L20 4.8L Engine Upgrade Guide: Expert Advice for L20 Engine Mods to Maximize Performance

[Editor’s Note: This L20 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 4.8L L20 Engines

The L20 appeared in model year 2010-17 GM 1500 series trucks and vans that offered between 285-302 horsepower and 285 ft.-lbs. of torque. It’s a Gen IV 4.8L iron-block engine, so you’re getting the better intake manifold, connecting rods, and ECM.

The engine—considered an updated version of the LY2—had 799 or 243 casting heads, which are closely related to the LS6. The engine also had flex-fuel capability, Variable Valve Timing (VVT), and pistons with valve reliefs, offering a compression ratio of 8.8:1.

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For L20 engine specs, read this:

• L20 4.8L Engine Specs: Performance, Bore & Stroke, Cylinder Heads, Cam Specs & More

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How to Get More Power From Your 4.8L L20 Engine

The L20, which began its life in trucks and SUVs, is gaining popularity for several reasons:

• It’s an iron-block alternative to older Gen III 4.8L, 5.3L, and 5.7L engines that are becoming more difficult to find. The intake and accessory drive swap over, but a Lingenfelter crank sensor trigger conversion is required.
• General Motors made about a bazillion of them, which has helped keep prices down.
• The flex fuel injectors will support 450+ horsepower at the wheels.

Do these engines have a downside? Arguably the VVT—but that’s because VVT isn’t for everyone. Fortunately, deleting it is easy if you’re so inclined.

Basic Bolt-On Upgrades for L20

There isn’t exactly a surplus of folks hot-rodding commercial vans, but the engine often is used in engine swaps due to the low price in wrecking yards. If you do want to modify the engine, you can take comfort in knowing it responds the same to mods as the others.

If you own a van, you’re not going to find much in the way of bolt-ons. If you have a truck, you’re in good shape. There are quite a few parts available that also bolt onto the 5.3L variant.

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

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

The 4.8L 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 some additional upgrades that will improve the performance of your L20 engine.

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Upgrading the L20 Camshaft and Valvetrain

When it comes to bang for your buck, cams are at the very top!

The first thing to decide is whether you want to maintain VVT. Though uncommon for all-out racing, VVT widens the power band in a street-driven vehicle. If that sounds good, a VVT limiter kit and VVT cam is just what you need.

What are the downsides to VVT?

More serious cams often require 400 lbs. of valve spring-open pressure. This makes it hard for the phaser to actuate. In addition, tuning VVT is a little more complex. If you don’t want to deal with that, a VVT delete kit is an option that opens the door to a wide selection of camshafts.

Generally, 3-bolt cams are more common and just requires the addition of a 3-bolt 4-pole upper gear to convert it over.

So how do you choose a non-VVT cam?

The price point of a LS3 or LS9 cam is good, but they make the engines pretty soggy up to 5,000 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 (at .050 inch) 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 4.8L truck cams in the 210-215 at .050 inch range if it’s still a daily driver.

If the engine is being swapped into a lightweight car with deep gears, converter, etc., the L20 responds well to bigger cams.

These cams 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. If you’re planning to turbocharge your 4.8L, you’ll want a dedicated turbo cam.

Drop-in .500-inch lift cams are popular, but LS6 springs allow you to run .550-inch lift cams 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 inch options aren’t a problem with 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 L20 cam swap, including an LS2 timing chain, LS7 spec lifters, LS2 timing chain damper, and .080-inch wall pushrods.

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L20 Power Adders

In general, the L20 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 four-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 perfect 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 that 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 breakage as a result of 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. While twins are a little more expensive out of the box, you’ll have more room to grow.

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Upgrading the L20 Fuel System and Tuning

We recommend looking at the injector’s part number before taking it to the tuner. The L20 injectors were rated at 36 lbs. and won’t support much more than 450 hp. Luckily, you have many options.

As for upgrades, the 1.730-inch O-ring centers L96 injector bolts in and bumps capacity to 49 lbs. The shorter 59 lb. LSA/LS9 injector also works, but a spacer will be required to fit the fuel rail.

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, but 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 L20 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, 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 that’s also a good value.

In most cases, a single plane is the wrong way to go; however, it might make sense if you’re running a lot of nitrous since it’s stronger and the cylinder-to-cylinder mixture distribution can be better. It’s a good choice when swapping it into an old car or 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 craving a bit more power and torque, the F.A.S.T. LSXRT intake allows for throttle bodies of 102mm and larger. Truck engine bays also accommodate tunnel rams. These trade a little bit of torque down low for more torque up high—and nothing looks cooler.

Ask your tuner about going with a Speed Density tune. Doing so removes the MAF restriction and gives 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 L20 Cylinder Heads

The stock heads can be CNC-ported for more airflow and milled up to .030 inch for more compression. Flow numbers can be as high as 325 cfm at .600-inch lift. Lightweight, hollow-stem LS3 valves can be cut to 2.040 inches to fit the seats. Between the light valves and better springs, the engines will pull cleanly to 7,000 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-inch intake valves are common. They flow great, and the cross-sections are great for boost.

When comparing heads, look at .400-inch lift numbers as a general indicator of how the heads will perform. With a medium-sized cam, 400+ whp naturally aspirated is common, even with the heavier truck drivetrain.

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Upgrading the L20 Rotating Assembly

Still looking for more?

The L20 has the lowest compression ratio of any LS/Vortec engine at 8.8:1. This is due to having the larger 65cc chambered heads and pistons with exhaust valve reliefs that net 3cc. If you go with VVT delete, true flat-top LY2 pistons can be used and the heads can be milled to 60cc to bump compression to 9.8:1, as long as you don’t use a cam that’s too big. It’s unfortunate that the valve reliefs aren’t for the intake side, as that’s where it’s typically needed first with a larger non-VVT cam.

Again, the pistons are a weak point—in fact, you probably know someone who has popped one. A set of forged pistons should be high on your priority list. They have stronger wrist pins, thicker ring lands, and added valve reliefs that allow you to run big cams.

There are exceptions, but Gen IV rods start getting dicey around 800 whp, and the bolts don’t like much more than 7,000 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-inch rod bolts.

The L20 cranks were cast but strong—they’ve been known to handle over 1,000 whp. The main reason for going with a stroker forged crank is for the added cubic inches. The extra cubes bring quicker boost, meaning you can use bigger turbos.

Performance rotating assemblies are available in many combinations.

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Upgrading the L20 Engine Block

There is no replacement for displacement.

If you’re running boost, it’s common to overbore by .020 to 3.800 inches. 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. Though LS9 gaskets don’t benefit from the optimal bore size, many people 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.

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(Information for this article originally appeared in this Upgrading the Gen. 4, 4.8L, Iron 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.)