L9H 6.2L Engine Upgrade Guide: Expert Advice for L9H Mods to Maximize Performance

Editor’s Note: This L9H 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 L9H Engines

Born in 2009, the L9H was a replacement to the legendary L92. It was similar in many ways, but added Flex Fuel capability.

It was a factory hot rod with 403 horsepower and 417 foot-pounds of torque. In many ways, it could be thought of as a truck version of the LS3 with the truck-specific cam, intake and accessory drive.

It also sported VVT to widen the torque curve. Better yet, unlike the L94, it has no Active Fuel Management system, which is known to fail when the engines get up in mileage.

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[Every L9H engine spec you’ll need can be found here: L9H 6.2L Engine Specs: Performance, Bore & Stroke, Cylinder Heads, Cam Specs & More.]

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How to Get More Power from Your L9H Engine (Bolt-Ons)

For many people, upgrading an L9H starts with adding a cold air intake and aftermarket exhaust. This will definitely free up a few horsepower and people will be pretty surprised to look over and see a full-size SUV making that sort of noise.

These mods can be done in your garage, but the tune won’t be optimized.

We recommend that you talk to your local 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. Raising the factory redline is a big part of this because it allows your vehicle to be in the meat of the powerband longer.

Torque limiting can be completely shut off, shift points can be raised and it makes it easier to tune for a bigger cam and injectors later on. Before beginning the tuning process, we recommend installing a colder thermostat to open up the L9H tuning window.

Below are some more upgrades you can make to improve the performance of an L9H engine.

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Upgrading the L9H Camshaft and Valvetrain

VVT is great in a street vehicle if you install a VVT cam and phaser-limiter along with a tune.

Properly done, it makes more power and torque than a much bigger engine-yet idles well and retains most of the gas mileage. If you don’t want to mess with it, you can easily convert to a standard non-VVT one- or three-bolt cam.

[Read LS Tech: How to Delete Variable Valve Timing (VVT) on an LS Engine for more info and part numbers.]

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Now let’s talk about a cam swap.

If the engine is still being used in a truck, you’re probably thinking you don’t want to sacrifice low-end torque—and you’d be right.

An LS3 or LS9 cam makes good power, but not where you want it. What you need is a cam that delivers a gut-punch right when the converter hits. We recommend a dedicated truck cam.

If the engine is going in a lighter car with deep rear-end gears and a high stall converter, you can be more aggressive with duration.

What’s the difference?

To maximize torque in the mid-range in a truck cam, aftermarket manufactures close the intake valve at about 33-38 degrees (@.050 in.) after bottom dead center and alter the intake valve opening point to set the idle quality.

The 6.2L engine can take a little more cam than a 5.3 or 6.0L engines and still idle well.

What if you already have a power adder?

Generally, supercharger cams and nitrous cams will have slightly more lobe separation and longer exhaust duration. Turbo cams reduce overlap with less exhaust duration split in relation to the intake.

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. After all, a C5 Z06’s LS6 had .555-in. lift and will happily go 150,000+ miles with normal use. Beyond that, .575 to .600 in. isn’t a problem with dual valve springs.

The stock rockers are good up to 175 lbs. of seat pressure and 450 lbs. open. You will want to install a trunnion kit for added reliability. When you’re pulling the cam, switch out the spring-loaded timing chain tensioner for the more-reliable wedge-style (early) LS2 damper.

Here are a few parts commonly used for an L9H cam swap: LS2 timing chain, LS7 spec lifters, LS2 timing chain damper and thick-wall chromoly pushrods. Keep in mind, LS engines accommodate 11/32 pushrods. These are stiffer than the factory 5/16 and greatly aids valvetrain stability and durability.

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L9H Power Adders

L9H engines are a popular choice for people that want to stay naturally aspirated, but if you do decide to boost, here are a couple of things to address:

• A 4-corner steam kit reduces hot spots that can cause the rings to butt and snap the piston’s ring lands.
• Serious power adders will put you well past the limitations of the fuel injectors. We’ve addressed those in the next section below on fuel system upgrades.

Okay, here’s the fun stuff:

• 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 want 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 are 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 are available with larger turbines and make plumbing easier. Although twins are a little more expensive out of the box, you will have more room to grow.
• A Roots-style supercharger is dependable and makes great torque in the low- and mid-rpm range. It’s great for melting tires. An original 6.2L is a rarity and owners usually turn them into hot rods and keep them for a long time. That being the case, a burly blower is a good investment.
• 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. They don’t have quite the curb appeal of a Roots, and are not as common.

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Upgrading the L9H Fuel System and Tuning

Good news! Because it was built with Flex Fuel capability, the L9H came with 54-lb. injectors. This is enough to support roughly 700 whp.

GM’s Flex Fuel-engine-powered vehicles received a high-flow electric fuel pump. If you max out the pump, you can maintain or increase pump pressure with electronic voltage controllers and hotwire kits.

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

One last note on tuning: The L9H has a 58x reluctor ring and is not compatible with the early Gen. 3 LS ECMs. It’s best to keep the E38 ECM as it’s faster and tunes better with bigger cams.

It’s always worth it to invest in a chassis dyno tune to find fuel, spark, and other issues that are harder to spot at the track.

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Upgrading the L9H Intake Manifold and Throttle Body

The intake is good and has a 4-bolt, 87mm throttle body. They were an improvement over the earlier truck manifolds and very similar to the Trail Blazer SS intake. The big plenum and long runners made better low-end torque and makes good power from idle through 6,000 rpm. It is also very good for boost, but not good for engines built for big nitrous.

If you have the L9H in a vehicle with deep gears, big converter, cam, and the rev limit bumped, you may want to trade a little of that low-end torque for some top-end horsepower. If so, go with a long-runner cast tunnel ram. Gains of 20 whp are possible with the crossover point starting at around 5,000 rpm.

Want it all?

Go with the Edelbrock cross-ram. The low-end torque of the factory and the top-end of the tunnel ram. Dual throttle bodies to top it off.

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.

Ask your tuner about going with a speed density tune. Doing so will remove the MAF restriction and will give you a bit more power.

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

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Upgrading L9H Cylinder Heads

The L9H cylinder heads are similar to the LS3 heads except they used a heavier solid stem intake valve. Because they needed to be compatible with E85 fuel, GM upgraded materials with SIL-1 nitride intake valves and Brico 3010 valve seats standard.

• The stock heads can be CNC-ported for more airflow. Flow numbers can be as high as 373 cfm at .700 lift. Lightweight hollow stem LS3 valves will drop right in. Between the light valves and better springs, the engines will pull cleanly to 7,000 rpm. If you’re looking to boost to 800+ hp; a heavy-duty aftermarket stainless intake valve is a bit tougher and will not tulip as quickly from the heat. Milling the heads .030 will bump compression to 10.8:1 and will increase power everywhere. Keep in mind, piston to valve clearance will be tight with cams beyond 230 at .050 in. of intake duration.
• A better option is aftermarket heads. They reduce down time, they are 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 for increased piston to valve clearance. They flow better and the cross-sections are great for naturally aspirated or boosted engines. When comparing heads, look at the .400 in. lift numbers as a general indicator of how the heads will perform. With a medium-sized cam, 475+ whp naturally aspirated is common even with the heavier truck drivetrain.

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Upgrading the L9H Rotating Assembly

On the L9H engine, the pistons are the weak link and you probably already know someone who has 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. If you are going over 1,000 hp, a set of .200 wall tool-steel pins is a good idea.

The L9H’s Gen. 4 rods are stronger than the Gen. 3 rods and have full floating pins. They can handle about 800 hp and 7,000 rpm in boosted applications (at least for a while). They are likely to bend before they break when subjected to real track conditions.

If you are getting forged pistons, upgrade to forged connecting rods at the same time. Big 7/16-inch rod bolts will go a long way to keep things together over 7,000 rpm.

The L9H crank was cast but strong. The main reason for a stroker crank is added cubic inches. With heads and manifolds available that breathe well above 7,000 rpm, more cubes can bring the power peak back into hydraulic roller territory for more power under the curve.

Performance rotating assemblies are available in many combinations.

A couple notes of caution:

• The 6.2L aluminum blocks had slightly longer cylinder sleeves than the iron blocks 5.500 in. vs. 5.430 on average). Much of the piston skirt drops out of the bottom of the cylinder at BDC. The best piston manufacturers have compensated for this by eliminating skirt taper until a point well above where it meets the bottom of the cylinder sleeve at BDC. Any skirt taper at this intersection acts as a razor blade and will wear out the piston quickly. This is especially true of the 427ci combination and the piston design is critical to long life.
• The blueprint deck height of the block is 9.240 in. It is best to measure deck height before ordering your rotating assembly. Thicker head gaskets or using an aftermarket 6.098-6.100 in. rod will ensure enough piston-to-head clearance.

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Upgrading the L9H Engine Block

The L9H engine block has a 4.065-in. bore diameter and can be safely bored to 4.080 in.

However, it is better to hone up in .005 increments to maximize strength.

You can make 850 whp with a couple of simple upgrades like head studs and LS9 head gaskets. Race gas, E85, or water-methanol injection is required.

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 can lift heads and cause the main caps to dance.

Main studs should be added anytime you are this deep in the engine.

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(Information for this article originally appeared in the “Upgrading the Gen. 4, 6.2L, Aluminum Block, LS Truck Engines” article at Summit Racing’s searchable database of FAQ tech info. Go there and search “LS engines” for a comprehensive collection of LS engine tech information.)