With a stout short-block filled with an all-forged rotating assembly, the remainder of a boost-ready engine assembly includes the cylinder heads and crucially important camshaft. All LS engines benefit from excellent cylinder-head airflow—some more than others—but it is the camshaft that is the key not only to optimal power but drivability characteristics.
This Tech Tip is From the Full Book, HOW TO SUPERCHARGE & TURBOCHARGE GM LS-SERIES ENGINES. For a comprehensive guide on this entire subject you can visit this link:
LEARN MORE ABOUT THIS BOOK HERE
SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://lsenginediy.com/build-ls-engine-cylinder-heads-camshafts-induction/
There isn’t sufficient room in this chapter to describe all of the cylinderhead choices available to the LS engine builder—and more seem to arrive every month. The use of heads with six bolts per cylinder isn’t an absolute requirement on an engine making less than 1,000 hp or less than about 15 pounds of boost, but they should be strongly considered for racing applications generating high boost and high horsepower.
The other crucial detail only applies to the installation of supercharger systems that mount the compressor in place of the intake manifold. For these applications, the cylinder-head intake ports must be compatible with the intake ports of the supercharger manifold’s ports. Mostly, that means the difference between LS1/LS6/LS2-style cathedral-port heads and later-style rectangular-port heads. In fact, even the later heads don’t all have matching port shapes—the LS7 head ports have a more square design, while the LS3/L92-style ports are taller and narrower.
Also, because of the comparatively small bores of LS1 and LS6 engines (3.89 inches), when compared with the later 6.0-liter and larger engines, they can only use LS1, LS6, and LS2 heads. Using the heads of 6.2-liter and larger engines causes valve-to-block interference. However, the larger 4.00-inch bore of the LS2 enables it to use LS1/LS6 heads, as well as L92-style heads (including LS3, LS9, and LSA engines). The 6.2-liter engines (LS3, L92, etc.) can use any head except the LS7 and C5R, while the 7.0-liter LS7 and C5R blocks can use any LS-series head. The C5R head is not recommended, as it is an expensive part with a unique intake port design that requires a custom intake manifold, as well as a small combustion chamber that promotes a highcompression ratio that’s incompatible with forced induction. The LS2, LS3/L92, and LS7 heads use production intakes, offer greater port volume and larger combustion chambers that are desirable with forced induction.
In general terms, higher-flow cylinder heads make the same horsepower with lower-boost pressure than a comparable engine combination with lower-flowing heads. Put another way, the boost of the supercharger or turbocharger overcomes the relative inadequacies of “smaller” heads, as the boost pressure fills the ports, and then some. That’s not to say the cylinder head isn’t important, but for most street/strip applications, the already high-flow characteristics of factory LS heads perform more than adequately with forced induction.
Later-style rectangular-port heads (whether LS7- or LS3/L92-style) are larger and have greater airflow attributes than cathedral-port heads. They are well suited to forced induction, but the ultimate selection may be influenced by bolt-on Roots/ Lysholm-type supercharger systems, because not all supercharger intakes are currently compatible with all cylinder-head port designs.
When it comes to combustionchamber design, the factory configuration is adequate, and conventional porting/blending provides a modest benefit. More crucial, however, is chamber volume, as it contributes to the engine compression ratio. A larger chamber volume lowers the compression ratio, while a smaller volume raises it. A good rule of thumb is somewhere in the 64- to 68-cc range.
Another consideration for engines projected to generate more than 15 pounds of boost is the use of cylinder heads with six head bolts per cylinder, rather than the factory-style four bolts per cylinder. When used with the appropriate cylinder block (see Chapter 8), the additional clamping power—at least 50 percent greater— of the six-bolt heads does much to prevent head-gasket failure. GM Performance Parts and Trick Flow offer six-bolt heads that fit the LSX cylinder block, although the Trick Flow heads are only available in LS1/ LS6/LS2-style cathedral-port styles. World Products also offers a six-bolt head, but it doesn’t match the bolt pattern of the LSX block, meaning it must be used with the company’s complementing six-boltcapable Warhawk cylinder block.
When it comes to other aftermarket cylinder heads, they are of the standard, four-bolt style. There are numerous choices, although most are based on the cathedral-port LS1/LS6/LS2-type head. They make great, higher-flowing choices for the 5.7- and 6.0-liter engines, but GM’s production and performance heads from GM Performance Parts are currently the best options for the largerdisplacement engines that use rectangular-port heads.
Here are some of the available aftermarket heads:
- AFR (Air Flow Research) offers “Mongoose” CNC-ported cathedral-port heads in 205-cc (66- or 76-cc chambers) and 225-cc (62-, 65-, or 72-cc chambers) intake runner sizes.
- Edelbrock’s cathedral-port heads are CNC-ported by Lingenfelter Performance Engineering and feature 202-cc intake runners and 65-cc combustion chambers.
- Pro-Filer cathedral-port heads are offered in 215- and 235-cc intake runner sizes, with 62-cc combustion chambers..
- Trick Flow offers a number of cathedral-port heads, including some with a six-bolt configuration that fit GM Performance Parts’ LSX cylinder block.
Additionally, a large number of companies offer modified versions of production GM heads that typically include CNC port work. These ported heads offer notable flow increases that are helpful on cathedral-port heads, but their relative expense doesn’t provide as much of a return on investment on the high-flowing rectangular-port heads. As-cast rectangular-port heads flow very well for supercharged and turbocharged engines.
Superchargers and turbochargers, especially turbochargers, generate more heat than a normally aspirated combination. Valves with the strength to withstand that heat are critical to the engine assembly. The factory-supercharged LS9 engine of the Corvette ZR1, for example, uses titanium intake valves and sodiumfilled exhaust valves.
The titanium valves are extremely durable and low in mass, but comparable performance and heat resistance can be found with stainless-steel intake valves, although they don’t have the weight advantage of titanium. As for the exhaust side, the use of sodium-filled valves have long been an effective way to combat exhaust heat, but like titanium intake valves, inconel-material exhaust valves offer a less-expensive, yet heavier alternative. Also, some builders shy away from sodium valves because of their multi-piece construction and resulting damage that could occur if the pieces separate at high RPM, but contemporary products have proven very durable.
Stronger, higher-rate valvesprings typically go hand in hand with higher-lift camshafts, but even if the camshaft profile remains relatively close to the stock cam, the boost pressure of a forced-induction engine generally demands stronger springs. That’s because the increased pressure in the cylinders wants to push the valves open or, at the very least, make it more difficult for them to close—and the greater the boost, the stronger the springs need to be.
Lifter type plays a role here, too, as a roller-type lifter can withstand about twice the valvespring pressure of a flat-tappet type. High spring pressure and high boost can affect the longevity of the camshaft, however, as greater pressure is transferred to it. So, a roller lifter is strongly recommended.
Production LS engines use nonroller rocker arms with, generally, a 1.5 or 1.6:1 ratio. Using roller-tip rocker arms reduces friction and a higher-ratio arm (typically 1.7 or 1.8:1) delivers the effect of a mild increase in cam lift. But because of the intake-side pushrod position on LS7/LS3/L76/L92 heads, the rocker arms are offset. There are few choices currently for roller-tip rockers for these offset-style arms, but SLP Performance has a set that fits LS3, L76, and L92 engines (PN 50189). The rockers have a 1.85:1 ratio.
Stronger-than-stock valvesprings are recommended with increasedratio rockers.
Because cylinder head sealing is crucial for forced-induction LS engines, the head gasket is a vital component in building a durable engine that stands up to great pressure. The factory-style gasket is a multi-layer steel (MLS) design that has proven very strong and reliable in naturally aspirated conditions and lower-boost engines of up to about 700 to 750 hp. However, the pressure of a supercharger or turbocharger, particularly when used with production-style fourbolt cylinder heads, often makes the head gasket one of the primary failure points on otherwise-stock engines.
The MLS head gaskets from Ohio-based Cometic Gasket have proven to be very strong and durable in forced-induction engines—and most importantly, they’re available in a variety of bore sizes for the sixbolt pattern of the GM LSX block and the RHS LS Race Block. They are constructed of two embossed, Vitoncoated stainless-steel outer layers sandwiched around a variable stainless inner layer.
Also, Fel-Pro’s four-layer gaskets have proven to be very durable at higher horsepower. Copper gaskets or O-ring-style gaskets are suggested for racing engines producing more than 2,000 hp.
The camshaft needs for forcedinduction engines are different than those for naturally aspirated engines. And while supercharged and turbocharged engines both feed pressurized air to the combustion chambers, there are differences that can affect the optimal camshaft profile, particularly the exhaust duration.
Before getting into the specifics of camshaft selection, here’s a quick primer on common cam terms.
Lift: The distance the valve head is raised off its seat when the camshaft lobe is at its highest position (and in combination with the rocker arm). Lift is measured in fractions of an inch or millimeters. In theory, greater lift enhances performance by creating a wider opening to the combustion chamber and allowing a larger air/fuel charge to be packed in it.
Duration: The amount of time a valve is held open, measured for both the intake and exhaust valves in degrees of crankshaft rotation; i.e., 250 degrees on the intake valve and 255 degrees on the exhaust side. In general terms, longer duration enhances performance at higher RPM or helps extend the RPM range of the engine
Lobe/Lobe Ramp: The part of the camshaft that interfaces with the lifter, with the ramp section being the part that initiates the lifting and descending of the lifter. The profile of the lobe determines the speed or rate at which the valve opens and closes.
Symmetrical/Asymmetrical Lobe Ramps: A camshaft with symmetrical lobes has matching opening and closing ramps on both the intake and exhaust. An asymmetrical cam has different opening and closing ramp profiles on each lobe.
Base Circle: The lowest point of the camshaft lobe and the place in the camshaft’s rotation when the valve is completely closed.
Lobe Separation Angle: The distance in camshaft degrees between the maximum lift points of both the intake and exhaust valves. It affects performance by affecting valve overlap to impact the overall performance range of the engine. In a nutshell, a narrow angle promotes a steep, immediate power curve and a wider angle spreads the power out across the RPM band.
Valve Overlap: Measured in degrees of crankshaft rotation, it’s the amount of time both the intake and exhaust valves are opened in a combustion chamber. It takes place as the exhaust stroke ends and the intake stroke begins. In general terms, greater overlap builds power at higher RPM by helping pull the fresh air/fuel charge into the chamber. Great lift and duration increases overlap, as does reducing the lobe separation angle.
Intake Centerline: The point of greatest lift on an intake lobe, measured in crankshaft degrees after top dead center. In an assembled engine, it is measured by the crankshaft degrees between top dead center and the point of maximum valve lift.
Hydraulic Flat Tappet vs. Hydraulic Roller: All production LS engines feature hydraulic lifters, but not hydraulic roller lifters, which are the friction-reducing rollers that interface with the cam’s lobe ramps.
In general, a camshaft for a supercharged or turbocharged engine should have a wider lobe-separation angle (LSA) than a naturally aspirated engine in order to spread the power across the RPM band and deal with the increased cylinder heat that comes with forced induction. The rule of thumb for the LSA is 112 to about 114 degrees, although several LS engines’ stock cams feature a 112- degree LSA—and the LS2 cam’s LSA spec is a wide 116 degrees, for example, while the factory supercharged LS9’s lobe-separation angle is an extra-wide 122.5 degrees.
The optimal LSA must be matched with the appropriate exhaust duration to better handle exhaust pressure. Again, in general terms, a supercharged engine needs more exhaust duration than a naturally aspirated engine, while a turbocharged engine needs less duration.
Generally speaking, a turbocharged engine benefits from a milder cam (one with lower duration) in order to make the most of the exhaust-gas pressure that drives the turbocharger’s turbine. In a nutshell, the camshaft should be used to keep heat out of the cylinders. (For a good example of the camshaft needs of a turbocharged engine, refer to Chapter 1, page 17, “Real World Project: Steve Gilliland’s 1,000-hp TwinTurbo Corvette Z06.”)
Ideally, the boost pressure of a turbo engine should be greater than the exhaust pressure at the low end of the power band, as the engine nears its peak torque. The boost pressure and exhaust-gas pressure are nearly equal when the engine approaches its peak horsepower. However, at peak horsepower there is typically greater exhaust back pressure than boost.
For optimal efficiency and maximum effectiveness of the turbo system, the boost should exceed (or at least be equal to) the exhaust back pressure over the RPM range. When this doesn’t occur, a too-small turbocharger can be the culprit.
The original LS7 engine’s cam had .591/.591-inch lift, 211/230 degrees duration and 121-degree lobe separation angle specs. Katech Performance used a camshaft with greater lift (.615/.613-inch) to maximize airflow; slightly more intake valve duration (220 degrees) and less exhaust valve duration (229 degrees). The cam’s lobe separation angle was also dialed back to a more appropriate 116 degrees.
The reason a turbo engine needs less duration than the supercharger cam is because the turbocharger itself is an exhaust restriction that increases exhaust gas pressure— and it’s the exhaust-gas pressure that spins the turbo. Therefore, a milder cam with lower duration helps exploit boost-enhancing exhaustgas pressure.
A supercharged engine’s boost is generated at the front of the engine’s air stream, so a cam with greater duration than what would be used with a turbo engine helps expel exhaust gases more completely, clearing the chamber and promoting unrestricted airflow. But as Comp Cams’ Billy Godbold pointed out, the camshaft for any forced-induction engine should have less overlap to prevent boost from escaping through the exhaust port.
An important note about the above samples: They are meant to provide an illustrated comparison and shouldn’t be considered recommendations for any LS forcedinduction engines. Factors determining the “perfect” camshaft grind include the size of the supercharger or turbo compressor; the displacement of the engine; cylinder-head changes over stock; and even the type of performance expected—i.e. street, street/ strip, or dedicated racing.
Ignition and Ignition Controller
The production-style coil-nearplug ignition system is surprisingly good for even moderately high boost pressure—up to about 12 to 15 pounds. As boost pressure increases, a higher-energy spark system is required because the higher pressure can effectively blow out the spark before it can jump the gap on the spark plug (much like trying to light a match in the wind). A few aftermarket companies, including MSD Ignition and ACCEL, offer replacement coil packs that provide greater energy than GM’s factory units. That greater energy helps light the mixture under pressure, while also ensuring a more complete burn.
On racing engines, where high boost and high RPM define the engine’s primary operating parameters (those making approximately 1,500 hp and more) the sequentially triggered individual-coil system has neither the energy nor speed to deliver adequate and dependable ignition control, even when using hotter aftermarket coils. The alternative is converting the engine from the production-style crank-triggered system to a conventional distributorbased system that is also linked to a high-energy coil.
Fortunately, the tools for the conversion are already on the market, thanks to circle track series’ that require distributor-driven ignition systems. To support them, GM Performance Parts offers a conversion kit (PN 88958679) that happens to be perfect for force-inducted racing engines. The kit provides the distributor-mounting fixture for the front of the engine, as well as the distributor drive gear that is attached to the camshaft. From there, a standard distributor and coil are used.
“It’s the only way to ensure adequate spark energy, with the cylinder pressure and speed that the engine achieves so quickly,” says experienced LS engine builder Brian Thomson.“The factory coil system is very good for even 1,000-hp combinations, but at this level, something stronger is needed.”
Thomson used the distributor conversion system on the 2,000-hp, twinturbo LS engine project on page 129.
In a general sense, the intake manifold doesn’t have as great an impact on the output of a forcedinduction engine, as the boost pressure largely overrides a manifold’s plenum volume and runner design. That said, the intake should offer low restriction and a straight path to the combustion chambers.
For vehicles used primarily on the street, a production-style intake manifold with greater plenum volume helps maintain low-end torque for greater low-speed drivability on the street, when the engine isn’t under boost. Bolt-on supercharger and turbo kits—even those used on modified, purpose-built short-block assemblies—perform very well on the street and drag strip with production intake manifolds, including factory-style intakes from aftermarket companies, such as F.A.S.T.
Vehicles designed primarily for drag racing, where the engine will spend most of its time at higher RPM, benefit from a carbureted-type “spider” aluminum manifold that has shorter, direct, and straight intake runners. These manifolds are readily available from several aftermarket companies, as well as GM Performance Parts. Using one with a high-boost turbo or supercharger system requires an adapter and/or elbow on top of the manifold, as well as an intake tube, to feed the manifold. These are generally custom-built parts, as the engines that employ such induction systems are typically custom built.
When selecting a spider-type intake manifold, be sure to match it with the cylinder-head intake-port design. The manifolds are designed to match the cathedral-port and different rectangular-port configurations of LS heads. Also, adapter plates are needed (perhaps requiring custom fabrication in some cases) when using any intake manifold with a tall-deck cylinder block, as the heads are pushed out farther than on an engine with a standard deck block.
For safety reasons, an aluminum or sheet metal intake should also be considered for racing-oriented engine combinations that will see 20 pounds of boost or more. Rather than being cast as a single part, the nylon/plastic production-style intake manifolds are generally comprised of multiple pieces that are assembled with adhesive bonding agents. They weren’t designed for the extreme pressure that comes with high boost and inadvertently discovering the pressure point at which the components separate or the brittle nylon material shatters is not something a racer wants to discover on the starting line.
Most off-the-shelf aluminum manifolds for LS engines are of the carbureted/spider style, although Holley offers an aluminum, production style manifold (PN 300-111) for LS1/LS6 engines.
The throttle body must be compatible with the engine-control module, particularly when using a GM controller that is calibrated for either a cable-operated or electronic throttle. For most street/strip combinations of up to 1,000 hp, or so, a production GM throttle body, such as LS7’s large, 90-mm unit, works and flows just fine.
It is possible to adapt an electronically controlled throttle to a vehicle originally equipped with a conventional cable throttle and vice versa, but modifications are required, including changing the engine controller and pedal assembly, and is generally not worth the time. If the vehicle was originally equipped with a cable throttle, use a larger, cable-actuated throttle body if necessary; and if the vehicle came with an electronic throttle, continue using the same type.
Fortunately, GM cable-operated and electronic throttle bodies are generally interchangeable on intake manifolds, so a cable throttle body can be used on an LS7 intake manifold, allowing for example, this high-flow intake to be used in a fourth-generation F-body with a cable-operated throttle system.
Dedicated race cars should use a cable throttle for maximum driver control. In fact, for safety reasons, many sanctioning bodies require it. Also, the airflow requirements of a 1,500- to 2,000-hp engine can’t be met with current production-based electronic throttle bodies.
Electronic Throttle Bodies and Blow Off Valves
Some builders have reported a condition with supercharged LS engines that use electronically controlled throttle bodies where excessive boost pressure pushes on the throttle blade after it is closed— such as when the driver takes his foot off the gas—forcing unwanted air into the engine. At the least, it can cause stumbling and other drivability issues, but it could also lead to engine damage if the excessive airflow causes a lean condition.
The problem can be due to an inadequate supercharger blow-off valve (in many cases, the issue has been reported with Eaton-type Roots superchargers) that doesn’t bleed off enough boost pressure when the throttle closes. It can also be due to a throttle body with a throttle blade spring that doesn’t have sufficient strength to keep the blade closed. Or it could be a combination of both issues.
Some enthusiasts have had success swapping the throttle body with a strong factory unit, such as the LS3 unit from the C6 Corvette; however, it requires additional tuning to match the throttle body with the engine controller. A stronger or larger bypass valve is another solution.
Real-World Project: Building a 2,000-hp Twin-Turbo LS Engine
In an experiment backed by GM Performance Parts, Detroit-area engine builder Thomson Automotive decided to push GM Performance Parts’ LSX block to see whether it lived up to the advertised claim of supporting 2,000 hp. Thomson did it with some custom parts and a couple huge turbochargers.
Because of projects like this one and a symbiotic relationship with GM Performance Parts, Thomson is rapidly becoming one of the country’s foremost experts in highhorsepower LS engine development. The turbo LSX project was launched in 2007, when the LSX program was in its infancy and parts for it were being custom built.
GM Performance Parts was eager for an independent party to verify its claims for the LSX platform, so it donated the cylinder block, as well as a set of prototype LSX racing heads.
“One of the keys to the success of the LSX block is the additional cylinder head bolt provisions,” says Brian Thomson, president of Thomson Automotive. “Production LS blocks have four head bolts per cylinder, but the LSX blocks accommodate six bolts per cylinder. It makes a huge difference in clamping power and, frankly, with the amount of boost this engine makes, it wouldn’t survive without that added clamping power.”
Those prototype heads were all the more necessary, too, because when the project started, they were just about the only six-bolt heads Thomson could locate. Today, GM Performance Parts and a couple other aftermarket manufacturers offer ready-built, six-bolt heads.
So, Thomson had the block and heads secured, but the rest of the assembly was still up for debate, including the displacement. A previous experiment with a boosted, 454-ci LS engine brought about concerns of crankshaft flex, so the decision was made to go a little conservative on the stroke.
“We planned to rev the engine pretty high and throw a lot of boost at it, so we felt we could overcome the displacement deficit without too much trouble,” says Thomson. “What we gave up in cubic inches, we’d hopefully make up in longevity and durability.”
Thomson’s caution paid off. The engine has made nearly 2,050 hp on the dyno—more than 5 hp generated for every cubic inch of displacement —and has survived approximately 150 full-load dyno pulls without so much as an oil leak.
“We’ve never lost a head gasket or had any real issues with it,” Thomson says. “We inspected the bottom end numerous times and it all looked great. We replaced some bearings for good measure after so many dyno pulls, but the engine has been very reliable.”
When you first look at the twinturbo LSX engine, it looks as if a couple Caterpillar loaders were robbed of their turbochargers. A pair of 88-mm Turbonetics turbochargers dominate the assembly, along with a custom intake system that looks somewhat like a Mad Max version of a tunnel ram. There’s also a front mounted distributor in place of the typical LS-engine coil packs. Here’s an overview of the basic parts and why they were selected.
Rotating Assembly: A Callies 3.750- inch-stroke forged crankshaft is connected to a set of GRP forged aluminum connecting rods. The pistons are from Diamond and in order to keep the compression ratio at a boost-friendly 9:1, they feature large, 50-cc dishes.
“You could pretty much drink coffee out of them,” says Thomson about the custom pistons.
The rotating assembly is housed in a tall-deck version of the LSX cylinder block. Its 9.70-inch deck height enabled the racing-style connecting rods to swing freely, without the need for internal block clearancing. The bores measure 4.125 inches. With the 3.750-inch stroke, that makes the displacement just a hair less than 401 ci.
Cylinder Heads: As mentioned earlier, the heads are prototype LSX racing heads that were ported by Utahbased Chapman Racing Heads. They’re filled with 2.200-inch titanium intake valves and 1.600-inch Ferrea Super Alloy exhaust valves. The intake port design is patterned after the high-RPM flow characteristics of the C5R head, but the chamber volume is a tight 45.6 cc—hence the need for the deeply dished pistons to keep down the compression ratio.
Camshaft and Valvetrain: A custom grind from Bullet Racing Cams was used, with comparatively mild .714/.721-inch lift and 266/268- degrees duration specs. Lobe separation is 113 degrees, which is appropriate for a forced-induction engine. The rest of the valvetrain is pretty standard stuff, including Comp Cams springs, titanium retainers, and keepers, along with 1.8:1-ratio roller rocker arms.
Intake System: During Thomson’s experiment with a force-fed 454-ci LS engine, it used a custom, CNC-carved tall runner intake system with a conventional front-mounted throttle body. It found unequal distribution among the cylinders when the boost was turned up (it was blowing past the front cylinders and getting crammed in the rear cylinders), so it redesigned the intake. It now features a pair of ACCEL/DFI 2,100-cfm throttle bodies mounted on top of the intake, with a custom sheetmetal air box on top of it. Three-inch tubing from the turbos feeds the air box.
Turbochargers: Two 88-mm Turbonetics compressors are used and blow through a custom-built intercooling system. They’re fed exhaust pressure from custom headers designed by GM Performance Parts to fit a specific project vehicle (more on that later). A pair of Turbonetics wastegates are also part of the system. Through the intercooler, the turbos deliver about 25 pounds of boost, with a maximum of 27 pounds recorded on some dyno pulls.
Ignition System: With the engine capable of generating more than 25 pounds of boost at high RPM, the production-style individual coil ignition system would not be effective or reliable in delivering the required spark energy, so it was replaced with a conventional distributor system. The engine uses the GM front-mount distributor kit described earlier, along with an ACCEL dual-sync distributor and high-energy ignition coil.
Additional Details: Other engine details include 160-pound Bosch injectors, custom valve covers, a Wagner racing-style water pump, and a dry-sump oiling system that uses a Moroso five-stage oil pump. One of the pump’s stages is used to draw oil out of the turbos to prevent unnecessary buildup that could lead to blown seals or worse.
Thomson handled the engine parts and assembly, but leaned on ACCEL/DFI’s Joe Alameddine to help with the engine control system. It was a coordinated effort that was both challenging and rewarding. Alameddine experimented with a prototype version of ACCEL’s Gen 8 control system, a standalone system that enabled him to batch-fire the injectors (it also handles productionstyle sequential firing). It was also designed to support tuning of forcedinduction engines generating up to 40 pounds of boost. Thomson’s turbo engine would make about 25 pounds, so it was well within the controller’s limits (see Chapter 7 for more details).
“It’s a great tool for completely custom engine combinations like this one,” said Alameddine. “We designed it to support the wildest racing engine configuration, but it is also great for street/strip engines with more conventional combinations.”
Capabilities notwithstanding, Alameddine spent many hours “sneaking up” on the turbo engine’s tune. It wasn’t a simple, “plug-andplay” operation. The fuel trim was kept safely flat and both RPM and boost were initially limited, as dyno pull after dyno pull revealed what the engine was capable of handling.
The engine recorded its best performance of 2,048 hp at 7,140 rpm and a neck-straining 1,507 ft-lbs of torque at the same RPM level. And while the engine makes big power even at lower revs (about 700 hp at 4,000 rpm), the power comes on like a sledgehammer from about 5,000 rpm onward. In fact, power jumps from about 980 hp at 4,800 rpm to nearly 1,400 hp at 6,400 rpm.
Similarly, torque leaps from the merely super-strong 906 ft-lbs at 4,800 rpm to nearly 1,700 ft-lbs at 6,400. That’s almost an 800-ft-lbs jump— nearly 90-percent more torque—in the time it takes the tach to swing only 1,600 rpm higher. Talk about driveline shock.
Thomson says there was even more power to be extracted from the engine, but the dyno sessions ceased when GM Performance Parts asked to put the engine in a 1996 Impala SS that had been constructed especially for it.
“There was definitely more power to be found in it,” says Thomson. “All we needed was more time with it.”
Written by Barry Kluczyk and Posted with Permission of CarTechBooks
GET A DEAL ON THIS BOOK!
If you liked this article you will LOVE the full book. Click the button below and we will send you an exclusive deal on this book.