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.
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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.
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.
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.
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.
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.
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.
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 smallerdisplacement 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 thirdparty 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.
Edelbrock stunned the hot rodding community when it released a dualplane carbureted intake manifold for the LS1 in 2003. The Performer RPM LS1 intake manifold, designed for cathedralport 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.
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 fullrace 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 carbstyle 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, singleplane 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 midrange torque isn’t a big deal, a singleplane intake manifold offers huge dividends in horsepower.
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.
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 Barry Kluczyk and Posted with Permission of CarTechBooks