Of course no book on cylinder heads would be complete without discussion of its supporting components.
Available intake manifolds and rocker arms should also be considered when choosing cylinder heads. It may add to the cost or help save money; it may add to the performance or take away from the performance of the heads.
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Unlike with other engines, aftermarket does not necessarily mean better. The OEM designs are durable, light, and well made. However, the more modified the engine—meaning higher RPM and higher-lift cam—the more likely OEM parts hinder the performance.
There are now a wide range of intake manifolds available for LS-series engines of all power levels. Both front-fed and top-fed (carb-style) intakes are offered. The factory intakes and several aftermarket offerings are made of composite, which is light-weight and sheds heat. Traditional aluminum castings also exist, as do high-end custom sheet-metal units. No matter what your needs, there is certainly an intake available to suit it.
For most street cars, it is hard to go wrong with an OEM GM intake manifold. All are made of a very durable composite material (glass-fiber-reinforced nylon) that disperses heat much better than aluminum, which helps keep heat soak and resulting detonation to a minimum. The original LS1 intake manifold used in the 1997–2000 Corvette and 1998–2000 F-Body is one of the least desirable for cathedral ports; however, it is still quite capable. It features long, 262-mm runners with a decreasing cross-section to increase velocity as the air moves into the cylinder head; this design is considered to be of little sacrifice to high-RPM power with a gain in low-RPM torque.
Like the LS1, the LS6 intake manifold, originally designed to complement the LS6 head for the 2001 Corvette Z06 and completely replace the LS1 intake on all models, was also designed for low hood clearance. In addition to the lack of EGR provisions and steam-tube clearance, the LS6 intake differs from the LS1 in its plenum volume, runner shape, and volume. Jumping from .536 to .541 liters proved much more suitable for higher-revving applications such as the new LS6, and with the revised camshaft and cylinder head specs on the later LS1s. The increased plenum volume (5.19 liters in the LS6) made it the largest among cathedral intakes.
Though not the most aesthetically pleasing, many of the truck intake manifolds have some of the most desirable runner lengths for producing torque. Unfortunately, the longer runner lengths most often mean decreased plenum volume. On the LQ4, LR4, LM7, and LQ9 (4.8, 5.3, 6.0, and 6.0 liters), the plenum volume is only 4.0 liters, though the runners are 263 mm and only .513 liters. Although great for throttle response and velocity, this design strains the engine to pull air through the entire intake system instead of having it readily available (in the plenum).
However, all of this may spell the perfect complement to a centrifugal supercharged application. The rectangular-port L92 truck intake follows this exact same formula, and as a result gives up as much as 43 cfm at high lift when bolted to a set of ported heads.
If higher RPM power is your goal, the L76 and LS3 intake manifolds clearly have the advantage. Designed for a 7,200-rpm redline, the LS7 intake also excels at high RPM. However, both the LS7 and LS3 leave room for improvement in even street-based combos.
Among the factory intake manifolds, it is important to note three very important differences. All Gen IV engines (LS2, LS3, L76, L92, etc.) utilize electronic, or “drive-by-wire,” throttle bodies. All of these intakes also utilize larger 90-mm throttle bodies, up from the Gen III’s 75 mm. Last, but not least, Gen IV intakes are constructed utilizing a traditional plastic-injected molding process with three separate pieces, instead of the single piece, that are vibration (or sonic) welded. Time and again, the LS2 and LS6 intake manifolds have been dyno’d back to back, and the LS6 always comes out on top despite the LS2’s substantially larger throttle body, slightly larger plenum, and revised runners.
Pete Incaudo at VMAX Motor-sports has conducted several tests comparing the LS2 and LS6 intake manifolds, from which he concluded that a miniscule amount of air leaks past those welds internally and this effect is amplified with increased head flow and higher RPM. His solution was to simply port the intake to allow it to flow more air since repairing (by sealing it) was deemed too difficult, and adhesives would have too high a rate of failure.
There are plenty of alternatives to running an OEM intake manifold that are still suitable for a street car, and that can even keep a similar look and fitment. The perfect example, and quintessential upgrade for most people, is the FAST LSX series of intakes. Originally, Wilson Manifolds had developed a 78- and 90-mm LSX intake, which became the gold standard for street cars and even worked well for many race cars. FAST improved upon the design years later, adding the 92-mm Big Mouth throttle body. However, a complete revamp stepped up the game considerably. The 102-mm LSXR intake for the LS7 and LS3 was the first to market, followed by the LSXRT cathedral truck manifold and LSXR cathedral.
Unparalleled plenum volume and removable and tuneable individual runners separate the LSXR from every other intake on the market. Stock cubicinch combos typically see gains of 8 to 15 hp, and boosted or large-cube strokers can see as much as 50 hp with no loss in low-RPM power. If you simply don’t like plastic, BBK Performance and Weiand each make a great upgrade to the factory LS1 that is made from aluminum but with the factory profile. They are both available in several finishes, offer a 10- to 15-hp improvement out of the box (over an LS1 intake), and are also able to be ported for further gains.
As good as the stock-style intakes are, they have several inherent flaws. The front entry is known to cause airflow distribution issues that can pose a serious problem on high-boost or nitrous combinations. Those constructed of plastic are also known to come apart at high boost and during a nitrous backfire. Even the 102-mm FAST LSXR fails to keep up at 7,800-plus-rpm. To solve all of these issues, and even allow the use of a carburetor, several companies are producing some great cast-aluminum alternatives.
Chevrolet Performance makes several 4-barrel intake manifolds for the LS2 (cathedral), LS3/L92, and LS7 that come with (untapped) injector bosses cast in, and a whole new line-up of LSX manifolds that are thicker to allow more material for porting. Chevrolet Performance has race-only intake manifolds to accompany the unique port design of the LSX-CT and LSX-DR heads, which have an extra set of injector/nitrous bosses and a 1-inch raised mounting pad.
All Pro and Mast Motorsports each make two-piece cast intake manifolds to accompany each of their own unique port designs, and Mast also has intakes for standard-port designs (LS1, LS3, and LS7). Both are designed for purpose-built race engines and ease of porting.
Edelbrock’s line of manifolds is a little more flexible in use. The Performer RPM is designed for 1,500 to 6,500 rpm (as is the RPM Air-Gap Dual-Quad), and the Victor Jr. (3,500 to 7,500 rpm) and Super Victor (3,500 to 8,000 rpm) hit a higher range and are more competition-only.
Holley also has a few manifolds in this ilk, such as the 2X4 Dual Plane Mid-Rise for mid-range and top-end power (up to 7,000 rpm).
All of these intakes (All Pro, Mast, Edelbrock, and Holley) are available for EFI or carbs.
Let your head start to spin when you realize that we haven’t even hit the tip of the iceberg yet when it comes to manifolds. The latest generation of intakes combines race car and OEM technology to help build the most extreme street cars, and some just beyond the mark. Harrop and Kinsler both make individual throttle body (ITB) setups that look amazing and perform equally well. You can’t beat an ITB intake for throttle response and “wow” factor. Kinsler makes a cross-ram and vertical style, depending on hood clearance, for all standard ports and even the C5R. The Harrop Hurricane intake is only available for cathedral port (currently), but is compatible with factory drive-by-wire electronics or mechanical throttle cables. The ITB intakes are very tunable, complicated, and therefore expensive. However, the latest trend seeks to reduce cost.
Wilson Manifolds and Holley have been producing more cost-effective alternatives to custom manifolds. The Wilson Billet Bank intake is essentially a CNC-made sheet-metal intake that utilizes F1-style trumpets, and is ideal for forced induction.
The Holley Modular Hi-Ram intake is a cast version of what is typical of custom sheet-metal intakes. The Hi-Ram is revolutionary in its flexibility of use. The base, flange with runners, is available with or without injector bosses and can be used with a variety of tops. Whether you plan on running dual 4150 or dual 4500 Dominator carbs, dual 4150 or 4500 throttle bodies, a single 92- or 102-mm throttle body, or want a blank top to do whatever you want, Holley has you covered. Depending on the configuration, the Hi-Ram can handle high RPM, high boost, or whatever you want it to.
Last but not least the custom sheet-metal option. For extreme racing (or oddball) combinations, it is hard to compare to a custom manifold. And if you have certain cylinder heads, such as the Chevrolet Performance C5R or Edelbrock LSR, you have little choice. Artists, such as Tom Nelson (Nelson Racing Engines), John Beck (Beck Mechanical), and John Marcella (Marcella Manifolds), have been crafting some of the most beautiful pieces of aluminum to ever sit atop an engine for decades. Things like runner length and taper, plenum volume, carb or throttle body location, and injector location can all be customized to suit your application. Their years of experience can help determine which best suits your engine size, RPM range, use, and choice of induction. To field a competitive heads-up drag racing effort, their services are highly recommended when class legal.
The LS-series engines, unlike so many engines before them, can actually boast of a really good factory rocker arm setup. The stock units are commonly retained on most street-based performance engines and are good for hundreds of thousands of miles. But, once you begin making serious power or spinning to high RPM levels (6,500 plus) for extended periods, it’s time to upgrade.
Traditionally the LS crowd has made very good use of factory rocker arms. However, despite being very light and strong, factory rocker arms suffer two fatal flaws. The first and most alarming problem with factory rockers is that they contain needle bearings in the trunnion that tend to fail with increased RPM, spring pressure, and lift. When these needle bearings fail, the result can be catastrophic as they escape into the oil galley. Both Comp Cams and Harland Sharpe have captured roller trunnions to upgrade factory rockers, which solves this problem. More than 1,500 hp has been made using this setup.
The second problem is that with high-lift camshafts, the factory rocker arms are known to cause excessive wear on the valveguides, even more so with certain aftermarket castings.
Aftermarket rocker arms with a roller tip not only have less friction, but they also push more evenly on the valve tip. Economical steel-body, stud-mount rockers are available from Comp Cams; however, a stud-mount aluminum rocker may be worth a little bit of extra dough to reduce valvetrain weight. The technology has come a long way, so no longer are aluminum rocker arms a liability when it comes to durability or stiffness. Comp Cams, Crane Cams, Scorpion, Yella Terra, Harland Sharpe, and Summit Racing all make quality pedestal or stud-mount aluminum roller rockers.
The next step up from a pedestal-mount rocker arm is a shaft-mount version, which is required for all high-RPM builds. With shaft-mount rockers, a shaft connects the rocker arm bodies, which acts as the fulcrum, increasing mounting stiffness and creating a more stable valvetrain. There is also quite a bit of adjustability, which can help optimize geometry.
Some aftermarket castings, especially in the race port section, require shaft-mount rockers and have plenty of clearance built in. Factory castings usually require milling off the pedestals and sometimes clearancing along the valve cover rail. Taller valve covers or spacers are usually required as well. With the machining, aftermarket valve covers, and the actual cost of shaft-mount systems, they are usually reserved for high-end builds. T&D, Lunati, Mast Motorsports, Comp Cams, and Jesel are some of the most popular and well-made examples.
Written by Dave Grasso and Posted with Permission of CarTechBooks
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