If you're trying to squeeze more power out of your GM supercharged engine, you're probably staring at an lsa pulley chart trying to figure out which combination won't blow your head gaskets. It's the classic rabbit hole for ZL1 and CTS-V owners. You start off just wanting a little more "oomph" for the weekend, and before you know it, you're calculating blower speeds and looking at upgraded heat exchangers.
The LSA engine is a beast, honestly. It's basically a slightly more polite version of the LS9 found in the C6 ZR1, and it responds to mods like a dry sponge in a bucket of water. But the quickest way to get that power is by changing the pulley ratio. The problem is that there are so many combinations of upper and lower pulleys that it's easy to get overwhelmed. That's where a good chart comes in handy—it keeps you from spinning that 1.9L Eaton supercharger into oblivion.
Why the Ratio Matters So Much
At its core, your supercharger is just an air pump driven by the crankshaft. When you look at an lsa pulley chart, you're looking at the relationship between the size of the crank pulley (the lower) and the supercharger pulley (the upper). If you shrink the upper or grow the lower, the supercharger spins faster at the same engine RPM.
More speed equals more air, and more air (plus more fuel) equals more horsepower. But there's a catch—there's always a catch. Superchargers have a "sweet spot." Once you spin them past a certain point, they stop being efficient air pumps and start being very expensive space heaters. They generate so much heat that the engine's computer will pull timing to keep things from melting, and you end up losing the power you were trying to gain.
Most guys agree that the stock LSA blower likes to stay under 20,000 to 22,000 RPM. If you go much higher than that without serious cooling mods, you're just fighting a losing battle against heat soak.
Upper Pulleys: The Gateway Mod
Most people start with an upper pulley swap because it's relatively cheap and easy to do. You'll see sizes like 2.85, 2.55, or even 2.4 on various charts. The stock size is around 3.0 inches. Moving down to a 2.55-inch upper is a super popular move because it gives a noticeable bump in boost—usually around 3 to 4 PSI—without requiring you to change out the entire front of the engine.
However, there's a downside to small upper pulleys: belt slip. Think about it—the smaller that wheel gets, the less surface area the belt has to grab onto. If the belt slips, your boost will drop off at high RPMs, and you'll be left wondering where your horsepower went. This is why a lot of people eventually move toward changing the lower pulley or doing a combination of both.
The Case for the Lower Pulley
If you look at an lsa pulley chart, you'll see the "lower" options listed in percentages or actual diameters, like 8.6-inch, 9.1-inch, or the massive 9.55-inch and 10-inch rings. Replacing the lower pulley usually involves swapping the entire harmonic balancer. It's more work and more expensive, but it has some massive benefits.
First, you get way more belt wrap. Since you're making the bottom pulley larger rather than the top one smaller, the belt has a much easier time staying stuck to the pulleys. Second, it's a more "stable" way to make big boost. If you run a stock upper with a 9.1-inch lower, you're getting a solid boost increase with zero worries about the belt sliding around. It also looks a bit more "sleeper" if that's your thing, though most LSA owners aren't exactly trying to hide the fact that they have a blower under the hood.
Reading the Chart Like a Pro
When you actually get your hands on an lsa pulley chart, you'll notice it's usually set up as a grid. You find your upper pulley size on one axis and your lower size on the other. Where they meet, you'll see a number—that's your drive ratio.
For example, if you have a stock 7.8-inch lower and you put on a 2.55-inch upper, your ratio is about 3.06. If you take your engine's redline, say 6,200 RPM, and multiply it by 3.06, you get a blower speed of about 18,972 RPM. That's a very safe, "daily driver" friendly setup.
Now, if you go crazy and throw a 9.55-inch lower on with that 2.55-inch upper, your ratio jumps to 3.75. At the same 6,200 engine RPM, your blower is now screaming at 23,250 RPM. At that point, you better have upgraded your injectors, your fuel pump, and probably your entire cooling system, or things are going to get spicy in a bad way.
Don't Forget the Belt Length
One thing that doesn't always show up clearly on every lsa pulley chart is the belt length. This is the part that drives people crazy. You finish the install, you're excited to start the car, and then you realize the stock belt is either three inches too long or way too short to even fit over the tensioner.
Whenever you change pulley sizes, you're changing the path the belt has to take. There are lists online that cross-reference pulley combos with specific belt part numbers (usually Gates Green Stripe or RPM belts). Always buy two belts. Seriously. Keep a spare in the trunk, because if you're pushing high boost, that belt is under a lot of stress.
Supporting Mods are Non-Negotiable
You can't just look at an lsa pulley chart, pick the highest boost combo, and call it a day. The LSA is a great engine, but the stock fuel system and cooling system have limits.
If you're moving to anything beyond a basic 2.55 upper, you're probably going to max out the stock injectors. Most guys jump straight to 850cc or 1000cc injectors just to be safe. You don't want the engine leaning out at the top of third gear because the injectors couldn't keep up.
Then there's the heat. The LSA's stock heat exchanger is "okay" for stock boost, but once you start spinning that blower faster, the intake air temperatures (IATs) will skyrocket. Upgrading to a larger heat exchanger and a better pump (like the Varimax or EMP) is pretty much mandatory if you want to keep that power consistent. Some guys even run "chillers" that use the car's AC system to freeze the intercooler bricks. It sounds like overkill until you see the power gains on a hot summer day.
Common Mistakes to Avoid
One of the biggest mistakes I see people make when using an lsa pulley chart is ignoring the "total" package. They pick a pulley combo because it says "15 PSI," but they don't realize that 15 PSI on a stock exhaust is way different than 15 PSI with long-tube headers.
Headers actually drop your boost pressure because they make the engine more efficient at moving air out. But don't let the lower number on the gauge fool you—you're actually making more power. Boost is just a measure of backpressure. If you have a high-boost pulley combo and a restrictive exhaust, you're just creating a lot of heat and pressure without much actual flow.
Another mistake is not checking the "brick." Inside the LSA supercharger is a water-to-air intercooler "brick." On the older versions, these were known to collapse under high boost. If you're going for a more aggressive pulley setup, it's a smart move to have the brick reinforced or "widened" to handle the extra pressure.
Wrapping It Up
At the end of the day, an lsa pulley chart is your best friend when planning your build. It helps you visualize where you want to be on the spectrum of "reliable daily" to "track monster." Just remember that every action has a reaction. More boost means you need more fuel, better cooling, and a tuner who knows what they're doing.
Don't just chase the biggest number on the chart. Think about how you actually use the car. If it's a street car, a moderate setup that stays within the blower's efficiency range will usually be more fun and way less headache than a max-effort setup that's always pulling timing due to heat. Take your time, do the math, and enjoy the extra torque—because let's be honest, that's why we're all here anyway.