If you've ever popped the hood of a vintage sports car and seen a side draught carburettor sitting there, you know exactly why they're so legendary. They don't just look like a serious piece of machinery; they change the entire personality of an engine. While most standard cars from back in the day used down draught setups—where the air falls vertically into the intake—the side draught changes the game by letting air flow horizontally, straight into the cylinders. It's a simple change in direction, but it makes a world of difference when you're chasing performance.
There is something deeply satisfying about the way these things work. They aren't hidden under giant plastic covers or buried beneath a mess of sensors and wires. They're right there, usually in pairs or triples, showing off their polished velocity stacks. If you're a fan of classic motoring, the sight of a side draught carburettor is usually a sign that you're looking at something built for speed rather than just getting to the grocery store.
Why the horizontal flow actually matters
The big question most people ask is why you'd bother with a side draught in the first place. The answer really comes down to physics and space. On a lot of high-performance overhead cam engines, there just isn't much room above the cylinder head. By mounting the carbs on the side, you save vertical space. But more importantly, you create a much straighter path for the air and fuel mixture to travel.
In a traditional down draught setup, the air has to make a sharp ninety-degree turn to get into the intake manifold. Air doesn't really like doing that, especially when it's moving fast. It gets turbulent, it slows down, and you lose efficiency. With a side draught carburettor, the air basically has a straight shot. It enters the horn, passes through the venturi, picks up fuel, and goes right into the combustion chamber. That "straight-shot" design is what gives these cars that crisp, immediate throttle response that makes them so much fun to drive.
That unmistakable induction roar
We can't talk about these carbs without mentioning the noise. If you've ever heard a classic Alfa Romeo or an old Escort RS2000 at full tilt, that deep, guttural growl isn't just coming from the exhaust. A huge part of that soul-stirring sound is the "induction roar" from the side draughts. Because there's usually no restrictive air box—just some mesh filters or open trumpets—you can actually hear the engine breathing. It's a mechanical symphony that modern fuel-injected cars just can't replicate, no matter how much fake engine noise they pipe through the speakers.
The big names: Weber, Dell'Orto, and SU
When you start looking into buying a side draught carburettor, you're going to run into a few specific names pretty quickly. The undisputed king of the hill is Weber. The Weber DCOE is probably the most famous carburettor ever made. It's the gold standard for Italian and British sports cars, and for good reason. They are infinitely tunable, which is both a blessing and a curse. You can change every single part of the internal calibration—jets, chokes, emulsions tubes, you name it.
Then you've got Dell'Orto. Their DHLA models are very similar to the Webers, and some enthusiasts actually prefer them because they're known for having slightly better progression circuits, which makes them a bit smoother for driving around town. Then, of course, there's the SU. The British-made SU (Skinners Union) side draught is a different beast entirely. It's a constant depression carb, meaning it uses a sliding piston to regulate airflow. It's simpler than a Weber, having far fewer moving parts, but when they're set up right, they provide a smooth, torque-heavy power delivery that's perfect for classic roadsters.
The dark art of tuning
I won't lie to you—getting a side draught carburettor to run perfectly is a bit of a dark art. It's not like a modern ECU where you just plug in a laptop and click a few buttons. This is old-school mechanical engineering. You have to listen to the engine, look at the spark plugs, and maybe even use a bit of "mechanical intuition."
The hardest part for most people is synchronization. If you have two twin-choke Webers, you basically have four individual carburettors that all need to be pulling exactly the same amount of air at the same time. If one is slightly off, the engine will stumble, vibrate, or pop through the intake. You'll need a tool called a synchrometer to measure the airflow at the mouth of each carb. It takes patience, a steady hand, and probably a few burnt knuckles, but when you finally get them "in sync," the engine will idle like a Swiss watch.
Jets, chokes, and headaches
The beauty of a Weber-style side draught carburettor is that you can tune it for almost any engine displacement. But that's also why they can be a headache. You have to choose the right venturi (choke) size first. Too big, and the air speed drops, making the car bog down at low RPMs. Too small, and you'll choke the engine at high speeds. Once you get the chokes right, you move on to main jets, air correctors, and idle jets. It's a lot of trial and error. Most guys end up with a small box of brass jets that cost a fortune, but finding that "perfect" combination is incredibly rewarding.
Living with side draughts every day
Is a side draught carburettor practical for a daily driver? Well, that depends on your definition of "practical." If you live somewhere with massive temperature swings, you might find yourself tinkering with the idle mixture screws more often than you'd like. Carburettors don't have sensors to tell them it's -5 degrees outside or that you've just climbed 3,000 feet in altitude. They just keep dumping fuel based on how they were set up on the workbench.
Cold starts can also be an adventure. Most performance side draughts don't really have a traditional "choke" butterfly like a lawnmower or an old truck. They have "enrichment circuits." Usually, it's a matter of a few pumps of the throttle to squirt some fuel in, a turn of the key, and a prayer to the gods of internal combustion. But honestly, that's part of the charm. You're more connected to the machine. You have to learn its quirks—how it likes to be started, how long it needs to warm up before it'll take full throttle, and how it sounds when it's happy.
Why we still love them
With modern electronic fuel injection (EFI) being so efficient and reliable, you'd think the side draught carburettor would be a relic of the past. But walk through any vintage racing paddock or a local "cars and coffee," and you'll see they're still everywhere. Why? Because they have soul.
An EFI system is essentially a computer managing an appliance. A side draught setup is a mechanical masterpiece. There's something about the smell of unburnt fuel on a cold morning and the way the engine responds to a mechanical linkage that just feels real. You aren't just driving; you're operating a machine.
If you're thinking about putting a side draught carburettor on your project car, go for it. Just make sure you buy a good manual, get a synchrometer, and prepare to spend some quality time under the hood. It's a steep learning curve, but once you hit that first tunnel and drop a gear, hearing those trumpets scream, you'll realize it was worth every bit of the effort. There's simply nothing else like it.