For maximum power production, it’s hard to beat boost from a turbocharger. The cheap-date method for LS owners is to find a 5.3 truck engine in the junkyard then install a cam, springs, and turbo system. This combination has powered some serious street machines, and the same philosophy can be applied to LS3 and LS7 applications. Most serious LS3 (or LS7) efforts tend to be dedicated buildups. The question now is, How do turbo LS applications make such tremendous power?
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Boost is really nothing more than a power multiplier. When I add boost from a turbo to a typical 430-hp LS3, it is important to understand that the NA LS is already running under boost, which comes courtesy of the atmosphere and equates to 14.5 psi at sea level. This atmospheric pressure obviously changes (as does the power output) with alterations in things such as elevation, temperature, and humidity, but the mechanics do not. As the piston races downward with the intake valve open, the external positive atmospheric pressure forces air into the negative pressure created by the piston.
This scenario creates plenty of power potential as you increase the external pressure applied to the engine above atmospheric. If an LS3 produces 430 hp at an atmospheric pressure of 14.5 psi, then you can theoretically double the power output (to 860 hp) if you supply an additional 14.5 psi of boost pressure. In truth, there are a number of reasons why this power/boost formula doesn’t always work, but it is nonetheless a good indicator of potential power from a turbo engine.
Another great thing about the formula is that it can be applied at any given boost level. If you apply just 7.25 psi (.5 bar or 50-percent atmospheric pressure), you get a corresponding 50-percent increase in power (430 to 645 hp). The same goes for running 10 psi (430 x 1.689 = 727 hp), or even 2 bar (29 psi), where your 430-hp LS3 becomes a 1,290-hp monster. This example illustrates the importance of combining a powerful NA combination with boost because the power gains are simply multiplied by the original output. The more you start with, the more you finish with. Having more power to start with also allows you to reach any given power level at a lower boost level.
Although the boosted power output is a function of the original power multiplied by the boost (actually pressure ratio), know that all boost is not created equal. The advantage turbochargers have over superchargers is that very little power is required to drive the compressor of the turbo. The impeller or rotors of a supercharger are driven directly off the crankshaft. This mechanical coupling can provide immediate boost response, especially with positive displacement superchargers. However, as with the power steering, A/C, and alternator, the parasitic losses associated with driving the supercharger reduce the power output offered by the engine. This means that for nearly any given boost level, the turbo should produce more power than a comparable supercharger.
This power differential increases with boost (and flow), but know that 10 psi from a supercharger does not produce the same power curve or output as 10 psi from a turbo. The sacrifice for this efficiency can be boost response, but proper sizing can produce amazing results because factory turbo engines are able to provide peak boost pressure as low as 1,800 rpm (lower than you would want for almost any performance application).
Turbos have offered this type of performance since their inception, but one of the major reasons for a sudden surge in popularity is availability. Like it or not, the advent of affordable, offshore products has helped create the current turbo craze. Before the China connection, turbo kits were few and far between, primarily because of their expense. The average Joe could not or would not spend $5,000 to $6,000 on a turbo kit, but thanks to knock-off turbos, intercoolers, and the associated couplers and tubing, turbo pricing has dropped dramatically.
Obviously it pays to shop wisely, but putting together your own turbo kit can be done for less than half of what it cost not long ago and even less if you shop around. With $300 to $400 turbos, $125 intercoolers, and aluminum tubing bends readily available, it is possible to piece together a DIY turbo system for less than $1,000 if you start with factory exhaust manifolds. This type of kit is not going to put a scare in the Street Outlaw boys, but it is capable of boosting the power of an LS by 50 to 100 percent or more.
Another area where superchargers and turbochargers sometimes differ is in the intake manifold design. Turbos and centrifugal superchargers tend to use the factory (or equivalent) long-runner intake design. Positive displacement superchargers often replace the factory manifold to mount the blower. Primarily for packaging reasons, the supercharger is combined with some type of ultra short-runner intake because it is often difficult to get the supercharger, intercooler, and intake manifold under the hood of your average Camaro or Corvette.
There is some merit to the fact that the immediate boost response overcomes the torque losses (from charge filling) associated with optimized runner length, but using long-runner intakes is one advantage turbos (and centrifugal superchargers) have over positive displacement superchargers. As indicated in Chapter 1, runner length is one of the major factors that shape the entire power curve. Even on turbo applications, selecting the correct runner length tunes the combination to the desired engine speed. If you want your turbo LS3 or LS7 to run well up to 6,500 rpm, stick with a stock, FAST, or MSD Atomic intakes. If you are looking to elevate engine speeds, short-runner intakes such as the Holley Hi-Ram can push power production on a turbo engine past 7,000 rpm.
When it comes to turbochargers, size really does matter. Like cam timing and intake manifold design, turbos should be selected to maximize power over a given RPM range, balancing response rate with ultimate boost and power potential.
The heat generated by turbo systems needs to be managed properly. Make sure to shield components positioned near the exhaust system.
Whether running a supercharger or turbocharger, intercooling is an effective way to improve power and eliminate harmful detonation.
Because they control the boost pressure supplied to the engine, make sure to purchase quality waste gates such as this unit from Turbo Smart.
Test 1: Effect of Ignition Timing on a Turbo 4.8/LS3 Hybrid
Nothing wakes up an NA engine like a small dose of boost. The critical element when running boost is actually making sure the air/fuel and timing values are correct because a turbo engine runs thousands of trouble-free miles when treated to the proper tune. Typically, turbo engines require additional fuel and a slightly richer mixture than its NA counterpart. For maximum (safe) operation, an NA engine is typically tuned to achieve an air/fuel ratio near 13.0:1. By contrast, a turbo engine runs its best and is safer with a richer air/fuel ratio closer to 11.5:1. It is possible to run the turbo engine leaner than 11.5:1, but this is an effective air/fuel mixture for safe operation.
In terms of ignition timing, an NA engine runs best with more total timing than a forced-induction application. A good strategy is to have a drop of 1/2 to 1 degree of total timing per pound of boost. For the 6-psi application, this means a decrease of 3 to 6 degrees of total timing, but the actual amount is dependent on available octane.
To illustrate the power gains offered through changes in timing on a turbo LS, I installed a single Precision turbo on an LS3 equipped with a 4.8 crank. The turbo kit used a set of JBA headers feeding a custom Y-pipe equipped with a T4 turbo flange. The package also included a Turbo Smart wastegate, air-to-air intercooler, and 114-octane Rocket Brand race fuel. Set to run just 6 psi, the combination of the intercooler, safe air/fuel mixture, and race fuel allowed me to safely dial up the ignition timing.
Running 18 degrees of total timing, the turbo LS produced 539 hp and 502 ft-lbs of torque. Stepping up to 20 degrees resulted in 551 hp and 507 ft-lbs; 22 degrees brought 557 hp and 517 ft-lbs. The final test at 24 degrees resulted in 575 hp and 523 ft-lbs of torque. Each step up in timing brought additional power, but there is a limit to how far you can go with the available octane, and the timing increased peak torque less than peak power (less timing is required at the torque peak than the horsepower peak).
Precision supplied a 67-mm turbo for this low-boost test.
Optimizing ignition timing is important for an NA LS application, but it is critical on a turbo engine.
Increasing the total timing from 18 to 24 degrees increased the power output of the turbo LS from 539 to 575 hp. This shows the importance of ignition timing on a turbo application, but don’t get greedy or you will just as quickly ruin a perfectly good engine.
The change in ignition timing offered serious power gains on the turbo hybrid engine (actually all turbo engines). The additional ignition timing was more beneficial at higher engine speeds (typical for timing), but the torque gains were sizable as well. Care must be taken not to add too much timing because detonation can rear its ugly (and destructive) head. Running just 6 psi on race fuel and with an efficient intercooler allowed me to maximize timing on this turbo engine.
Test 2: 6.0 LS3 Hybrid: NA vs Single Turbo at 6.8 and 9.8 psi
This test involved turbocharging and applying boost to an LS hybrid. The hybrid was built by combining a 6.0 bottom end with an LS3 top end. The 6.0 short-block was boost-ready thanks to a forged rotating assembly that included a SCAT crank, K1 (6.125) rods, and JE Asymmetrical pistons. I combined a flat-top piston with the 70-cc combustion chambers for this turbo combination. The 6.0 hybrid was assembled using Fel Pro MLS head gaskets and ARP head studs. Although I ran a number of cams on this combination, this test was run with a stock LS2 cam. The stock LS3 heads were treated to a valvespring upgrade from BTR. I also ran 75-pound FAST injectors, the stock LS3 intake, and a FAST (manual) throttle body.
In essence, this 6.0 was an LQ9 or LS2 equipped with high-flow LS3 heads. Run on the dyno with a Holley HP Management system, long-tube headers, and a Meziere electric water pump, the LS produced 480 hp at 6,000 rpm and 472 ft-lbs of torque at 4,800 rpm. After adding the single turbo kit that included a Precision turbo (PN PT6766), CX Racing ATW intercooler, and Turbo Smart wastegate, I applied boost to the hybrid. Running 6.2 psi (6.8 psi at the torque peak), the turbo-hybrid produced 615 hp and 619 ft-lbs of torque. After stepping up to 9.8 psi, the power output jumped to 718 hp and 687 ft-lbs of torque. The increase in boost improved power production through the entire rev range.
This hybrid started life as a 6.0 truck engine but was upgraded with JE pistons, K1 rods, and a SCAT crank. For this test I retained the stock LS2 cam.
The changes in ignition timing increased the power output of the turbo test engine by 36 hp.
This test shows that even small amount of boost can have a dramatic effect on power production. Equipped with a 6.0 short-block, stock LS2 cam, and LS3 heads, the NA hybrid produced 480 hp at 6,000 rpm (as high as I revved it during the test). After adding the single Precision turbo, power jumped first to 615 hp at 6.2 psi (6.8 psi came at the torque peak) then to 718 hp at 9.8 psi. There was still more power to be had from the turbo, but this test wasn’t designed to max out the engine or turbo.
The torque gains offered by the single turbo were impressive. Although the engine dyno artificially loaded the engine to provide a better boost curve than you might see on the street or strip, adding 6.8 psi (actually just 6.2 psi at the peak) of boost increased torque production from 472 ft-lbs to 619 ft-lbs. Adding another 3 psi pushed the torque peak to 687 ft-lbs.
Test 3: Turbo Cam: LS9 vs BTR Stage II 4.8/LS3 Hybrid
The right cam is critical for any LS turbo application, including the short-stroke hybrid used for this test. Typical LS3 applications combine a 4.065-inch bore with a 3.622-inch stroke. This 6.2 combination works well and accommodates turbocharging. The combination used for this test replaced the stock 3.622-inch stroke with a smaller 3.267-inch stroke from a 4.8. With the exception of the smallest (4.8) and largest (7.0) engines in the family, all other LS engines (5.3, 5.7, 6.0, and 6.2) share the same stroke crank. The 4.8 shared the block with the 5.3, but the reduced displacement came from a shorter stroke. For this test, I combined the 4.8 crank with custom Lunati rods and forged JE pistons then stuffed it all inside an LS3 aluminum block. I then topped it off with a set of TS Gen X 255 heads and Holley Hi-Ram intake.
The turbo system consisted of a single 76-mm turbo from Precision Turbo fed by a pair of DNA turbo manifolds into a custom Y-pipe. Controlling the boost was a pair of Turbo Smart waste gates. Boost was fed through an air-to-water intercooler from CX Racing. This test was a comparison between the most powerful factory cam (LS9) and a Stage II turbo cam from BTR. The boost was limited to a maximum of 9 psi using just the waste-gate springs (no controller).
Run with an LS9 cam, the short-stroke turbo engine produced 701 hp and 598 ft-lbs of torque. The boost curve (on the spring) started at 8.5 psi, rose to a maximum of 9.4 psi, then dropped to 8.0 psi. After swapping in the BTR Stage II cam, the power output jumped to 733 hp and 621 ft-lbs of torque. The boost curve started at 8.2 psi, rose to 8.9 psi, then dropped to 8.0 psi. The BTR turbo cam improved peak power and offered more than 60 ft-lbs lower in the rev range.
Feeding the short-stroke turbo engine was a Holley High-Ram intake and TFS Gen X 255 heads.
What looks like an LS3 with forged pistons was actually a 4.8/LS3 hybrid. The aluminum LS3 block was stuffed with a 4.8 crank, custom Lunati rods, and JE forged pistons to produce a (high-RPM) short-stroke LS3.
The great thing about the BTR turbo cam was not just that it added 34 hp, but that it improved the power output through the entire rev range. More peak power is good, but more power everywhere is even better!
We all love big horsepower gains but torque is more meaningful in real-world street driving. In addition to adding 34 hp, the Stage II BTR cam dramatically improved torque production over the LS9 cam. Down low, the BTR cam offered as much as 62 ft-lbs of torque, which is a sure indication that the BTR guys understand the needs of a turbo LS engine.
Test 4: Turbo Sizing: Big vs Small 76-mm
Turbocharging has been around since the birth of the internal combustion engine because it is a tried-and-true method of improving the power output. The basic concept is to force-feed the engine more air than it would ingest of its own accord. This air contains power-producing oxygen molecules, which when combined with fuel, ignite to provide the downward force on the crank. More air equals more oxygen, which in turn equates to engine power. Of course, this assumes the turbo is sized correctly for the intended application and desired power level. This also assumes you have sufficient fuel flow to meet the needs of the engine, something I neglected to do on this test (the results nonetheless demonstrate proper turbo sizing).
The 417 stroker test engine featured forged internals from Speedmaster and JE, along with an aluminum LS3 block from Gandrud Chevrolet. Topping the stroker was a FAST LSXR intake and throttle body, but I could only get my hands on a set of 60-pound injectors in time for testing. This ultimately limited the maximum power output, but the effect of turbo sizing was still clearly evident.
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The 417 stroker was configured with a single turbo kit that consisted of a pair of tubular headers feeding a common Y-pipe. The Y-pipe was equipped with a pair of 45-mm Turbo Smart waste gates to control boost. The Y-pipe also featured a T4 turbo flange to readily accept a T4 turbo.
For this test, I ran a pair of T4 76-mm turbos, one from CX Racing and one from Precision Turbo. Although both advertised at 76 mm, the Precision unit was capable of supporting as much as 1,200 hp, and the CX turbo topped out under 800 hp. It bears mentioning that there was a substantial price difference between the two turbos ($450 to $1,800).
Run with the smaller CX turbo feeding an air-to-water intercooler, the turbo managed to produce just 7.3 psi at the power peak of 761 hp. The boost pressure rose as high as 9.9 psi early on, but fell off rapidly as it ran out of flow. The larger Precision turbo suffered no problem, but available fuel flow limited boost to just 12.5 psi, whereas the turbo stroker produced 913 hp and 925 ft-lbs of torque. The CX turbo would be great for a lower power level on a stock or mildly modified engine, but if you plan to crank up the boost on a stroker, better get the good stuff.
The test engine was a 417 stroker that included a Speedmaster 4.0-inch stroker crank and rods with a set of JE forged pistons. The stroker assembly was stuffed inside a new aluminum LS3 block from Gandrud Chevrolet.
A pair of ported LS3 heads from TEA feed the LS3 stroker. The heads were combined with a custom cam from BTR.
Despite the lack of fuel flow, the graph shows gains offered by proper turbo sizing. Both are designated 76-mm turbos, but the Precision turbo offered significantly more flow than the unit from CX Racing. The boost pressure and power curve fell off rapidly at the top of the rev range with the smaller turbo.
The falling boost curve is even more apparent in the torque curves because the fall off in torque is even more pronounced. The smaller CX Racing turbo was about maxed out at 761 hp, but it managed to increase torque production by 260 ft-lbs over the NA engine. The Precision turbo was up more than 100 ft-lbs and only fuel flow kept me from easily eclipsing the 1,000 ft-lbs mark.
Test 5: Effect of Boost on a Turbo LSX B15 (14.6 vs 19.5 psi)
Boost from a turbo can be both a blessing and a curse. The blessing comes in the form of additional power because each extra pound of boost brings with it a substantial jump in power. The curse comes from the extra power as well because owners often become greedy after sampling all that wonderful power. If some boost is good, then more must be even better, right? Well, there is a limit to just how much fun is available without something getting hurt. The important point here is, don’t get greedy.
This test was run on a GM B15, boost-ready crate engine from Gandrud Chevrolet. Equipped with forged internals and LSX LS3 heads, the crate engine was perfect for this boost test. Because the B15 came with an intake manifold, I installed a Holley Hi-Ram intake and FAST 102-mm throttle body, along with Holley 120-pound injectors. Tuning for the turbo combo was controlled by a Holley Dominator EFI system
The key to a successful turbo engine is knowing its limits. The tune is important, especially at elevated boost levels. Forged internals are slightly more forgiving in terms of detonation, but even the toughest components snap without the proper air/fuel and timing curves. I made sure to dial in the timing and air/fuel (kept constant for each boost level) using the Holley system.
Running the single Precision 76-mm turbo through the air-to-water intercooler, the turbo B15 produced 951 hp at 6,300 rpm and 891 ft-lbs of torque at 5,200 rpm. The boost curve started at 16.0 psi, rose to 17.5 psi, then fell to 14.5 psi at the power peak. I relied on a manual boost controller, but an electronic version would have kept the boost consistent. After cranking up the boost to 19.6 psi (at the power peak), the peak power numbers jumped to 1,083 hp and 979 ft-lbs of torque. Once again, the boost curve started out at 20.1 psi, rose to 22.7 psi, then dropped to 18.8 psi (19.6 psi at the power peak of 6,300 rpm).
Keeping things cool during testing was this single-pass, air-to-water intercooler from CX Racing. I ran dyno water through the core during testing.
Once again I relied on the single 1,200-hp 76-mm turbo from Precision.
Boost always has a positive effect on the power curve, and this test was no different. Cranking up the boost from 14.6 to 19.5 psi increased the peak power numbers from 951 to 1,083 hp.
The manual waste-gate controller did not allow me to dial in the boost curve precisely through the entire rev range. Since I was getting close to the maximum output of the single Precision turbo and the back pressure was escalating, the boost curve was not consistent through the rev range and therefore, the LSX with 19.5 psi spiked above 979 ft-lbs early in the run. Despite this fact, the change in boost offered some serious torque gains.
Test 6: 4.8 LS3 Hybrid: NA vs Single Turbo at 9.8 psi
This test proves that turbo boost can be applied to any LS3 combination, including a short-stroke engine. This short-stroke engine was used extensively with both cathedral and rectangular-port heads and ran safely to 8,000 rpm with a hydraulic roller cam (see Chapter 3). The hybrid was the result of combining a 4.8 crank with a big-bore, LS3 aluminum block. Gandrud Chevrolet supplied the new GM block and stuffed it with forged internals from Lunati and JE. The short-stroke LS3 was topped for this test with TFS Gen X 255 LS3 heads that flowed more than 380 cfm. I liked the fact that the TFS heads featured even smaller port volumes than the stock heads (good for reduced displacement). The engine also featured an ATI dampener and complete Moroso oiling system (both critical at high RPM). Tuning came from a Hol- ley EFI management system controlling 120-pound injectors.
For this test, the LS3 hybrid was equipped with a factory LS9 cam. (For details on how much a turbo cam might be worth, see Test 3 in this chapter.) The aluminum test engine was equipped with a single 76-mm Precision turbo capable of easily exceeding the intended power level for this comparison.
I ran the test engine in NA trim before subjecting it to boost. The hybrid produced 512 hp at 6,900 rpm and 415 ft-lbs of torque at 6,200 rpm. Credit the lack of displacement and Holley Hi-Ram intake for the elevated engine speeds (despite the mild cam timing).
Run with the single turbo system pushing out 8.5 psi at the power peak, the turbo-hybrid produced 701 hp at 6,500 rpm and 598 ft-lbs of torque at 5,500 rpm. Given the forged internals and 1,200-hp capability of the turbo, there was plenty more left in the combination, but this 700-hp turbo LS idled like a stocker and would provide thousands of trouble-free miles at this boost level.
Tuning for the short-stroke turbo engine was provided by a Holley Dominator EFI system. The Holley was used to control timing and fuel from the 83-pound Holley injectors.
The test engine was an LS3 aluminum block stuffed with a 4.8 crank, forged Lunati rods, and JE pistons. I topped it with a set of TFS Gen X 255 heads and Holley Hi-Ram intake. Note also the use of a Moroso oil pan and ATI dampener.
The NA short-stroke LS3 was no slouch at 512 hp, but things really got serious once I added boost. Running a peak of 8.5 psi, the turbo LS produced 701 hp, with plenty left in both the engine and turbo.
Despite the reduced displacement (compared to an LS3) and the use of a rather large 76-mm turbo from Precision, the boost response was very good and so were the torque gains. The boost increased torque production by as much as 192 ft-lbs, with consistent gains through the rev range.
Test 7: Turbo LS: Effect of Snow Water/Methanol Injection
You may be wondering why I decided to include a test on water/methanol injection in this chapter. In reality, water/methanol injection is a form of inter- cooling, which should be employed on any turbocharged (or supercharged) engine, almost regardless of the boost level.
Although the Snow Boost Cooler water/methanol injection does not provide additional power in the same way as (say) nitrous oxide, it does dramatically decrease the inlet air temperature. This combined with the extra octane offered in the methanol portion of the mixture allows you to be more aggressive on the ignition timing, air/fuel ratio, and/ or boost pressure to improve the power output.
The higher the intake charge temperature, the higher the risk of the fuel self-igniting. Having the mixture ignite prior to the piston being in the proper position can result in the expanding mixture working against the upward moving piston. At the very least, this has a detrimental effect on power production; at the very most, it can cause catastrophic engine failure.
In addition to minimizing the chance of detonation, a cooler inlet charge temperature can also provide additional power thanks to the increase in air density. Cooler air has more oxygen molecules per volume, so getting cool air to your engine should be considered mandatory. This is especially true of turbocharged (and supercharged) engines, where the compression (boost) has an elevated charge temperature well above ambient. In the case of the turbocharged LS running 8.5 psi of boost, the inlets air temperatures exiting the turbo exceeded 185 degrees. The Snow water meth system dropped the air temperatures to 95 degrees and improved power by as much as 29 hp and 32 ft-lbs of torque. Water/methanol injection is especially important when running pump gas because it allows for changes in timing and air/fuel that dramatically improve power (see Test 1).
This system featured dual nozzles, but I employed only minimal pressure and the smallest nozzle sizes on the low-boost LS.
The Boost Cooler from Snow Performance can be thought of as chemical intercooling. The injection of a water/methanol mixture dramatically cools the intake air temperature while decreasing the chance of detonation.
Adding the Snow Boost Cooler water/ methanol injection to the turbo LS dramatically decreased the inlet air temperature. The super-cooling system dropped the charge temps from 185 to 94 degrees at 8.5 psi of boost. This drop in temperature not only increases the power output, but allows for additional timing and changes in air/fuel to further improve power production.
Gains offered by the combination of the cooling effect and changes in timing were substantial on this turbo LS application. The changes in charge temperature would be even greater on higher boost applications. The Snow system netted an additional 29 hp and 32 ft-lbs of torque on this turbo LS.
Written by Richard Holdener and republished with permission of CarTech Inc
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