LH8 5.3L Engine Upgrade Guide: Expert Advice for LH8 Engine Mods to Maximize Performance

[Editor’s Note: This LH8 engine upgrade guide is part of a series of LS engine upgrade guides assembled by a team of LS experts at Summit Racing that we are sharing at OnAllCylinders. For a primer on the entire LS engine universe, read LS Engines 101: An Introductory Overview of the Gen III/IV LS Engine Family.]

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Intro to the LH8 Engines

The Gen. 4 5.3L LH8 was introduced in 2008 as the V8 option for the Hummer H3. It was the most basic engine of its family. The LH8 was available in the H3 and GM mid-size pickups through 2010. It was also known as the Vortec 5300.

The LH8 was an odd one, but in a good-weird sort-of way. On one hand it has an aluminum block, which is nice. One the other hand, it came without AFM, VVT, or flex fuel capability. That’s not all bad because it saves the cost of an AFM delete kit. A lot of people end up nixing the VVT if they put a cam in anyway, so that could be another plus.

One reason the LH8 is valued is it’s an aluminum block alternative to older Gen. 3 5.3L and 5.7L engines that are getting more difficult to find. The intake and accessory drive swap over, but a Lingenfelter crank sensor trigger conversion is required.

Being a Gen. IV, you’re getting the better intake manifold, connecting rods, and faster ECM. The engine also had 799 or 243 casting heads which are closely related to the LS6. The engine wasn’t a slouch at 300 horsepower and 325 torque right out of the box.

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For LH8 engine specs, read this:

• LH8 5.3L Engine Specs: Performance, Bore & Stroke, Cylinder Heads, Cam Specs & More

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How to Get More Power From Your LH8

Basic Bolt-On Upgrades for LH8

Trucks are the everyday hotrods of the 21^st century and you can think of the LH8 as the modern equivalent of the old 327—a long-time favorite of hot rod builders.

For many people, it starts off with a cold air intake and aftermarket exhaust. The problem is that the engine starts to sound REALLY good and owners find themselves wanting to go faster. All these can be done in the garage, but the tune won’t be optimized.

At this point, we recommend talking to your chassis dyno tuner and decide on a computer programmer. Whether you’re towing on low-octane or springing for good fuel, a tuner can dial the ECM and take it to the next level. They can also remove the torque limiting function.

The 5.3L loves to rev, so raising the rev limit and shift points is common. It also makes it easier to tune with a bigger cam and injectors later on. Before you go, make sure to install a colder thermostat to open up the tuning window.

Below are more upgrades that will improve the performance of the LH8 engines.

First off, the ubiquitous LS cam swap.

The price point of a LS3 or LS9 cam is good, but they make the engines pretty soggy up to 5000 rpm. This is due to the intake valve closing (IVC) point being late and bleeding off a lot of compression down low.

What you want is a dedicated truck cam.

What’s the difference?

To maximize torque in the mid-range, manufactures close the intake valve at about 40 degrees (@.050 in.) after bottom dead center and alter the intake valve opening to set the idle quality. For a good idle and low end, you’ll often see the 5.3L truck cams in the 210-218 @ .050 range if it’s still a daily driver.

If the engine is being swapped into a lightweight car with deep gears, converter, etc., the LH8 responds well to bigger cams.

If you’re planning to turbocharge your 5.3L, you’ll want a dedicated turbo cam.

They reduce overlap to keep high turbine inlet (backpressure) from flowing back into the cylinder. Generally, supercharger cams and nitrous cams will have slightly more lobe separation and longer exhaust duration.

Drop-in .500 in. lift cams are popular, but LS6 springs allow you to run .550 in. lift and extend the rpm range. Spring life isn’t a problem because trucks generally don’t spend a lot of time at high rpm. Beyond that, .575 to .600 in. isn’t a problem with single beehives or dual valve springs.

The stock rockers are good to 175 lbs. of seat pressure and 450 lbs. open. You will want to install a trunnion kit for added reliability.

There are a few other parts needed for a LH8 cam swap such as an LS2 timing chain, LS7 spec lifters, LS2 timing chain damper, and .080 in. wall pushrods.

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LH8 Power Adders

In general, the LH8 will see boost or nitrous before any serious head work. Before we get into the power adders, there are a couple things to address:

• A 4-corner steam kit reduces hot spots that cause the rings to butt and snap the piston’s ring lands.
• Any power-adder will put you well past the limitations of the stock injectors and pump. We’ll address those in the next section.

With that out of the way, let’s get to the fun stuff.

• A roots-style supercharger is dependable and makes great torque in the low- and mid-rpm range. It’s great for melting tires.
• A centrifugal-style supercharger is lightweight and makes more power at high rpm. This is partially due to a larger intercooler mounted in front of the radiator.
• A nitrous oxide kit (at low settings) is great for street driving with stock internals. Up to a 200 shot is common. Keep in mind the tight piston ring-gap is the limiting factor beyond that. If you’re wanting to get serious, a single plane intake is less prone to break from a nitrous backfire. A plate system has better distribution than the original intake, but an eight-nozzle fogger system is even better. Running higher octane fuel is advised.
• Truck engine bays make fitting turbos easy. Single turbo systems using turbo exhaust manifolds are an inexpensive way to make big power. If you’re running a single turbo, the T4 hot-side fits well, but the small turbine diameters limit exhaust flow. The 650 whp begins to feel like 400 did in a hurry, so take this into consideration. V-band style exhaust housings open up the turbine options and make plumbing easier. Although twins are a little more expensive out of the box, you’ll have more room to grow.

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Upgrading the LH8 Fuel System and Tuning

We recommend looking at the injector’s part number before taking it to the tuner.

The LH8 injectors were rated at 30 lbs. and won’t support much more than 440 hp. Luckily, you have many options. They are a USCar EV6 style and 1.730 in. long between O-ring centers. The GM 12613412 Flex fuel injectors interchange and flow 49 lbs. They were used in the L96 and LC8.

Keep in mind, the best injectors are fully characterized which helps your tuner maximize idle quality, etc.

When running boost, you can use a water-methanol system to supply extra fuel and lower charge air temps.

The factory pump is good to about 430 whp. Many fuel system upgrade options are available. Drop-in fuel pump modules and external pumps are popular. Other options to maintain or increase pump pressure includes electronic voltage controllers and hotwire kits.

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Upgrading the LH8 Intake Manifold and Throttle Body

If you have a power adder, the intake and throttle body can take a backseat for a while longer. If you are naturally aspirated though, it’s commonly done before the heads.

The factory truck-style manifold has long runners for better low-end torque. Porting the intake is one option and a good value.

In most cases a single plane is the wrong way to go, but might make sense if you’re running a lot of nitrous because it’s stronger and the cylinder-to-cylinder mixture distribution can be better. It’s also popular if you are swapping it into an old car and is the way to go if you’re not familiar with injection. You can slap your trusty carb on it and run a programmable ignition box that plugs right into the factory coil harness and cam sensor.

If you’re looking for more power and torque, the F.A.S.T. LSXRT intake allows a bigger 102mm+ throttle body. Truck engine bays also accommodate tunnel rams. These trade a little bit of torque down low for more up high—and nothing looks cooler.

Ask your tuner about going with a Speed Density tune. Doing so removes the MAF restriction and will give you a bit more power.

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[Trying to find an LS engine for a swap or build? Check out Part 1 and Part 2 of our LS Spotter’s Guide.]

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Upgrading LH8 Cylinder Heads

• The stock heads can be CNC ported for more airflow and milled up to .030 in. for more compression. Flow numbers can be as high as 325 cfm at .600 lift. Lightweight hollow stem LS3 valves can be cut to 2.040 in. to fit the seats. Between the light valves and better springs, the engines will pull cleanly to 7000 rpm.
• A better option is aftermarket cathedral port heads. They reduce down time, they’re all new, and you can usually offset the added cost by selling your original heads. Valve angles are typically laid over to 13.5 degrees and 2.100 in. intake valves are common. They flow great and the cross-sections are great for boost. When comparing heads, look at .400 lift numbers as a general indicator of how the heads will perform. With a medium-sized cam, 425+ whp naturally aspirated is common even with the heavier truck drivetrain.

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Upgrading the LH8 Rotating Assembly

Still looking for more?

As mentioned before, the pistons are a weak point and you probably know a guy that’s popped one. A set of forged pistons should be high on your priority list. They have stronger wristpins, thicker ring lands, and the added valve reliefs allow you to run big cams.

There are exceptions, but Gen. 4 rods start getting dicey around 800 whp and the bolts don’t like much more than 7000 rpm. They are likely to bend before they break when subjected to real track conditions. If you’re getting forged pistons, it’s best to also get forged connecting rods with 7/16 rod bolts.

The LH8 cranks were cast but strong. They’ve been known to handle over 1000 whp. The main reason for going with a stroker forged crank is for the added cubic inches. The extra cubes bring boost on quicker which means you can use bigger turbos.

Performance rotating assemblies are available in many combinations.

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Upgrading the LH8 Engine Block

There is no replacement for displacement.

If you’re running boost, it’s common to overbore .020 in. to 3.800 in. The blocks can be taken to 3.905 (.007 oversize LS1) if it’s not going to see a lot of power.

When combined with a 4.000-inch stroke, this will increase displacement to 383 cubic inches.

The blocks have been known to withstand 850+ hp at the wheels with proper machining, racing fuel, and an excellent tune. Head and main studs are advised if you’re making more than 850 whp.

At that point, a set of head studs are a good idea. Although LS9 gaskets don’t have the optimal bore size, many have used them with success.

The factory main caps aren’t doweled. It’s better to reduce ignition timing and compensate with added boost to reduce the cylinder pressure spikes that lift heads and cause the main caps to dance.