mounted rockers is recommended over 0.600 lift.
Intake Manifold and Throttle Body
The factory truck-style manifold has long runners for better low-end torque. However, they are about 3 in. taller than car-style manifolds. LS1/LS6-style manifolds will fit. Many Aftermarket Manifolds are also available.
The aftermarket car-style manifolds will increase the rpm range and power. The factory LS6, LS1, and LS2 intakes lose a bit of power compared to the truck manifold. They can provide hood clearance when swapping the engine into a different vehicle. Keep in mind the F-Body or Corvette front accessory drives will be required as well.
When changing intakes, also take note of the Throttle Body. The stock manifold is 78mm and uses a 3-bolt mounting flange. Many aftermarket manifolds use a 90mm or larger throttle body and use a 4-bolt flange.
Depending on the year and model the engine came from, the throttle could be controlled differently. Some vehicles used a cable to control the throttle. Others used a drive-by-wire set up.
Fuel System and Tuning
Upgrading to larger Fuel Injectors is often needed to meet the fuel demand of increased power. Custom tuning will be required to properly adjust the fuel and ignition systems.