Intake Manifolds: Born To Rev

Intake manifold
Intake manifolds
are a fascinating part of the internal combustion engine. Their design has a great deal of influence on how the engine performs. The simplest change can drastically alter how the engine feels under power.

Your engine in its most basic form is an air pump. As the piston moves down the cylinder during the intake stroke, it’s pulling a fuel/air mixture through the intake valve. Above that, your fuel system (unless your car has direct injection) is delivering fuel through the open intake valve. The oxygen supply needed for combustion is coming in at the same time, via the intake manifold.

And why should you know this? Because the design of the intake manifold has a significant effect on the output of your engine.

The Long and Short of It

Back in the days of prohibition, moonshiners started modifying their cars with the purpose of getting away from the law. One of the quickest ways to get more power out of a car is to allow the engine to breathe more efficiently. If an air intake is like your nose, then the intake manifold is like a pair of lungs. You can sniff all you want, but if your lungs aren’t up to the task of taking on that air, you’re going to have trouble.

Intake manifolds are designed to evenly distribute air to each cylinder of the engine. The more cylinders an engine has, the more complex this becomes. Older vehicles were pretty uniform in the way their manifolds were designed. Each cylinder has its own dedicated “runner” that delivers the air to the cylinder through the intake valve(s).

The tricky thing is, the length and diameter of the intake runners affect where you get your power. If your intake runners have a larger diameter, you’ll have higher horsepower, while a smaller diameter has less power but will allow you to reach that peak power more quickly. Longer runners are good for low-end power, while short ones are best for when you need the power in the upper registers of your power band. This is where modern technology comes in handy.

Power Where You Want It

Engine bayOlder cars had to find the happy median with their intake manifold design to perform the best for their typical scenario of use. Many new cars can have the best of both worlds — or at least a broader range of the two. Commonly called the DISA valve, a butterfly valve is built in to their intake manifolds to adjust the length of the intake runners depending on the throttle position. This ingenious little device is quite common on BMWs, for example. It helps bring a wider range of performance to a vehicle without having to swap the intake manifold out for specific power needs.

If you’re modifying an older car and you want more power, you’ll have to stick to the more traditional method. Depending on where you want your power, you’ll want a specifically designed manifold for that purpose. Take this Edelbrock Performer intake for example. You’ll see that in the product description, it’s designed to run at idle to a 5500 RPM limit and will provide a broad torque curve with excellent throttle response and mid-range power. This particular setup would be good for a muscle-car owner who is looking for good power on the street. Good throttle response and mid-range power is what you want if your goal is to be the stoplight drag king. This Edelbrock Performer RPM intake, in contrast, is built with high-end power in mind and would be better suited for situations in which top speed is the end goal.

When To Replace Your Manifold

You may not be looking to soup up your daily driver, but knowing how your car works is always a benefit to a car owner and can save time and money. Most intake manifolds on late-model cars are made of plastic. Over time they may crack, warp, or have a bad gasket. Typical symptoms of a faulty intake manifold would be hard starting, stumbling during acceleration, and often a “check engine” light. A leak in the intake manifold would likely set off a code that your engine is running too lean or getting too much air. A lean running engine could lead to premature detonation in the cylinder, which leads to major damage of the engine.

Have you found the perfect setup for your car? Let us know what you’re running in the comments below!

The Weird World of Intake Manifolds

 

Intake manifolds are often a hot rodder’s upgrade part but are otherwise mostly ignored. Every minivan on the road has an intake manifold feeding an air and fuel mixture to the cylinder heads, so they don’t have the sexy and complex reputation of a turbocharger. Still, throughout the history of internal combustion, there have been several intake manifolds that left us scratching our heads. Here are a few of the weirdest.

Source | Andy Jensen

If You Can’t Dodge It, Ram It

This one causes a puppy-head-tilt reaction in everyone who sees it for the first time. The Chrysler B-block was a standard and unexciting people-moving engine by 1960 until it was topped by the unique cross-ram manifold. The dual four-barrel carbs sit way out over the exhaust manifolds and run the air charge through a gigantic, 30-inch runner to the opposite side intake port. Yup, the driver’s side feeds the passenger side cylinders, and vice versa. Chrysler rated the 361 cross ram at 310 horsepower, which wasn’t bad considering the muscle-car wars hadn’t really started yet. While it wasn’t a drag strip warrior due to losing power in higher RPMs, the cross-ram-equipped car had an impressive 435 lb-ft of torque down low, thanks to the extremely long runners.

Defying Gravity

What do you do when the traditional intake manifold world gets boring? Turn it upside down — or in this case, sideways. Sidedraft carbs were needed due to packaging constraints on cars with average-size engines in a small engine bay, like the Jaguar XK120 and Datsun 240Z. While North America was familiar with a standard Holley sitting directly on the manifold, the sidedraft style meant the Weber or SU carbs were mounted 90 degrees sideways, feeding a vertically mounted intake manifold. It’s easy to assume that gravity pulls fuel from the carb bowl into the manifold, which means sidedrafts shouldn’t work. Fortunately, the Venturi effect, which draws the air and gas mixture into the engine, is far more influential than gravity, meaning the intake manifold works just the same as if it were installed on top of the engine. If you want really weird-looking, there’s aftermarket kits to put sidedrafts on a rotary.

Truck Engine in a Sports Car

Remember the ’80s? No? Well, lucky you. The rest of us suffered for a bit while the manufacturers tried to figure out how to balance horsepower with emissions. GM’s solution was electronic-fuel injection with the tuned port intake (TPI) manifold. The distinctive long curved runners connecting the plenum to the lower manifold are a source of the engine’s torque, with a tuned length that takes advantage of pulses in the air charge at low and mid RPM. Right as the pulse of air is about to slam into the closed intake valve, it opens, sending a blast of slightly compressed air into the chamber. While only generating 245 horsepower, the TPI could make an impressive-for-the-time, 345 lb-ft of torque. If that isn’t oddball enough for you, the ’85 to ’88 V8s had nine fuel injectors.

Looks Like a Bad Day at the Factory

A transverse (sideways) mounted intake manifold make sense on a transverse mounted engine, like the modern Toyota Corolla. The cylinders are in a line between the wheel wells, and the intake manifold lines up with the cylinders left to right. Things get quite a bit more confusing when looking at the engine bay of the Infiniti Q45. The Nissan VH series engines were longitudinal (front to back) V8s driving the rear wheels but topped by a spider-like intake manifold sitting sideways as if it were front wheel drive. The reasoning behind the strange layout is unclear, but it was probably for packaging or emissions. This reminds us that the orientation of the intake manifold does not always determine the drive wheels. For further proof, look to the ’90s Acura Legend. While the engine drives the front wheels, the longitudinally mounted manifold suggests the rear wheels are driven. Oddly, this layout in a modern Japanese EFI sedan recalls the classic Oldsmobile Toronado.

While these oddities are no longer in production (excluding some as aftermarket upgrades), they solved an engineering dilemma of their times.

If you know of any other unusual intake manifolds that should be on this list, make sure to let us know in the comments.