Virtually all drag racers have a choice when it comes to selecting an intake manifold. Between carbureted and EFI intakes, there are literally endless of options, sometimes there are dozens for a single engine type. This can make selecting the right one an arduous task. Get it right and your engine will make great power in the range you desire, get it wrong and you may wonder why the power curve is flat. But more than just peak power, the correct intake will produce the most average output for your application.
Simply stated, the intake manifold takes air entering the engine (and sometimes fuel, too) and distributes it into the intake runners and then in to the cylinder heads. To do this in the most efficient fashion, engineers have designed manifolds with varying runner lengths, shapes and with different plenum sizes. Engineers work with constraints like hood clearance, cylinder head inlet flange angles along with carburetor or throttle body location.
Thusly, when selecting an intake, consider fitment for hood clearance, accessories or sensors that may be attached, material type and style as it relates to the actual design.
As you well know, increased airflow means the engine can burn more fuel and make more power. So, the goal is to get more air into the cylinders so more fuel can be burned per engine cycle, but it’s not as easy as just installing an intake with giant runners.
With this in mind, savvy racers, engine builders and manufacturers have developed improved designs that deliver more air and fuel to the engine. Over time there have been intake designs from mild to wild, all in the name of increased airflow. In almost every case, developers are faced with challenges based around carburetor placement, engine style (inline, 60-degree V, 90-degree V, opposed, etc.), plus other parameters such as hood clearance or class rules. Familiar designs include single-plane, dual-plane, and individual runner (IR). Of course, there are variants, including cross rams and tunnel rams, plus high- and low-rise intakes.
Most OEM (and many aftermarket) EFI manifolds found in production today are made from composite materials and incorporate individual runners with a common plenum. In drag racing, however, cast aluminum is the most common material. Reason being, it’s far lighter than steel, it’s economical, durable, and easy to produce and it dissipates heat nicely. Sheetmetal and billet aluminum are other choices, but they’re more expensive to produce. Most OEM manifolds are formed from composite materials, which are very light and do a fantastic job of resisting heat.
Common traits between all intakes will be a mounting flange for the throttle body or carburetor, plenum and runners (which connect the plenum to the ports in the heads). Of course, IR intakes do not use a plenum, but have runners and some form of throttle control.
Plenum and Runners
For any intake to be efficient it must be capable of providing adequate airflow for the displacement and rpm range of the engine. This requires a properly sized plenum with runners that are matched to the power plant.
As a general rule of thumb, long-runners promote lower-to-mid-rpm power, while shorter runners are more efficient at high rpm. “The runners are the brain of the intake,” said Tim Hogan of Hogan’s Racing Manifolds. “Changing the length and taper of the runner can move hp and torque up or down as well as broaden your torque curve. When designing an intake, we look at tuning with the firing order, including changing cross-sectional areas of individual runners, entrance angles to cylinder heads, and most importantly changing velocity from runner to runner.”
Remember, the intake valve is open for just milliseconds so in addition to volume, you need that column of air traveling in the intake runner to move at maximum velocity to achieve efficient cylinder filling. And, as rpm increases, this window of time shrinks, placing more importance on the intake manifold runner design.
Get Your Fill
Filling the cylinder starts with the downward stroke of the piston, which creates a pressure drop in the cylinder. When the intake valve opens, air rushes in, because pressure in the intake is higher than in the cylinder, and pressure always tries to equalize.
“Clearly, there is ‘magic’ in every manifold designer’s choice of runner taper (more tunes over a wider range and less tunes to higher peaks) and bell-mouth design,” said Billy Godbold, Valvetrain Engineering Group Manager, COMP Performance Group (FAST, RHS, COMP Cams). “However, these fall into the last percent of performance and are not something people are really going to tell you. But choosing the proper cross-sectional area and length is incredibly important.
“The manifold exit cross-section needs to match the head entry very closely. Going over a small cliff is fine, but air doesn’t like hitting a wall. This is going to be the first consideration on the runner cross-section when designing a manifold for a given head. Then you go back to that taper magic to work back to the opening. Once you have the cross-section, the runner length can be chosen following three considerations,” he explained.
“You also have to look at Helmholtz resonance equations,” Godbold added. “This will give you the proper tuning lengths based on rpm and other engine configuration parameters.” Remember, the column of air [in each runner] has momentum, and it’s still moving towards the cylinder when the intake valve shuts. When this occurs, the air slams into the valve and returns back up the port, before it is pulled back when the intake valve opens again. Tuning runner length can take advantage of this resonance resulting in improved volumetric efficiency and power. You want to take that information and work around some simulation programs to further research how longer or shorter runners should shape the torque curve and change overall mass flow,” Godbold added. “Finally, you want to test numerous runner configurations to see exactly how the performance changes with different runners with different camshafts and exhaust systems.”
As far as drag racing goes, the perfect example of intake tuning can be seen in Pro Stock. “NHRA Pro Stock has given us a great example of just how important it is to tune intake runner length to the rpm and cam timing events for any application,” said Godbold. “As rpm has been reduced to the currently mandated 10,500 rpm maximum spec, the intake runner lengths that were optimum for well over 11,000 didn’t work for proper shift recovery from the lower rpm.” You can also note the large plenum area needed to feed these 500-inch engines and the short runners that work best in the desired power range.
Today, racers do more than just select the type of manifold—they also look at materials. The process of 3D printing has given manufacturers the ability to design, build, and arrive at a finished product in hours, versus days or weeks. Composites also save weight and offer great resistance to heat. But composites are not suited for all combinations.
As we mentioned before, cast aluminum is still the most common material for aftermarket manifolds. This is due to cost and ease of manufacturing, plus they hold up well to boost. One example is the line of Holley Sniper intakes, crafted out of 1/8-inch thick T6061 aluminum. They are available for popular late-model applications, like LS and Ford modular engines. Holley also offers the MSD Atomic Airforce intake manifold that’s constructed from composite polymer material, so they’re extremely lightweight and use trick rubber O-ring style reusable seals.
Air Flow Research is also on the cutting edge of intake technology, with many exciting high-tech products. In addition to Titon composite manifolds, AFR offers a line of aluminum manifolds matched to its cylinder head offerings.
“Make sure you aren’t just buying an intake to fit, but rather an intake for specific application,” said Tim Hogan. “Some of the biggest advances we have seen in recent years come in injector technology, as well as EFI systems. The tunability and power has accelerated so greatly in this area over the past five years. And we use mostly 6061-T6 for strength, machinability and lightweight.”
How To Select Your Intake
Unless you’re strapped by class rules, an aftermarket intake is the way to go. Factors to consider include displacement, camshaft specs, cylinder head flow, gearing, weight and desired rpm range. And don’t forget hood clearance.
Godbold recommended finding answer to these questions: Are the runners and cross section appropriate for my application? Is the overall flow appropriate for my performance goals (including throttle area)? Is the plenum sized appropriately for my application? Are all the flow paths well laid out and the bellmouths properly shaped?”
Naturally, horsepower and torque are the main concerns for any racer, but different racers want a different power curve. “For example, if you’re a footbrake racer shifting at 5,500 rpm, you aren’t doing yourself any favors by choosing a big single-plane intake designed for 8,000 rpm,” said our source at Holley. “Even if that engine makes the highest peak horsepower on the dyno with that intake, you’re potentially sacrificing average horsepower and torque, which is what helps get your car off the line and through the gears. A lot of folks seem to forget that a good mid-rise dual-plane manifold will usually yield the most torque and horsepower through the rev range on your typical street-going or street-strip hot rod.”
Ultimately, arriving at the best intake takes research and proper testing. R&D is educational, and to many, the learning process is part of the challenge or “racing experience.” Thanks to technology and computer simulation and with years of experience, most manufacturers can get you home with off-the-shelf intakes that work quite well on most popular applications. But don’t forget help is out there. If you really want max power, phone the experts who are there to help.