Turbo 101 Part II
By Marlan Davis
Photography: Marlan Davis
Inlet Ducting
Power level defines the inlet duct sizing. Generally the compressor inlet should be about 1 inch larger in outer diameter than the outlet. You can't go too far wrong if the inlet ducting is the same size as the compressor inducer orifice size.
If you were with us last month, you should be fairly well versed in selecting a basic turbocharger configuration for a given application. But the turbocharger is only one piece of an integrated system including inlet ducting, exhaust piping, wastegates, and other components. This month we'll take a look at how everything fits together.
Fuel Management
On the inlet side, a turbo installation is classified as a blow-through system if the turbo is located ahead of the carburetor or fuel-injection system throttle-body; in this configuration, the turbo compressor wheel pressurizes air only, with fuel being added to the system after the air leaves the compressor. In a draw-through system, the turbocharger is located after the carb or throttle-body, and both air and fuel pass through the compressor housing. With the advent of modern electronically managed fuel injection systems, virtually all current turbo systems have gone to the blow-through configuration. Unlike cumbersome old-school carbureted blow-through systems, throttle-valve pressurization isn't needed because fuel is injected directly into the intake manifold.
Turbochargers
Mount the turbo(s) as close to the engine's exhaust ports as possible. Twin AiResearch TO4E turbos were selected for this application. On a 350 Chevy V-8, the twins spool fast and make for a cleaner installation. See last month's issue for detailed turbo sizing and selection guidelines.
On a factory-designed EFI/turbo system or premium custom-built and calibrated aftermarket turbocharged engine package, boost compensation and fuel enrichment are handled electronically through the engine- management system and computer. A special three-bar MAP sensor (one that can read pressure up to two atmospheres, or roughly 30 psi) is required to read accurately under boost as well as vacuum conditions. Overall, this is the best way to control the fuel curve because the proper air/fuel ratio can be mapped for every condition. As a final fail-safe, a knock-sensor is often used to retard ignition timing if the engine starts to detonate.
Complete recalibration and fuel mapping may be ideal, but to reduce cost and complexity, many aftermarket bolt-on turbo kits that are intended for use with otherwise stock factory engines and engine management systems rely on some sort of auxiliary fuel enrichment system to compensate for turbo boost. The effectiveness of bolt-on systems can vary widely, so be careful and thoroughly evaluate the kit, particularly its compatibility with any additional engine mods. Probably the best bolt-on system is an auxiliary FMU (fuel management unit) that actuates extra injectors located in the intake plenum. Nevertheless, a really serious turbo system generating over 8-10-psi boost should dispense with fuel-system crutches and move up to the larger port injectors and a complete computer remap.
Wastegate
Short of buying stock in a piston company, the primary means of preventing engine-destroying over-boost is an adjustable wastegate. Located in the exhaust collector downstream of any individual primary tubes, a wastegate keeps boost at a preset level by routing excess exhaust gas around the turbine and out the tailpipe.
Intercooler
Also known as a charge-air cooler, an intercooler is a heat-exchanger that mounts between the turbo and the engine. It draws heat from compressed air exiting the turbo before it reaches the engine. Air-to-air 'coolers like this should generally be mounted in front of the engine-coolant radiator; the air that's passed through the latter is already at least 40 degrees F hotter than ambient.
Boost Management
Boost needs to come in early to prevent turbo lag, but the drawback of early boost initiation is that most turbo systems--which build boost exponentially--are capable of generating way too much boost on the top-end. This requires a boost-limitation device to prevent destroying the engine. The most popular boost-limiter is a wastegate, a spring-loaded valve operated by a diaphragm assembly actuated by a boost-reference signal. The valve releases excess exhaust pressure when a predetermined boost level is attained, thereby maintaining the turbine wheel's speed and ultimately stabilizing boost.
Many different wastegate sizes are available in both integral (built into the turbo) and remote-mount designs. Most universal aftermarket designs mount remotely, which eases packaging considerations and offers a slight performance advantage. Wastegate valve size should be matched to the engine's power output for best results. Too big a wastegate may open early, making it hard to get up on boost; if too small, the engine may over-boost or be otherwise hard to control. The following wastegate selection guidelines work well for most single-turbo installations (divide power level by two on dual turbo/wastegate installations):
Marmon Clamps
Common worm-drive clamps don't cut it under boost. Duct and pipe junctions are held together with beefy flexible ducts held on by aerospace-style stainless steel V-band adjustable-latch couplings, aka "Marmon clamps."
Valve Size Typical Power Level
1.250 inches Up to 400 hp
1.500 inches Up to 500 hp
1.625 inches 800-900 hp
2.000 inches 1,000-1,400 hp
Exhaust Outlet and Wastegate Discharge Pipe
Smaller exhaust outlets help a turbo spool up faster on a street car; larger outlets help flow on the top-end. In any event, the turbine outlet pipe should be no smaller than the exducer size. Discharge pipe size is determined by the combination's power level. On a high-power street car with full exhaust, the discharge tube diameter should at least equal the valve orifice size.
Engines
Blow-Off Valve
In a blow-through system sudden throttle closure when the engine is under boost can cause the turbo to surge or choke. On a road-race, rally, or street car, this can slow response time if you need to quickly get back on the throttle. A blow-off or bypass valve quickly relieves pressure when you back off the throttle. It may be mounted on the compressor outlet duct (after the ducts merge on a twin-turbo setup), between the intercooler and throttle-body, or even directly on the intercooler.
Charge-Air Management
Since it's fed by engine exhaust, obviously the turbo should be mounted near the exhaust side of the engine--ideally, as close to the engine's exhaust manifold flange as possible. This lets the maximum amount of exhaust heat enter the turbine housing; the expansion of the hot gases helps provide additional turbine rotational impetus.
Unfortunately, some of this turbine heat gets transferred to the compressor (induction) side. Inlet charge-air heating becomes a serious problem when boost levels reach 10 psi or higher. It needs to be cooled back down to maintain system efficiency. That's where an efficient aluminum-core intercooler comes in. Every 100 degrees F reduction in air temperature increases air density 12-13 percent, greatly increasing engine power output.
Exhaust Headers
Standard mild steel fatigues, so headers must be fabricated from at least Type 304 stainless steel tubing with extra-thick 0.065-inch wall thickness. Note the relatively small (for this power level) 15/8-inch-od primaries (about 11/2-inch id)--small tubes help retain beneficial heat on the turbo's exhaust side.
Intercoolers may use either ambient air or liquid coolant to reduce charge-air temperature. Assuming equivalent efficiency levels, an air-to-air cooler's surface area must be much larger than a liquid exchanger's. But because a liquid coolant like ice water has a heat transfer coefficient into aluminum that's up to 14 times greater than air into aluminum, real-world packaging constraints preclude most air-to-air installations from approaching a liquid 'cooler's efficiency level in actual service. On the other hand ice melts, so liquid coolers are only really effective in drag racing, land speed racing, or marine use. For road-racing or on the street, air-to-air designs remain more practical.
The intercooler must be large enough to achieve the requisite temperature drop while minimizing any pressure drop. Turbo system designers try to shoot for 70 percent or greater intercooler efficiency and no more than a 1.0-psi boost pressure drop through the unit, although on a street car packaging constraints may force you to accept up to a 2.5-psi pressure drop and efficiencies as low as 60 percent. The target 70 percent efficiency means, for example, that if the charge-air temperature from the turbo going into the intercooler is 300 degrees F, and the ambient temperature of the coolant medium is 100 degrees F, the charge-air temperature exiting the intercooler will be 160 degrees F. This is shown by the equation ...
The Nelson Racing 350 Chevy kit shown here has been validated for the '67-'69 Camaro/Firebird, '70 Camaro, '68-'72 Nova, and '65 Chevelle. Kit parts include the turbos, inlet ducting, exhaust headers and pipes, wastegates, intercooler, cam, Electromotive EFI and coil-ignition system, and related parts. Nelson will also build you the engine to go with it. Got $17,500?
E = T2-T3/T2-T1, where: E = efficiency (percent) T1 = ambient air temperature T2 = compressor discharge temperature (intercooler inlet temperature) T3 = intake manifold temperature (intercooler outlet temperature)
Naturally, this formula can be algebraically reshuffled to solve for any missing variable. Why worry so much about pressure drop; after all, you could simply compensate by adjusting the wastegate, right? Wrong-o! If the wastegate is adjusted to raise boost by more than about 1 psi to compensate for pressure losses, it produces a slight increase in exhaust-side turbine pressure, further raising the temperature of the air going into the intercooler. That reduces package efficiency, so in effect you're chasing your tail. And if you were to actually catch your tail, you'd end up biting yourself on the ass. Ouch--that's gotta hurt!
Next month, we'll charge ahead and follow the buildup and testing of a state-of-the-art dual/purpose street/drag V-8 turbocharged engine. HR
Mount the wastegate downstream in the header collector to ensure accurate load balance, optimum gate response, and uniform turbine inlet pressure. Ideally the gases should not have to radically change direction when headed toward the turbine in order to flow through the wastegate; in other words, a split Y configuration is best, and a right-angle takeoff is worst. Although factory wastegates are typically preset to a fixed boost level, most aftermarket units are user-adjustable on the unit itself. Variable boost control regulators have been developed that allow the driver to change boost levels from the cockpit. Innovative Turbo has even deployed a computerized electronic multistage boost controller. These units permit, for example, the end-user to change the amount of boost generated in each gear (boost over gear), over six segments of time (boost over time).
Regardless of your fuel enrichment strategy, the rest of the fuel supply system must also be capable of supporting the additional fuel-flow requirements. You'll need to add a high-volume electric fuel pump (or auxiliary booster pump), at least 1/2-inch feed-line, and boost-referenced fuel pressure regulators.