A turbocharger is the best power-adder by far. Not only do turbos harness otherwise wasted exhaust energy to drive the compressor, their modular nature lets compressor and turbine housings and wheels be mixed and matched within a given turbo frame size to optimize the unit for a particular application. But this level of choice can also be daunting, introducing complexity somewhat akin to selecting the best cam for a normally aspirated motor.
As with selecting a cam, you really need to consult an expert before you take the turbo plunge. So that's what HOT ROD did: We came up with three hypothetical, entry-level, single-turbo applications, then consulted leading turbo industry sources ranging from practical engine builders to hard-core turbo company engineers for their recommendations. To simplify things, we asked our advisers to spec a single-turbo system capable of running on pump premium unleaded gas for a daily-driven generic muscle car in the 3,200- to 3,800-pound range with an auto trans and mild rear gears. All will have EFI and sophisticated engine management controls. Well, let's get crackin'.
Turbo geek-speak may sound like a foreign language, but here's what the fancy terms mean in standard English:
Area/Radius (a/r) ratio: A term defining the properties of the compressor or turbine housing's curved scroll. Compressors are relatively insensitive to A/R changes, but turbine performance is greatly affected by changing the A/R. Proper A/R is determined by application, engine size, and, ultimately, real-world tuning.
Blow-off or bypass valve (bov):
A valve on the compressor's outlet side that quickly relieves pressure in a blow-through system when the throttle is suddenly backed off.
Blow-through system: A turbo system in which the throttle blades are located on the compressor's outlet side. Most modern efi setups are blow-through configurations.
Boost: The amount of pressure above atmospheric pressure, expressed in pounds/square inch (psi) in the U.S. system or bars (b) in the metric system. 1 psi is about equal to 0.0689 B. A turbo EFI setup usually requires a 2B map (manifold air pressure) sensor.
Choke flow: The maximum flow for a compressor wheel trim and housing combo at a specific efficiency, usually 60 percent. This is the righthand boundary on a compressor map.
Compressor: The cold-air-into-engine side of the turbo. Intake air passes through this side of the turbo on its way to the engine.
Compressor surge: Reversion of air out of the front of the turbo--either when the throttle is suddenly slammed shut and air is caught between the compressor and throttle plate or under boost if there's too much pressure in relation to the amount of airflow through the system. Cure the former with a bov and the latter by getting the right compressor for your combo. Draw-through system: A system in which the throttle blades are on the inlet side of the compressor. Rarely seen today, it was mostly found on some old-school carbureted setups.
Intercooler or charge-air cooler: An air-to-air or air-to-liquid heat exchanger mounted between the turbo and engine that draws heat from the air exiting the compressor under boost before it reaches the engine.
Lag: Any delay between a change in throttle position and the production of usable boost.
Ported shroud: Antisurge relief channels used in certain high-tech compressors that allow some airflow to bypass the wheel.
Pressure ratio: The ratio of the total absolute pressure produced at the turbo outlet divided by atmospheric pressure. Turbo engineers usually prefer this standard to an outright expression of boost pressure.
Turbine: A turbo's exhaust or "hot" side. Exhaust pressure exiting the engine passes through the blades on the turbine wheel, causing it to spin. Through a shaft, it spins the compressor wheel.
Wastegate: A boost pressure–actuated valve in the exhaust collector that allows only enough exhaust gas into the turbine to achieve desired boost, venting excess gas around the turbine.
Wheel trim: An area ratio defining the relationship between the inducer and exducer of both the turbine and the compressor wheels. A higher-trim-number wheel means more airflow; a smaller trim results in faster spool up. Changing the compressor wheel trim in particular can dramatically alter performance.
Combination 1: Chevy 250ci L6
This is a classic 8.5:1 Chevy straight-six in an early '60s Nova. It has a mild cam as well as EFI and a corresponding intake manifold. The goal is 300 to 350 hp at 5 psi of boost. Would the selected unit also work on similar-size sixes besides the Chevy, or even a Ford flathead V8?
Turbo engine builder Ken Duttweiler (KD) has built some of the world's fastest turbo engines, but for entry-level apps, he spec'd a practical Precision Turbo unit that won't drive you to the poorhouse. With its 55mm compressor inducer and a T3-sized turbine housing with a 0.63 A/R, Precision's 3254E is capable of supporting up to 470 hp. Isn't that overkill? Sooner or later, most of us can't resist turning up the wick. KD leans to turbos capable of supporting at least 30 percent more power than the originally stated goal. He adds that you can generally double a stock, standard-performance engine's short-block design power output without undue durability concerns. As a stock Ford flathead starts out about 85 hp and only has three main bearings, don't exceed 300 hp--choose a smaller, 48mm turbo, such as Precision's 4854B.
Rick Head, owner of Exile Turbo, a small yet growing turbo outfit, still has the time to personally interact with his customers. He took a budget approach to this entry-level combo. He says, "Knowing it's your first rodeo and assuming a tight budget, I would suggest a T3/T4 hybrid turbo. The low boost pressure and subsequent lower 1.34 pressure ratio can make the compressor choice a bit tricky. Assuming 100 percent VE, you will need 35 lb/min airflow to reach 300 to 350 hp. The TO4E-57 compressor stays within the map throughout the operating range, thus providing decent efficiency. Assuming 5,500 to 6,000 max rpm, drive the compressor with a TA31 turbine wheel in a 0.63 A/R housing. This will be a very responsive turbo on the street and will have little or no lag. A 38mm wastegate will be plenty; a BOV isn't needed. This combo will work with similar-size engines except for the flathead, which may require a tad more boost to compensate for lower VE; more boost mandates an intercooler."
Industry giant Garrett supplies massive quantities of turbos for OE factory applications. But it also has an aftermarket division, Turbo By Garrett, that sells through performance-oriented distributors. For the Chevy six, we asked Garrett to do a full engineering analysis, just as if it were specing a turbo for a factory job. Along with our 300hp, 250ci stipulations, for the purposes of the exercise, Garrett engineer Rob Symonds assumed a 3,000-rpm midrange torque peak, a 5,200-rpm power peak, no intercooler, pump gas, and Torrance, California (near sea level) ambient conditions. The target air/fuel ratio at WOT on pump gas was 11.5:1, with a 0.46 BSFC (brake-specific fuel consumption) number. That means the turbo needs to flow 27.20 lb/min to make 300 hp. At 5,200 rpm, 5.50-psi intake manifold pressure and a 1.38 pressure ratio are required to achieve this flow level. In the midrange, you're looking at about a 15.70 lb/min airflow requirement. "The two operating points should straddle the main efficiency island on a compressor map as closely as possible. Garrett's 53mm GT3071R (PN 700177-23) assembly meets this criteria, plus it has a ball-bearing driveshaft and water-cooled housing for optimum durability and quick spool up. The three different available A/R ratio turbine housings are sold separately; use the midrange 0.82 A/R as a starting point, and fine-tune boost response versus ultimate power with either the smaller 0.63 or larger 1.06 as needed." Symonds adds that any changes in the selection criteria or engine design might alter the turbo selection. An intercooler isn't needed if intake temperature stays under 150 degrees F.
Hellion specializes in kits for late models as well as special custom jobs. Owner John Urist says that to make 300 to 350 hp at 5 to 7 psi, you need a 54mm compressor. "This supports the power level but still has great response. A larger, 61mm turbo will support 400 to 500 hp," he says. A 24x6x4-inch air-to-air intercooler could add 50 hp at the same boost level. Urist says that "because it runs hotter, that old Ford flathead definitely should run an intercooler, but other similar engines should be OK without one." As for turbine A/R ratio, he says that "on an initial custom build, you're shooting from the hip. We tend to go larger to avoid restriction, but different people have different desires depending on the response they want. I'd start with an 0.81 A/R in a 62mm turbine housing. A 35 to 38mm wastegate should be adequate; a 30 to 50mm BOV is optional."
Premier competition turbo maker Precision Turbo's Joe Krivickas spec'd a high-tech solution: "With 250 ci and only 5 psi to work with, we need a turbo able to move a lot of air with cool charge temps and exceptional spool-up characteristics"--namely, a PT6266B. "These units can handle about 20 times more thrust load than their hydrostatic bearing counterparts. By utilizing ceramic ball bearings, we are also able to take advantage of their excellent heat resiliency properties, eliminating the need for the liquid-cooling provisions commonly found on competitor's products," Krivickas says. He is referring to products that use steel ball bearings or old-school hydrostatic bearings (although Precision does offer roughly equivalent turbos with hydrostatic bearings at a considerable cost savings). Mate the unit's 62mm compressor with a 0.58 A/R T4 tangential turbine housing that includes a 35?8-inch V-band inlet/outlet turbine housing to ease installation. Use a Precision PW46 wastegate plus a BOV. Krivickas says this unit should work for similar engines in this displacement range.
Turbonetics caters to the replacement aftermarket and the hard-core competition markets. Engineer Dave Austin's compressor match spits out a GT-K 500 turbo. An improved descendant of the classic TO4, it has an HP-61 compressor wheel and an 0.82 A/R turbine housing with its proprietary F1 wheel. Normally, these matches are generated after a customer fills out the worksheet on Turbonetics' website. After filling in the blanks--VE and desired horsepower are two big variables--Turbonetics experts take over, dialing in the boost pressure needed to meet the goals, then looking for a "turbo map" that generates the highest efficiency in meeting them. "That picks the compressor," Austin says, but "the turbine comes
from a chart we have or what I know works from our tribal knowledge. Sometimes you have to use your best guess and cut and try different A/R ratios." Turbonetics' Evolution wastegate, and (optional) Duo-35 BOV complete the setup. At just 5 psi, Austin forecasts 320 hp without an intercooler or 335 hp with one. These recommendations apply to similar-size OHV engines, but the Ford flatty with its lower VE and airflow needs a smaller turbo with higher boost in addition to an intercooler.
Combination 2: 350 to 400CI Small-Block
Your classic 9.0:1 small-block has a decent bottom end with forged pistons and rods. On top are Edelbrock Performer RPM aluminum heads. Valve actuation is via a hydraulic roller cam with duration in the 230- to 236-degree range (at 0.050-inch tappet lift). Power flows through an auto overdrive trans to 3.55:1 rear gears. Installed in a '60s-era muscle car, the goal is 600 hp at around 8-psi boost.
Duttweiler spec'd a 72 or 76mm turbo: "At full boost (25 to 28 psi), the 72 is good for about 800 to 950 hp and the 76 for 950 to 1,150 hp--so either would do the job and make the needed power at 8 psi. In Precision's line, look at the relatively affordable PT7275 or PT7675. Definitely use an intercooler: "It's mandatory at 8 psi."
Head says that on a 383 or 400 with 100 percent VE and 6,000- to 6,500-rpm max engine speed, at 8 psi the pressure ratio is still a low 1.54, meaning 60 lb/min is needed to achieve 600 hp. That requires a compressor with a fairly broad map. "I recommend a GT-76 compressor and [to] drive it with a GT-42 turbine with a 0.96 A/R housing. This turbo will provide a nice linear power and torque curve on the street--with a bit more in it should you be inclined to turn it up. If you have only a 350, tighten up the A/R." Head says the same turbo should work on other small-blocks that are "equivalent in size and VE." To match the turbo, you'll need a larger 45mm wastegate. Although still optional, an inexpensive air-to-air intercooler would enhance system efficiency. "Use a 50mm BOV in venues like autocross, where you are on and off the throttle continuously."
10/14 Using an HP Performance C4 Vette 60mm turbo kit, a Comp XR270HR hydraulic roller cam, and a FAST engine management system, this old Tuned Port Injection L98 350 made 481 hp and 579 lb-ft on 7 psi of boost. Output was aided by the efficient air-to-air intercooler but hurt by the TPI's long-runner intake, which limited the combo's ultimate top-end potential.
We used TurboByGarrett.com 's online Boost Adviser tool. Besides the basic parameters noted above, we assumed near-sea-level (Los Angeles) ambient temperatures and pressures, a 4,700-rpm torque peak, and a 6,500-rpm power peak. A conservative 11.5:1 air/fuel ratio and 0.6 BSFC were used, along with 8 psi of boost and a 1.66:1 pressure ratio. The motor has a 70 percent effective intercooler with a 1-psi pressure drop. Of the four possible turbos spit out by the Boost Adviser, a GT4708R unit had the highest efficiency without getting near choke, permitting future growth. This unit has an 80mm compressor housing, dual-ball-bearing shaft, and an oil-cooled center housing. There are four different turbine A/R options: 0.96, 1.08, 1.23, and 1.39. On a street-driven 350, start with 0.96 (PN 769112-1 for a complete turbo assembly); for a 400, try an assembly with a 1.08 turbine A/R (PN 769112-2).
Hellion's Urist specs a 67 to 69mm compressor side with a 68 to 75mm exhaust turbine. The 350 should err on the small side, a 400 on the high side. "More cubic inches need more flow," Urist says. Likewise, turbine A/R should be 0.82 for a 350; 0.096 for a 400. "The wastegate, BOV, and intercooler should be the same size as the those spec'd for the Chevy six--they can handle up to 800 hp. But the intercooler is now mandatory because we need to move more air and the boost is higher. These recommendations also apply to similar small-blocks such as the old Ford and Chrysler." Hellion still offers a Ford 302 Mustang Fox-body kit.
11/14 Note how tight the volute's throat is on the righthand Turbonetics turbine housing. Its smaller A/R greatly improves turbine response--but may restrict the top end. Duttweiler suggests starting with the default A/R turbine housing.
Krivickas spec'd a PT7675B turbo with a T4 0.96 A/R turbine. The preferred version has the ceramic ball-bearing cartridge, CEA wheels, and a ported-shroud compressor cover. With 1,200-plus-flywheel-horsepower potential, there's plenty of room for growth. While not mandatory at 8 psi, Precision's 825hp air-to-air intercooler is recommended.
Turbonetics' Austin assumed a 383 Chevy. "The turbo will be a midframe 82mm compressor with an F1-83 turbine wheel. As displacement moves up, the turbine A/R needs to get bigger because you don't need the velocity. I'd probably use a 1.0 A/R for a 350; a 400 would probably stay with the 1.14." Couple that with an RG-45 wastegate plus a Duo-35 BOV. "At 8 psi, an intercooler becomes important; it's like the crossover point," Austin says. The turbo won't change for other small-blocks with similar engine displacement, RPM, VE, and horsepower.
Combination 3: GM LS-Series (Gen III-IV)
We know from HOT ROD engine dyno tests that stock GM's LS-series engines see tremendous power gains when turbocharged, so our third hypothetical combo specifies a 6.0L (364 ci) or 6.2L (376 ci) Gen III or IV engine that's totally stock, up to and including the rather high (for a turbo engine) factory 10.0:1 to 11.0:1 compression ratio. The engine is installed in a late-model vehicle with full-factory electronic controls. The goal is 800 hp at any boost level. We also wanted to know if a similar recommendation would work for late-model Chrysler Hemi or Ford modular engines. Our sources seriously questioned the long-term durability of a stock short-block at this power level.
Duttweiler again specs a 76mm Precision PT7675 with (to start) an 0.81 A/R turbine housing. You'll be running this unit at about 17 to 18 psi to make the power, so "water/alcohol injection is mandatory if staying with pump gas, " Duttweiler says. Don't forget the intercooler, either! While a stock LS short-block might live on the dyno—for a while—at 800 hp on the street, "they will fail! They need good rods and pistons to be durable."
"Assuming 100 percent VE, 6,000 to 6,500 max rpm, and 10.0:1 to 11.0:1 compression, it takes 15 psi minimum to achieve 800 hp," Head says. Besides a mandatory intercooler, he says to consider E85 fuel. Head suggests a GT4508R turbocharger with an 80mm compressor and a 1.01 turbine A/R. "A ball-bearing turbo is more durable than journal bearings, but the downside is cost and repair difficulty. Use at least a 50mm wastegate and corresponding BOV. The combo should work for similar-size, late-model, two-valve motors, but a smaller four-valve generally has less low-end torque, so I'd look at a GT42 for a Ford 4.6."
For the LS, we used the online Boost Adviser with parameters similar to the classic 350 small-block (Combination 2), but with peak torque and peak power now occurring at 3,900 and 5,900 rpm, respectively. Reaching 800 hp requires a 2.38 pressure ratio and 18 psi of boost. The Adviser spit out half a dozen possibilities, and after analyzing them, the GTX4508R seems the most efficient. This unit's 80mm compressor has a steady 76 percent efficiency rating from the torque peak through peak power, probably due to its ported-shroud compressor. There's plenty of room for future growth (up to 1,250 hp), and with dual ball bearings and an oil- and water-cooled center housing, this baby should both perform and last. The center housing and compressor assembly (PN 800270-1) are sold separately from the turbine housings, which are available in 1.01, 1.15, 1.28, and 1.44 A/R ratios. With a stock cam and highly efficient heads, a 1.28 A/R (PN 757707-7) is a good starting point.
Urist says to use "a T4-based frame with a 76mm compressor and a 75mm turbine with a 0.96 A/R." The recommended intercooler and BOV remain the same as the Combination 2 350, but since 18 to 24 psi of boost is needed, the 'cooler is now mandatory. The high stock compression ratio of GM's LS series or late Chrysler Hemi requires high-octane fuel or water/methanol injection. "If you lower compression to 8.5: 1 to 9:1, then 800 hp is easily obtainable on pump gas," Urist says. Ford 4V modular motors have a more detonation-resistant chamber, so they can reach 800 hp on pump gas with no problem. Variable valve timing (VVT) motors "will see gains by locking out the VVT." Urist adds that 800 hp is the crossover point between single- and dual-turbo setups. "You can get there with a single, but if you want room for future growth, consider a twin [offered by Hellion for most late models]."
Krivickas spec'd a turbo able to "move a lot of air without having to max out the unit or worry about having to run leaded race gas at all times." The PT8285 turbo with a T4 1.28 A/R turbine housing is capable of supporting 800 hp at moderate boost levels, and with its dual-ceramic-ball-bearing centerhousing and CEA technology, the combo "would have excellent spool-up characteristics and street manners." The now-mandatory air-to-air intercooler must be large enough to support at least 1,200 hp, so it likely needs to be custom-fabbed for the chassis. The same parts should work on Chrysler 6.1/6.4L Hemis. Most of the Fords are about 70 to 100ci smaller with multivalves, so you'll want to step back to a slightly smaller turbo, such as the PT7675B spec'd for Combination 2.
Turbonetics has a complete, smog-legal, single-turbo kit for the LS-powered '10 to '11 Camaro that comes complete with all necessary plumbing and an intercooler. Normally, the kit is set up for 550 hp to the wheels, but with fuel system improvements and a larger 82mm turbo configured for more boost, its possible to reach 800 hp or more if not subject to emissions requirements. Packaging a large single underhood could be problematic, so Austin prefers twin GT-K 550s with an F1-65 turbine wheel and 0.82 A/R, which collectively can support up to 1,000 hp. Use a single RG-45 wastegate with one large single or two Evolution wastegates with the smaller twins.