Cylinders heads are sometimes referred to as an engine’s lungs. That being the case, the intake manifold can accurately be described as an engine’s nostrils. As important as airflow through the cylinder heads is, it’s ultimately limited by the efficiency of the intake manifold. In fact, the only way an engine can use all of the cfm potential the cylinder heads have to offer is if they’re bolted to an intake manifold that operates at 100-percent efficiency. Realistically, that’s nearly impossible to achieve, so the goal when designing an intake manifold isn’t so much to improve airflow, but rather to minimize the loss in airflow the cylinder heads will experience once the intake manifold is bolted to them.
Intake manifold design is always a compromise. That’s because a manifold designed with airflow as the top priority would never fit under the hood of a car. Underhood installation constraints are the biggest obstacles to overcome during the R&D process of intake manifold design.
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Intake Dynamics
At its core, the purpose of the intake manifold is to distribute air evenly to all eight intake ports. Unlike cylinder head and camshaft dynamics, which most enthusiasts find quite fascinating, intake manifold design is very boring in comparison. That’s because the most important, yet least interesting, aspect of manifold design is hood clearance.
As with the location of the cylinder head’s intake ports in relation to the valves, a runner design with the highest, most direct approach into the intake ports will usually produce the most horsepower. In essence, the intake manifold’s runners are an extension of the cylinder head’s intake ports, and the cross-sectional area of the manifold’s runners determines the power potential of an intake. Establishing the proper cross-sectional area of the runners is a balance between airflow and air velocity.
Unlike a cylinder head, whose intake port lengths can’t be changed, the length of the runners on an intake manifold can easily be tweaked, provided there’s adequate underhood space. Changing runner length affects the RPM at which pressure waves inside the manifold best promote cylinder filling. Longer runners increase low- and midrange torque, and shorter runners promote improved high-RPM horsepower. That’s because longer runners increase the distance the air must travel, which promotes increased air velocity at low RPM. However, longer runners become restrictive at high RPM.
Plenum size and shape are other factors that impact intake manifold design. The plenum is the central chamber inside the manifold positioned directly ahead of the intake runners. A larger plenum promotes top-end power production, and a smaller plenum has the opposite effect. The shape of the plenum, incidentally, greatly affects cylinder-tocylinder air distribution.
Because EFI intake manifolds are designed to fit under very low hood profiles, most of the important elements of an intake’s design—such as runner length, cross-sectional area, and plenum volume—can’t be measured without cutting the manifold in half. Consequently, the only way to gauge an intake manifold’s airflow potential is through reading the manufacturer’s published specs or by studying the performance of various intakes on combinations similar to your own. It’s certainly not the most scientific approach, but it’s by far the most effective.
The stock LS7 intake manifold is quite possibly the best-flowing small-block manifold GM has ever built. It can easily support 650 to 700 hp, and it provides more than enough airflow for the vast majority of street engines.
Unlike those of a carbureted intake manifold, the inner workings of an EFI manifold can’t be seen without cutting it in half. To maximize runner length under a steeply sloped hood, the driver-side intake ports draw air from the passenger side of the plenum, and vice-versa.
Early Stock Intakes
The early days of modifying the Gen III small-block’s induction system were simpler times, as the LS1, LS6, and truck intake manifolds were the only choices available. With the high-flow potential of the LS cylinder heads, enthusiasts quickly reached the limit of these factory manifolds. Nonetheless, certain situations still warrant their use, so it certainly doesn’t hurt to examine their virtues and drawbacks.
All factory GM Gen III/IV intake manifolds are built from nylon, not of aluminum or iron as in small-blocks of generations past. This not only reduces weight, but it helps minimize heat absorption from the lifter valley into the intake manifold. Unfortunately, the nylon material is more difficult to port than metal, especially because it’s just 3 mm thick. With the assortment of factory and aftermarket intake manifolds on the market, it often makes much more sense to upgrade to a higher-flowing unit rather than to port a stock intake.
Dollar for dollar, it’s tough to beat the performance of the GM LS3 and L92/L76 intake manifolds. Both designs are nearly identical, and each manifold can feed plenty of air for a 600- to 650-hp stroker combo. The cost for that caliber of performance is just $250 through GMPP.
The production LS6 intake manifold is an excellent choice for an engine with cathedral-port heads producing up to 600 hp. GM installed it on all 5.7L LS1 and LS6 engines from 2001 onward, so they’re plentiful on the used market.
Fuel Air Spark Technology was one of the first companies to come to market with an aftermarket LS-style intake manifold. The design has since been superseded by FAST’s LSXR line of intakes, but there are a fair number of the old-school units floating around on the used market. Gains of 20 to 30 hp over the stock LS6 intake manifold are common. (Photo courtesy of Comp Cams)
The intake manifold found on the original 5.7L LS1 is best suited for mild, stock-displacement engine combinations. Although the manifold performs well in the 350-hp applications it was designed to support, simply porting the heads and installing a mild hydraulic roller camshaft in an otherwise stock 346 can leave it gasping for breath. The manifold’s biggest flaw is its lack of plenum volume that can’t provide enough airflow when matched with a set of quality cylinder heads. In fact, very early aftermarket LS1 camshafts relied on reverse-split patterns—with more intake duration than exhaust duration—to make up the deficiencies of the stock LS1 intake. At anything beyond 500 hp, the LS1 intake is a poor choice. Throw extra cubic inches and cylinder head airflow into the mix, and the LS1 intake is an even less appealing option.
Realizing the limitations of the LS1 intake manifold, GM improved upon it tremendously while designing the LS6. The main difference between the LS1 and LS6 intake manifolds is that the LS6 unit incorporates a dropped-floor design. This adds much-needed plenum volume and boosts airflow significantly. Additionally, the throttle body opening on the LS6 intake is enlarged from 75 to 80 mm. These changes enable the stock LS6 intake to easily support 600 hp, and at roughly $500, it’s a much more reasonably priced alternative to many aftermarket units. From 2001 onward, the LS6 intake was used on all LS1 engines, as well as the LS6, which means finding a good deal on a used unit isn’t that difficult.
The Weiand Air Ram intake manifold features a removable bottom panel for access to the runners and extra-thick walls for porting. According to the manufacturer, it’s good for 25 hp over a stock intake.
Edelbrock’s Pro-Flow XT intake manifold for cathedral-port heads does away with the factory-style, cross-ram setup for a more conventional runner design. The result is a manifold with copious plenum volume and long tapered runners for a broad powerband. Provided enough hood clearance, Edelbrock advertises a 30-hp gain over a stock LS6 intake. (Photo courtesy of Edelbrock)
GM originally planned on using a common intake manifold on all early LS-series small-blocks, but installation constraints forced engineers to raise the throttle body inlet 3 inches in order to clear the cooling fan in truck applications. This led to the development of a truck-specific intake manifold, used on 4.8L, 5.3L, and 6.0L Vortec truck engines. The truck manifold’s long runners should hint at poor high-RPM performance, but that simply isn’t the case. In back-to-back dyno testing, the truck manifold often produces even more power than the LS6 intake while boosting low-end torque. The downside is that its taller design requires more hood clearance.
The black sheep of the factory cathedral-port intake manifolds is the LS2 unit. Because the LS2 small-block uses LS6 cylinder heads, the LS2 intake manifold is very similar in design to the LS6 unit. The LS2 manifold features a larger 90-mm throttle body opening, re-contoured runners, and slightly more plenum volume than the LS6 unit, but it actually flows less air. So, unless you can find a very good deal on a used LS2 intake manifold, in the wake of stock intakes, the LS6 unit is a better option.
Stock Rectangle-Port Intakes
Perhaps the greatest asset of the LS-series small-block is the outstanding performance capabilities of its stock components. GM added to this mystique by creating the rectangle-port cylinder heads that made their debut in the LS7 and L92. Aware that these new head castings would flow substantially more air than their cathedral-port forebears, GM went to work and created intake manifolds that could keep pace. The result is a family of state-of-the-art intake manifolds that pack tons of performance for the dollar. These intake manifolds share the same nylon construction as previous LS-style units, but they have larger 90-mm throttle body openings that hint at their high-flow potential.
Professional Products’ Power Plus Typhoon intake manifold for cathedral port heads is available with 85- and 96-mm throttle body openings. It boasts a removable bottom cover for easy runner access and includes a set of fuel rails. In forced-induction and nitrous applications, aftermarket intakes are less prone to blowing apart compared to their factory GM counterparts.
If the stock LS7 intake manifold isn’t enough, FAST offers its LSXR unit as an upgrade. Independent dyno testing has proven that the FAST LS7 intake is good for an additional 25 to 30 hp over the stock unit.
A unique feature of the FAST LSXR intake is that the top cover and runners can be disassembled from the base of the intake manifold. This provides tremendous flexibility for porting. (Photo courtesy of Comp Cams)
FAST’s new LSXR intake manifolds feature several improvements over the models they replace, such as longer runners, increased plenum volume, and larger throttle body openings. They’re available for both cathedral- and rectangle-port cylinder heads. In applications that have enough hood clearance for an LSXRT manifold, the unit’s additional plenum volume and runner length have the potential to produce an extremely flexible powerband throughout the RPM range. (Photo courtesy of Comp Cams)
The production LS7 intake manifold was the first of the factory rectangle-port units, and considering that it was designed to feed a 427-ci engine that turns 7,000 rpm, it’s a very impressive piece of engineering. The LS7 intake has proven to be extremely effective in stroker combos putting out 650 to 700 hp. Just as impressive is its ability to deliver a very broad torque curve from idle to peak power. Available for less than $400 through GMPP, the LS7 intake manifold is a raging bargain. In reality, very few stroker combos need more airflow than the LS7 intake can provide.
Thanks to Edelbrock and MSD, it’s now possible to run a carburetor on an LS-series small-block. Edelbrock’s carbureted LS intake manifolds feature an integrated MAP sensor. When a manifold is combined with the MSD ignition box, which has a harness that plugs into the factory coil packs and camshaft and crankshaft position sensors, the result is an LS small-block that doesn’t need a factory computer. (Photo courtesy of Edelbrock)
Like Edelbrock, GMPP offers a full line of carbureted LS-style intake manifolds. The LSX-DR intake loudly broadcasts its high-winding intentions with enormous runners and a mounting pad for 4500-series Holley carburetors.
Just because you bolt a carbureted intake to an LS small-block doesn’t mean you have to run a carburetor. Mast Motorsports’ Retro LS intake manifold includes an adapter that allows for the mounting of a throttle body onto the carburetor pad. The single plane intake comes with runners drilled for fuel injectors. Mast offers the complete kit with the manifold, adapter, injectors, and fuel rails.
Raising the carburetor allows for much more flexibility when designing a single plane intake manifold. The runners can be made very large for high-RPM breathing, but they can also be made very long to promote low-RPM torque production. Tapering down the cross-sectional area of the runner also helps enhance this dual purpose effect.
When converting a single-plane intake for EFI duty, there are two options for mounting a throttle body. Companies, such as FAST and Wilson Manifolds, offer carb-style throttle bodies that mount directly to the carb pad. Wilson also offers 90- and 100-degree elbow adapters that mount to the carb pad. This allows for the attachment of a conventional forward facing EFI throttle body, which is often necessary for hood clearance concerns.
Just as the LS7 cylinder heads inspired the design of the L92 heads, Gen IV small-blocks equipped with the L92 castings feature an intake manifold that doesn’t flow quite as well as the LS7 unit, but it is an exceptional performer in its own right. The LS3, L92, L99, and L76 all share a common intake manifold. Although there are very subtle variations between the LS3 and L92/L76 units, such as the use of noise-reduction covers on the LS3 intake and the mounting of the MAP sensor, the basic design and performance of both manifolds are virtually identical. Compared to the LS7 intake, the LS3 and L92 manifolds have runner and plenum designs better suited for the smaller-displacement short-blocks they’re bolted to. Providing enough flow to support 600 to 650 hp, at just $250 through GMPP, the LS3 and L92 intake manifolds are great values.
Designed for factory hydraulic roller small-blocks, it’s not surprising that both the LS7 and LS3 intake manifolds are very well matched to the RPM range associated with the hydraulic roller stroker small-block. The LS7 intake provides enough air for 427- to 454-ci motors to pull up to 6,500 to 7,000 rpm, and the LS3 intake works well in 396- to 416-ci motors in the same RPM range. Because the ports on the LS7 head and LS3/L92 heads are shaped differently, intake manifolds must be matched up accordingly. For instance, the LS3 intake manifold isn’t compatible with LS7 heads, and the LS7 intake won’t work on LS3 heads. Furthermore, even though the L92/L76 intake manifold is used in truck applications, it has a low-profile design that fits under steeply sloped car hoods.
LS6-Style Aftermarket Intakes
Whether it’s a solid roller screamer that turns 8,000 rpm or a 500-ci hydraulic roller behemoth that makes more than 725 hp, there comes a point where even the high-flow factory intake manifolds run out of breath. Fortunately, companies, such as FAST, Holley, Wilson, Weiand, BBK, Professional Products, and Edelbrock, offer an assortment of intake manifolds for both cathedral- and rectangle-port cylinder heads. Early aftermarket intake manifolds were based on the stock LS6 design. Because of that, they offer slight improvements in airflow and horsepower, but exactly how much is debatable. Holley, Weiand, BBK, and Professional Products all offer aluminum intake manifolds that fall within this design category, and independent dyno testing results on both 5.7L and stroker motor combos are all over the map. Performance gains over the stock LS6 range anywhere from 10 to 25 hp.
By raising the carburetor or throttle body, air can travel in a direct path into the intake ports. Air doesn’t like to bend or change directions, so this arrangement yields dividends in airflow and power.
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A dual-plane intake manifold produces more low-end torque than a single-plane intake, at the expense of high-RPM power. They’re best suited for heavy street cars that will rarely exceed 6,500 rpm.
If readily available factory or aftermarket intake manifolds are deemed unsuitable for an application, there’s always the option of stepping up to a custom sheetmetal intake. Each one is custom fabricated for a specific application, so although a custom intake is extremely expensive at $1,500 to $2,000, it’s capable of performing extremely well.
Common features among these LS6-style intake manifolds include aluminum construction, thicker runner walls, and larger throttle body openings. Compared to the plastic factory manifolds, plumbing in nitrous injection nozzles and porting the runner is much easier on the cast-aluminum units. Furthermore, they’re more durable under high-boost, forced-induction applications. Considering that these manifolds were designed before aftermarket blocks enabled the huge displacement figures that are common today, they’re best suited for smaller engines between 346 and 396 ci. Even so, LS6-style aftermarket intakes only cost $100 to $200 more than a stock LS6 intake, so they can work quite well on a smaller displacement stroker combination.
Without question, Fuel Air Spark Technology’s (FAST) cathedral-port intake manifolds have set the performance benchmark for LS6-style intakes. FAST was the first to release an aftermarket LS intake, and its manifolds have a consistent track record of proven performance in a diverse range of applications. They feature high-strength polymer construction that’s 30 percent stronger than stock and a unique three-piece design that allows engine builders to disassemble the intake for easy access to the runners should porting be necessary. The runners on the FAST manifold have a large cross-sectional area that tapers as the runners approach the cylinder heads. This affords excellent top-end power while retaining air velocity to preserve low-end torque. Gains of 20 to 30 hp are common over a stock LS6 intake in hydraulic roller stroker combinations. Furthermore, the FAST intake is offered with 78-, 90-, 92-, and 102-mm throttle body openings.
FAST’s 102-mm LSXR intakes are the latest units in the company’s original LS manifold design evolution. It would be somewhat inaccurate to call them LS6-style intakes, because their design has several significant improvements over the standard LS6 architecture. In addition to their larger throttle body openings, they feature an increase in plenum volume and longer runners. Additionally, all eight runners can be removed individually for greater porting flexibility. Building upon these improvements, FAST’s 102-mm LSXRT intake offers the same basic improvements as the LSXR, but with even more plenum volume and runner length. Given enough hood clearance, the LSXRT is tough to beat in high-end street/strip or race applications that require a broad powerband with outstanding high-RPM airflow.
Aftermarket Rectangle-Port Intakes
The factory LS7 intake is an exceptional piece of engineering, but it was never intended to be pushed beyond the 700-hp mark that’s becoming more common with 450-plus-ci short-blocks and serious cylinder heads that now define stroker Gen III/IV small-blocks. Furthermore, because the stock LS7 intake isn’t compatible with L92/LS3 cylinder heads, enthusiasts opting for these castings needed a quality aftermarket intake manifold. FAST responded again with its 102-mm LSXR intakes, which are designed specifically for rectangle-port cylinder heads. Two versions are offered, one for LS7 cylinder heads and another for L92/LS3 heads. Independent third party dyno testing of the LS7 LSXR intake on a 500-ci hydraulic roller small-block showed an increased output from 720 to 750 hp over a factory LS7 manifold.
As a company famous for its diverse range of intake manifolds, it’s hardly surprising that Edelbrock offers a full line of dual- and single-plane LS-style intake manifolds, as well as EFI intakes. Edelbrock’s Super Victor intake manifolds are famous for their high-RPM breathing capabilities. (Photo courtesy of Edelbrock)
Since cylinder heads are only part of the induction equation, GMPP released a new line of LSX intake manifolds to go along with its LSX-LS3, LSX-LS7, LSX-CT, and LSX-DR heads. As with most carbureted LS-style intakes, they can also be drilled for fuel injectors. (© GM Corp.)
Porting the stock throttle body is a common trick that yields noticeable improvements in airflow. Katech’s LS1 units feature a smoothed throttle body radius, and the company claims that they flow an additional 5 to 10 percent of air over stock units.
Factory throttle bodies are either actuated by a throttle cable or an electric motor. A drive-by-wire throttle body can be used in an engine swap application, but it must interface with a matching computer and a stock gas pedal.
Carbureted Intakes
Edelbrock stunned the hot rodding community when it released a dual plane carbureted intake manifold for the LS1 in 2003. The Performer RPM LS1 intake manifold, designed for cathedral port heads, works in conjunction with an MSD ignition control module that replaces the stock computer; the manifold also allows replacing the factory EFI system with a carburetor. As with most dual-plane intakes, the Performer RPM places a priority on low- and mid-range torque over top-end power, with an operating range from idle to 6,500 rpm. More importantly, however, the success of the Performer RPM and its overwhelmingly positive reception by the hot rodding public set the quintessential paradigm shift in motion.
Almost all aftermarket throttle bodies are cable-actuated, which explains their popularity in engine swap applications. FAST’s billet Big Mouth units are available in 92- and 102-mm diameters. (Photo courtesy of Comp Cams)
Professional Products’ 80- and 85-mm throttle bodies for early Gen III engines are budget priced at $190. Each one includes an adapter plate that fits between the intake manifold and throttle body. This is said to smooth the throttle-body-to-manifold transition for improved airflow.
Ideally, the diameter of the throttle body and the intake manifold opening should be the same. Not surprisingly, FAST offers 102-mm throttle bodies to go along with its 102-mm LSXR intake manifolds. (Photo courtesy of Comp Cams)
Shortly after the launch of Edelbrock’s ground-breaking carbureted intake, companies, such as GM Performance Parts and Performance Induction, got in on the action with carbureted manifolds of their own. From these companies, there are now dozens of carbureted intakes in single-plane and dual-plane configurations for everything from stock-displacement motors all the way up to 10,000-rpm full race applications.
The significance of these intakes is two-fold. Not only do they enable hot rodders to enjoy the simplicity of carburetors on late-model LS engines, but they also allow EFI to be retrofitted onto these carb-style intakes. This is easily accomplished by drilling holes into the intake runners for fuel injectors and bolting a throttle body onto the carburetor pad. A conventional forward-facing throttle body can also be used by holding a 90-degree elbow adapter to the carburetor pad.
For high-end street or race applications where hood clearance isn’t an issue, a single-plane intake offers irrefutable advantages in horsepower. They typically have very large plenum volumes and generous cross-sectional area to assist with high-RPM breathing, as well as long runners to help minimize low-RPM power loss. In the average 440- to 460-ci hydraulic roller stroker combo with 250 to 260 degrees of duration at .050 inch, a single-plane intake usually sacrifices 20 to 30 ft-lbs of torque in the mid-range, compared to an EFI intake, but it makes up for it with an additional 50 to 60 hp beyond 6,500 rpm. Furthermore, single plane intakes extend the useable powerband by as much as 500 rpm. So, in applications with lots of hood clearance where giving up some low- and mid-range torque isn’t a big deal, a single plane intake manifold offers huge dividends in horsepower.
GM stepped up to a larger 90-mm throttle body in the 6.0L, 6.2L, and 7.0L Gen IV small-blocks. These units are capable of moving lots of air, and they can support more than 700 hp.
If using a 90-degree adapter on a single plane intake, it must be matched with a throttle body recommended by the manufacturer. Wilson Manifolds’ square flange throttle bodies bolt to the company’s elbows, but they aren’t necessarily LS-specific parts.
Throttle Bodies
As GM continually upped the horsepower ante with successive iterations of the LS-series small-block, throttle body diameters increased accordingly. The original 5.7L LS1 came equipped with a 75-mm throttle body, and the LS6, LQ4, and LQ9 received 80-mm units. By the time the 6.0L, 6.2L, and 7.0L Gen IV small-blocks entered production, the factory throttle bodies had grown to 90 mm.Additionally, aftermarket companies, such as Edelbrock, BBK, Wilson, FAST, Holley, and Summit, offer aftermarket units ranging from 80 to 102 mm.
FAST and Wilson Manifolds both offer throttle bodies that bolt directly to the carb pad on a single-plane intake. Naturally, they resemble a carburetor without a base plate. (Photo courtesy of Comp Cams)
Unlike a carburetor, the throttle body in an EFI engine doesn’t directly impact the amount of fuel that’s metered into the air intake charge. Consequently, EFI motors aren’t nearly as sensitive to changes in throttle plate diameter. A good rule of thumb to follow is that the throttle body diameter should match the inlet opening of the intake manifold. For example, if you are installing a factory LS3 intake manifold on a stroker engine buildup, its 90-mm inlet opening should be paired with a 90-mm throttle body. There aren’t any adverse affects of installing a throttle body that’s slightly larger than the inlet opening of the intake manifold, but it won’t improve performance, either.
Almost all aftermarket throttle bodies are cable-driven, whereas GM installed both cable-actuated and driveby-wire units in production cars. For stroker engine builds utilizing a cableactuated throttle body, switching to an aftermarket unit is a bolt-in affair. Alternately, a factory GM drive-by-wire unit can be retrofitted by installing a GM gas pedal that interfaces with the throttle body. For stroker LS buildups destined to power GM vehicles originally equipped with drive-by-wire throttle bodies, the stock 90-mm unit is the best option. This factory throttle body provides plenty of airflow, and it has proven to be effective beyond 700 hp. Other alternatives include converting to a cable-actuated throttle body or porting the stock unit. Smoothing out the throttle body radius and putting a bullnose finish on the leading edge of the throttle blades can boost airflow by 5 to 10 percent.
Written by Stephan Kim and Posted with Permission of CarTech Books
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