You've finally got your hands on a 400-cubic-inch Chevy, but now you're staring at a bench full of parts wondering about the best heads for a 400 small block. It's the biggest small block GM ever cranked out, and while it's a total torque monster, the factory heads were usually the weak link. Most of the original iron castings were prone to cracking and had tiny ports that choked the life out of those extra cubic inches. If you want that engine to actually breathe, upgrading the heads is the first thing you should do.
The 400 is a bit of a weird beast compared to the more common 350. It's got a bigger bore and a longer stroke, but it also has those "siamesed" cylinders, meaning there's no water jacket between the cylinder walls. This leads us to the most important thing you need to know before you even buy a set of heads.
The whole steam hole situation
If you're looking at aftermarket heads for a 400 small block, you'll notice that most of them don't come with "steam holes" pre-drilled. Because the 400 cylinders are pressed right up against each other, steam pockets can form in the water jackets. To stop the engine from overheating or blowing a head gasket, GM drilled small holes in the block and the heads to let that steam escape into the cooling system.
If you buy a set of high-performance aluminum heads designed for a 350, they probably won't have these holes. You'll have to drill them yourself using a head gasket as a template. It's not a hard job—you just need a steady hand and a drill press—but it's absolutely non-negotiable for a street-driven 400. If you're building a dedicated drag car that only runs for ten seconds at a time, you might get away without them, but for anything else, don't skip this step.
Thinking about combustion chamber size
One of the quickest ways to mess up a 400 build is picking the wrong combustion chamber size. Because the 400 has such a massive bore (4.125 inches), it naturally creates more compression than a 350 with the same heads.
If you grab a set of 64cc heads, which are standard for making power on a 350, your compression ratio might jump up into the 10.5:1 or 11:1 range depending on your pistons. That's great for a race car, but it's a nightmare if you want to run 91-octane pump gas without the engine pinging itself to death.
A lot of guys go with 72cc or even 76cc chambers to keep the compression around 9.5:1. This keeps the engine happy on the street and makes it way easier to tune. Of course, if you've got dished pistons, you can stick with those 64cc heads and get a nice, crisp throttle response. Just do the math before you click "buy."
Intake runner volume and airflow
Since the 400 is basically a 350 on steroids, it needs more air. If you put a tiny 170cc intake runner head on a 400, you'll have a lot of low-end grunt, but the engine will fall flat on its face by 4,500 RPM. It'll feel like a tractor engine—lots of pull off the line, then nothing.
For a healthy street 400, you should be looking at heads for a 400 small block with at least a 195cc or 210cc intake runner. The extra displacement can "swallow" a larger port without losing that bottom-end torque we all love. If you're building something more aggressive with a big cam, 220cc or even 230cc heads aren't out of the question.
The beauty of the 4.125-inch bore is that it unshrouds the valves. In a smaller engine, the cylinder wall is so close to the valves that it actually blocks airflow. On a 400, the valves have plenty of room to breathe, which is why these engines respond so incredibly well to high-flowing aftermarket heads.
Aluminum vs. Iron: Is the weight worth it?
Back in the day, if you wanted performance, you went and found a set of "double hump" iron heads and spent forty hours porting them. Nowadays, that's just not worth the effort. Aluminum heads have become so affordable that it's hard to justify sticking with iron.
Aluminum heads for a 400 small block save you about 40 to 50 pounds off the front of the car. That might not sound like much, but it helps with handling and weight transfer. More importantly, aluminum dissipates heat much better than iron. This actually lets you run about a full point higher compression ratio on the same fuel without knocking. If an iron-headed engine pings at 9.5:1, an aluminum-headed engine will usually be fine at 10.5:1.
That being said, if you're doing a "sleeper" build or you're on a super tight budget, there are some decent iron options out there from companies like Dart. But for most of us, the aluminum aftermarket is where the real power is hiding.
Choosing a brand that fits your goals
There are so many options when it comes to heads for a 400 small block that it can get a little overwhelming. You've got the big names like Edelbrock, AFR (Air Force Racing), Dart, and Trick Flow, plus a bunch of budget-friendly "no-name" castings.
- AFR (Air Force Racing): These are often considered the gold standard for street/strip builds. Their 195cc or 210cc Enforcer or Eliminator series heads flow incredibly well right out of the box. They aren't the cheapest, but you get what you pay for in terms of casting quality and valvetrain components.
- Edelbrock: Their Performer RPM heads are a classic choice. They might not flow as much as a high-end CNC-ported head, but they are reliable, they fit perfectly, and they've been proven on thousands of builds. They're a great "set it and forget it" option.
- Dart: If you want something beefy, Dart is the way to go. Their SHP (Special High Performance) heads are designed specifically for guys who want a rugged head that can handle some abuse.
- Budget Aluminum Heads: You'll see a lot of cheap heads on eBay or at big retailers. Some of them are actually decent castings, but the hardware (springs, valves, retainers) is often junk. If you go this route, it's usually smart to buy them bare and have a local machine shop set them up with good quality parts.
Don't forget the valvetrain
When you're picking out heads for a 400 small block, don't just look at the flow numbers. You need to make sure the springs are matched to your camshaft. If you're running a hydraulic roller cam, you need much stiffer springs than you would for an old-school flat tappet setup.
Most pre-assembled heads come with "standard" springs that are okay for mild cams, but if you're going with something high-lift, you'll need to check the max lift rating on those springs. There's nothing worse than finishing a build only to have a valve spring bind up and trash your brand-new engine on the first fire-up.
Also, check the rocker arm stud size. Most performance heads use 7/16-inch studs because they're stronger and flex less than the 3/8-inch ones found on stock heads. It's a small detail, but it means you'll need to make sure your rocker arms match.
Final thoughts on the 400 build
Building a 400 is a lot of fun because the torque is just instant. You don't have to scream the engine to 7,000 RPM to feel like you're moving. But because it's such a big, thirsty engine, the heads are really the gatekeeper for all that potential.
If you get the steam holes right, pick a sensible combustion chamber size for your fuel, and give it enough intake runner volume to breathe, your 400 will be a beast. It's all about balance. You don't need the biggest, most expensive heads on the market to have a fast car, but you do need the right ones for your specific setup. Take your time, do the math on your compression, and don't be afraid to ask a pro if you're unsure about the specs. Once those new heads are bolted down, you're going to feel the difference the first time you crack the throttle.