If you've spent any time lurking on Mopar forums or tinkering with an old Second Gen Ram, you've probably heard that a 5.9 magnum throttle body upgrade is the fastest way to get that lazy V8 to actually wake up. It's one of those classic "bang for your buck" mods that people swear by, and for good reason. The factory setup on the 360 (the 5.9L) and even the 318 (the 5.2L) was let's just say "conservative." Chrysler designed these engines to be reliable workhorses, but they definitely left some performance on the table by choking them out at the air intake.
If you're driving a Dakota, Durango, or a Ram from the late 90s, you know the feeling. You hit the gas, the engine makes a lot of noise, but it feels like it's breathing through a cocktail straw. Swapping out that restrictive stock unit can change the entire personality of the truck.
What's wrong with the stock throttle body?
To understand why the upgrade matters, you have to look at what Dodge gave us from the factory. The stock Magnum throttle body has these massive "horns" or "neck-downs" inside the barrels. If you take your air hat off and look down into the bores, you'll see that the opening at the top is much wider than the actual butterfly valves. This creates a weird turbulence and restricts the total volume of air the engine can pull in when you're really stepping on it.
The factory unit is roughly 48mm, which isn't terrible for a stock engine, but the way it's cast is the real problem. It's restrictive and inefficient. When you go for a 5.9 magnum throttle body upgrade, you're usually moving to a unit that has been "bored out" to 50mm or 52mm, and more importantly, those restrictive ridges have been machined away. This allows for a much smoother, higher-velocity airflow directly into the plenum.
Choosing the right size: 50mm vs 52mm
This is where things get a bit heated in the gearhead community. Everyone wants the biggest part possible, but bigger isn't always better when it comes to intake velocity.
The 50mm Option
For a mostly stock 5.9 Magnum—maybe you've just got a cold air intake and a muffler—the 50mm upgrade is usually the "sweet spot." It provides a noticeable increase in throttle response without killing your low-end torque. Since most of us use these trucks for daily driving or towing, you don't want to lose that grunt off the line. The 50mm flows significantly more than stock but keeps the air moving fast enough that the engine doesn't feel "doggy" at low RPMs.
The 52mm Option
Now, if you've gone down the rabbit hole and added a tuner, headers, maybe some 1.7 roller rockers, or a better intake manifold, the 52mm is the way to go. It flows a massive amount of air. However, if you put a 52mm on a bone-stock engine, you might find that the throttle becomes a bit "touchy," and you might actually feel a slight loss in torque right off the idle. It's all about balance. If the engine can't use the air, there's no point in shoving it in there.
Billet vs. Bored-Out Stock Units
When looking for a 5.9 magnum throttle body upgrade, you'll generally see two types of products.
First, you have the "bored-out" factory units. These are actual Chrysler castings that someone has put on a lathe, machined out the barrels, and fitted with larger butterflies. These are great because they're affordable and they look completely stock. If you're trying to keep things low-key or you're on a budget, these are a solid win.
Then you have the high-end billet units, like those from F&B Performance. These are works of art. They are machined from a solid block of aluminum and are usually much more precise than a modified factory casting. They don't have the "leaky" issues that some cheap bored-out units can have around the shaft seals. They cost more, sure, but the quality is night and day.
The "While You're In There" Problem
I can't talk about a 5.9 magnum throttle body upgrade without mentioning the infamous Magnum plenum gasket. If you're pulling the throttle body off, do yourself a favor: take a flashlight and look down into the intake manifold (the "beer barrel"). If you see a pool of oil at the bottom, your plenum gasket is blown.
This is a legendary flaw with the 5.2 and 5.9 engines. The steel plate on the bottom of the aluminum intake expands at a different rate, eventually sucking the gasket in and causing a vacuum leak and oil burning. If your plenum is blown, a new throttle body won't do much for you. Most guys do the "Hughes Plate" or a similar aluminum plenum fix at the same time they upgrade the throttle body. It's more work, but it's the only way to make sure your new mod actually performs the way it should.
Installation and the "TV Cable" Dance
Installing the upgrade is actually pretty easy. It's four bolts, a few vacuum lines, and two electrical connectors (the TPS and the IAC). Most people can swap one out in about 30 minutes in their driveway.
However, there is one tricky part: the Throttle Valve (TV) cable. If you have an automatic transmission (the 46RE), that cable tells the transmission when to shift based on how far the throttle is open. When you install a new throttle body, sometimes the bracket is slightly different, or the geometry has changed. You might find your truck shifting way too early or "hunting" for gears. Don't panic—you just need to adjust the clip on the cable to get the tension right. It's a five-minute fix once you know how to do it.
What will you actually feel?
Let's be real—you aren't going to gain 50 horsepower from a 5.9 magnum throttle body upgrade. If anyone tells you that, they're selling something. But what you will feel is a much "snappier" engine.
The truck will feel lighter on its feet. Passing on the highway becomes less of a chore because the engine doesn't have to struggle as hard to gulp in air when you drop a gear. It's about driveability. The "dead spot" at the beginning of the gas pedal travel usually disappears. Many owners also report a slight increase in MPG—usually 1 or 2 miles per gallon—simply because the engine is operating more efficiently and you don't have to bury your foot as deep to get the truck moving.
Supporting mods to make it better
To really get the most out of your new throttle body, you should consider a few other things. A free-flowing air intake is a must—there's no point in having a 52mm throttle body if it's still sucking air through a tiny factory air box.
Also, a tuner (like an SCT or Superchips) can help the computer understand the new airflow parameters. The stock PCM (computer) is pretty smart and can adjust for a 50mm swap on its own, but a custom tune will really sharpen the timing and fuel maps to take advantage of that extra oxygen.
Final Thoughts
Is the 5.9 magnum throttle body upgrade the single greatest modification in history? Maybe not, but for the price and the ease of installation, it's hard to beat. It addresses one of the fundamental "chokepoints" of the Magnum engine design. Whether you're trying to build a street sleeper or just want your work truck to stop feeling so sluggish on the hills, opening up that intake is a logical first step. Just make sure you check that plenum gasket while you're at it, or you'll be leaving half your gains on the table!