What's the Deal with 624 Chevy Heads? Unpacking the SBC's Black Sheep
Alright, let's chat about a particular piece of cast iron that often sparks a lively debate among small block Chevy enthusiasts: the 624 Chevy heads. If you've been around the SBC world for a bit, you've probably heard these mentioned, usually with a groan or a roll of the eyes. But what's the real story behind them? Are they truly the boogeyman of cylinder heads, or do they have a hidden charm for specific applications? Pull up a chair, grab a cold one, and let's dive into the often-maligned, sometimes misunderstood, world of the 624 casting.
The Introduction: A Reputation Precedes Them
First things first, when someone says "624 heads," they're almost certainly talking about the casting number 3338624. These weren't exactly performance jewels from the factory, and their reputation amongst hot rodders isn't exactly stellar. In fact, many people treat them like the automotive equivalent of a fruitcake at Christmas – you appreciate the thought, but you're probably going to re-gift it.
You see, for decades, gearheads have been swapping heads on their small block Chevys to gain horsepower, increase compression, or simply update an older engine. And in that grand tradition, the 624s are usually the ones you're swapping off, not onto. But why such a bad rap? Let's peel back the layers and understand where they came from and what makes them tick – or, perhaps more accurately, what makes them not tick for performance.
A Walk Down Memory Lane: The Era of Emissions and Economy
To truly understand the 624s, you've gotta put them in context. These heads largely came out of the late 1970s and early 1980s. What was happening in the automotive world back then? Well, a little something called the oil crisis and a growing emphasis on emissions regulations. Car manufacturers, GM included, were scrambling to meet stricter fuel economy and emissions standards, often at the expense of outright power.
The 624 heads were a product of this era. They were commonly found on 305 cubic inch small block Chevys, and sometimes on lower-compression 350s. These weren't the fire-breathing muscle cars of the '60s; these were cars and trucks designed for everyday driving, trying to squeeze out a few more miles per gallon while keeping the smog police happy. So, right off the bat, you can probably guess that their design priorities weren't exactly "max horsepower at 6500 RPM."
The Nitty-Gritty: Unpacking the Specs
Let's get down to the brass tacks and talk about what these heads actually are.
Casting Number and Chamber Size
As we mentioned, the key identifier is the casting number 3338624. One of the most significant characteristics of the 624 heads is their relatively large combustion chamber size, typically around 76cc. Now, if you know anything about compression ratios, you know that a larger chamber volume generally means a lower compression ratio for a given engine displacement. This was a deliberate design choice to help engines run on lower octane fuel and reduce pinging, which was a common issue with early emissions-controlled engines.
Valve Sizes
Stock 624 heads usually came with modest valve sizes, often 1.72" intake and 1.50" exhaust, though some might have had 1.94" intake valves. These aren't huge, especially when you compare them to older, higher-performance SBC heads that sported 2.02" intake valves. Smaller valves, coupled with that large chamber, weren't exactly screaming "flow like a river!"
The Dreaded Swirl Port Design
Ah, now we get to the real "culprit" (if you want to call it that) behind the 624's reputation: the swirl port design. This is the feature that really sets these heads apart from many other SBC castings, and not in a good way for performance applications.
What exactly is a swirl port? Imagine the intake runner of the cylinder head. Instead of a smooth, direct path for the air/fuel mixture to enter the cylinder, the swirl port design incorporates a "hump" or a "dam" that forces the incoming mixture to swirl or tumble as it enters the combustion chamber. The idea behind this was to promote better atomization of fuel and a more complete burn at lower engine speeds. This, in turn, helped with fuel economy and reduced emissions, especially at idle and part throttle.
Sounds good on paper, right? For a stock daily driver of the late '70s, it certainly was. However, there's a huge trade-off: airflow at higher RPMs. That swirl-inducing hump becomes a massive restriction when you're trying to shove a lot of air and fuel into the cylinder quickly. Think of it like trying to run through a maze versus running on a straight track. The maze might make you think more efficiently, but the straight track gets you to the finish line faster.
The Performance Dilemma: Why Hot Rodders Run Away
So, you can probably see where this is going. For anyone building a performance engine – be it a street machine, a drag car, or even a spirited cruiser – the 624 heads present a significant challenge.
- Poor High-RPM Flow: That swirl port design just chokes the engine when you try to spin it past about 4000-4500 RPM. You'll hit a wall, and no amount of cam, carb, or exhaust tuning is going to fix a fundamental restriction in the cylinder head itself.
- Low Compression: The 76cc chambers, especially on a 350, result in a very low compression ratio (often in the 8.0:1 to 8.5:1 range). Low compression means less power. It's just physics, my friend. While you could mill these heads to reduce chamber volume, you'd need to take off quite a bit of material to get into a desirable compression range for performance, and that costs money.
- Porting Challenges: Many heads respond well to professional porting, which can significantly improve airflow. With the 624's swirl port, however, effective porting is incredibly difficult and often not worth the effort or cost. Removing that hump basically involves re-sculpting a significant portion of the intake runner, and even then, the core design isn't ideal for flow. You're trying to turn a pig's ear into a silk purse, and sometimes, you just end up with a very expensive pig's ear.
Are They Ever a Good Idea? A Niche Application
Okay, so they're not for your nitrous-fed beast or your screaming track car. But does that mean they're utterly useless? Not necessarily! Every component has its place.
- Bone Stock Replacement: If you're restoring a numbers-matching late '70s or early '80s vehicle and you need to replace a damaged head, finding another 624 might be the most historically accurate and cost-effective option for a bone stock rebuild.
- Budget Truck Engine/Daily Driver: Let's say you've got an old work truck, a beat-up daily driver, or just need to get something running on the absolute tightest budget. If you stumble upon a set of good, used 624s for cheap (or free!) and your goals are simply reliable transportation, low-end torque, and decent economy (for an SBC, anyway), they can work. They'll offer good torque down low thanks to that swirl design, and if you pair them with a small, economy-oriented camshaft, you might be surprised how drivable the combo is. Just don't expect to win any drag races.
- Mud Bogger/Low RPM Torque Beast (with caveats): In some super niche applications where very low RPM torque is paramount and high RPM is never seen (think certain mud bogging or rock crawling setups where you're just chugging along at 2000 RPM), the swirl port might offer some benefits. But even then, there are usually better options available for similar money.
What to Do If You're Stuck with Them (or Find Them Cheap)
So, you found a set for free, or they came on a junkyard engine you snagged. What can you do if you really want to make them perform better?
- Valve Job: A good multi-angle valve job is always beneficial for any head. It won't overcome the swirl port, but it will improve what flow is available.
- Springs and Retainers: If you're putting in a slightly beefier cam (and by "beefier" for 624s, I mean very mild), you'll need valve springs and retainers that can handle the increased lift and RPM. But honestly, you won't be spinning these heads high enough for radical springs to be truly necessary.
- Milling for Compression: You can mill the heads to reduce the chamber volume and raise compression. However, remember the costs involved, and realize you're still fighting the swirl port. Every dollar you spend on 624s might be better invested in a different set of heads.
- Match with a Mild Cam and Intake: If you're going to use them, pair them with a camshaft designed for excellent low-end torque and peak power below 5000 RPM. A dual-plane intake manifold would also be a better match than a single-plane, which is designed for higher RPMs.
The Bottom Line: Know Your Goals
In conclusion, the 624 Chevy heads are what they are: a product of their time, designed for specific compromises. They're not the ideal choice for a performance build, and if you're looking to make serious horsepower, your money is almost always better spent on other SBC castings (like Vortecs, or older performance heads like the 461s or 186s, even the venerable 441s or 882s are often better candidates for porting).
However, they're not completely worthless. For a dead-stock restoration, a rock-bottom budget daily driver, or a utility engine where low-end grunt and economy are king, they might just fit the bill. Just remember their limitations, and approach them with realistic expectations. They're definitely not the head you dream about, but they might just be the head that gets your old beat-up project moving down the road without breaking the bank. And sometimes, that's all you really need.