Turbo Tech 101 & Turbo Vs Supercharger

[Neutrino]

Note:The pictures in this thread are an integral part of the article so please have patience while they download!!!


Engines 101-Turbos!
HOT ROD's guide to the ultimate power-adder

By Marlan Davis
Photography: Marlan Davis


Part I: Science & Selection

We've all heard the homilies, "There's no replacement for displacement," and "You just can't beat cubic inches." The basis for these statements is that the greater an engine's displacement, the more air and fuel can be squeezed into the cylinders, and the higher its potential power output. But they're not entirely accurate: There is another way to stuff more air and fuel into the cylinders--lots more, in fact--without increasing an engine's size. It's called supercharging, which is a way to force more air into an engine than it could normally take in by atmospheric pressure alone. Only the most efficient normally aspirated race engines with very specialized induction tuning can exceed 100 percent volumetric efficiency (VE), but a supercharger's forced induction makes exceeding 100 percent easy; 15 pounds of boost pressure (defined as pressure above the normal 14.7 psi atmospheric pressure) effectively doubles an engine's displacement--with correspondingly huge potential horsepower increases.

"Supercharger" is a generic term for any forced-induction compressor that is driven by a belt, gears, or a turbine. The turbine-driven version is known as a turbocharger, and it has the potential to be the most efficient power-adder for an internal-combustion engine on the planet. An internal-combustion engine is notoriously inefficient: Only about one-third of the energy released during combustion actually drives the crank. Of the remaining two-thirds, one-third goes into the cooling system, and one-third goes out the exhaust as heat. In fact, a 200hp engine dumps the equivalent of about 70 hp of raw heat straight out the tailpipe! However, a turbo's turbine-wheel is driven by the engine's own exhaust gases as they exit the motor, so some of the heat that normally goes to waste is now used to power a compressor that pumps more air into the engine.

Turbos have three major subassemblies: an exhaust turbine housing, a bearing housing, and a compressor housing. The exhaust and bearing housings each have a wheel with integral blades, and are connected together by a shaft mounted on bearings. Some turbos--like this Turbonetics Super Thumper that can support over 2,400 hp--offer a ceramic ball bearing option. A: Turbine wheel, B: Bearing and seal, C: Turbo shaft, D: Exhaust turbine housing, E: Backplate, F: Compressor wheel, G: Compressor housing, H: Bearing housing, I: Inducer bore, J: Exducer bore

Although a turbo's position in the exhaust stream does restrict exhaust flow potential to some extent, the pumping losses are much less than the parasitic drag induced by a conventional supercharger's belt or gears. In a typical gasoline-fueled engine, it's common to see 30 out of every 100 hp added by a beltdriven supercharger being wasted turning the drive pulleys and belts; this compares to about 5-10 hp per every 100 suffered as pumping losses by a typical well-designed turbo installation. Considered as a system, the turbo setup has less heat buildup than an old-style Roots blower, and its smaller size compared to a centrifugal supercharger permits higher compressor-wheel rotational speeds and more radical blade-tip curvature that collectively translate into greater pumping efficiency.

If turbos are so cool, why don't we see more of them on street machines outside of imports? In racing, it's discrimination, plain and simple. Turbos are dominant anywhere they're allowed to compete against beltdriven blowers (as well as nitrous oxide), so rule-makers almost always legislate against them, adding weight, reducing displacement, or relegating them to a separate class. On the street, it's due to perceived complexity and installation difficulty. While these issues certainly aren't trifles, in these pages--with help from Innovative Turbo, Turbonetics, and other turbo specialists--HOT ROD will attempt to demystify some of these complexities and get you started on the road to making some serious horsepressure.


The little brother of the TO4 (far right) is the T3 (far left). The T3's envelope is about 25 percent smaller, making it easier to package. Turbonetics says many street V-8s run OK with T3 or T3/TO4 hybrids, which is a real godsend in a tight engine compartment.

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The Feedback Loop
There are a bewildering variety of turbo configurations, but they're all similar in appearance and function: During engine operation, hot exhaust gases blow out of the engine's exhaust ports, into the exhaust manifold, through connecting tubing, and into the turbo's turbine housing. They strike the blades on the turbine wheel and make it spin. When the turbine wheel spins, so does the compressor wheel. As the compressor wheel rotates, it sucks air (or both air and fuel in the case of a draw-through carbureted setup) into the compressor housing. Centrifugal force throws the air outward, causing it to flow out of the turbo into the intake manifold under pressure.

There are four TO4 turbine wheel trims (rear row)--N, O, P, and Q. The large Q-trim reduces backpressure, but being heavier, it spools up slower. Each trim level is available in two shaft diameters and with a choice of ball or plain bearings. In the front and middle rows are just 10 of the literally dozens of TO4 compressor wheel trims; they're ID'ed by numbers.

As engine speed and boost increase, the turbo becomes self-feeding: The more air the compressor packs into the engine, the more exhaust gas is generated, which causes the turbine wheel to spin faster, in turn spinning the compressor faster and packing more air into the engine.

The key is getting the wheels spinning fast enough in the first place to start generating boost and a feedback loop. Turbos are load-sensitive and need energy to work. If the compressor and turbine wheels are not spinning fast enough when the accelerator pedal is mashed, there will be a slight delay before the turbo develops sufficient boost, a phenomenon known as turbo lag. Factors contributing to turbo lag include improper turbocharger selection, the turbo's physical location within the system, and the inherent limitations of nonelectronically managed engine packages


Turbo bearings are subject to tremendous heat. The water-cooled housing (above left) is recommended for prolonged highway use. Short-duration competition engines often use an air-cooled housing (above right) for less complexity.

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No Junkyard Dogs
The most critical aspect of a successful turbocharger installation is the proper selection of the basic turbocharger unit itself. Conventional superchargers come in only a few different size variations, and their output is easily adjustable by changing the drive-pulley ratio. Turbochargers come in an enormous array of sizes and shapes to confuse you, and if you select the wrong one, the engine won't function at anywhere near its potential.

First, you can't just go down to the salvage yard, pick up an OEM unit, and bolt it onto your hot rod. Its size and design characteristics almost certainly won't be right for your custom engine from a flow and efficiency standpoint. Its physical layout may also be hard to adapt: The wastegate may be integral with the turbo, making it hard to mate with other engines' exhaust systems, and the compressor and turbine halves may not be clockable as is the case with high-performance aftermarket units intended for use on custom installations.

This 358ci small-block Chevy is about as rad as it can get and still use conventional 23-degree-valve-angle heads. Electronically managed by ACCEL Gen 7 DFI, the 10.0:1 engine runs a single large-frame Innovative GTB88 turbo and makes over 1,400 hp on C-16 race gas. Big race turbos are typically identified by the inducer orifice size--in this case 88 mm.

Specifically intended for custom installations, aftermarket units like AirResearch's popular TO4 series are modular and assemble like an erector set, allowing for variable combinations of turbine housings, compressor housings, turbine wheels, and compressor wheels within a given turbo series. Just like cams, there are so many factors governing turbo selection that consulting an expert is highly recommended. However, the following overview will get you close.

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Compressor Housing

Turbo size selection begins with choosing the compressor housing (the air-into-engine side of the turbo). Racers operating with high-octane fuel usually base this on how much horsepower is required to be competitive in their particular racing venue. Street-driven cars operating on available pump gas are boost-limited, so their primary selection criterion is based on how much turbo their engine combination can accept at a specified boost level. Generally, 10 psi without an intercooler, or 15 psi with an intercooler (on a well-tuned, electronically managed 8.0:1-compression engine) is about the best a street guy can hope for on pump gas.

This graph can be used to determine engine airflow requirements for 10- and 15-psi boost levels.

Whether you're seeking to reach a desired power level (for racing) or a specific boost level (on the street), first determine how much airflow is needed to reach your goal at a given engine displacement and engine rpm. A normally aspirated four-stroke engine's cfm requirements are expressed by the classic formula: VE is at least 100 percent for a turbocharged engine, so use 1.0 for VE.

Generally on a high-performance EFI engine, every 1 lb/min of airflow is worth about 10 hp, so to find the required lb/min for a race-only application, start with the horsepower requirement, then divide by 10:

Lb/min = hp / 10

For any turbo series, the higher the A/R ratio, the better the top-end performance. The lower the A/R ratio, the better the low-speed response.

Every compressor has a definite combination of airflow and boost pressure at which it is most efficient. When choosing a compressor, you want to position the point of maximum efficiency in the most useful part of the engine's operating range. As efficiency drops off, heat transferred to the air-induction side of the turbo goes up. That's bad for both power and durability.

Turbo manufacturers publish compressor maps that establish the peak efficiencies of every turbo unit and its variations. These maps are an extremely important part of compressor selection because popular turbo series like the TO4 and its custom aftermarket derivatives have many different available wheel trims--a classification system that defines the relationship between the compressor's inducer (inlet orifice) and the compressor wheel overall diameter and tip shape. At first glance, these maps resemble a topographic contour map, and in a sense the map's bands are describing a turbo's output geography, but in terms of boost and airflow instead of elevation. They may look complex, but don't be put off. The accompanying sidebar shows how to read a compressor map and use it to select a compressor for some hypothetical engine combos.

A tangential turbine housing (left) offers about 4 percent higher flow but has less mounting and packaging flexibility than the on-center housing (right). In the ubiquitous TO4 line, each design is available in a choice of four different trim levels (which must match the turbine wheel trim), and up to eight A/R ratios.

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Turbine Housing



Because of the turbocharger's modular nature, in many instances it is possible to mix and match different turbine housings (the exhaust side of the turbo) with a given compressor housing. This permits tailoring the turbo specifically to the individual engine's operating characteristics and the vehicle's intended usage.

The turbine must make the compressor spin fast enough to produce the required airflow at the specified boost level. A small turbine spins faster than a larger turbine (which reduces lag), but develops more backpressure (which restricts exhaust flow). The goal is a turbine that spins fast enough to generate the necessary response and airflow while minimizing backpressure in the exhaust.


The turbine housing A/R (area/radius) ratio is the area (A) of any turbine inlet scroll cross-section divided by the distance from the center of that cross-section to the center of the turbine shaft (R). For any given turbine housing, A and R vary in the same proportions, so all As divided by their corresponding Rs yield the same dividend--which is the A/R ratio.

But brute size is not all that matters. The turbine's A/R (area/radius) ratio basically determines where the turbo starts to accelerate. A turbine housing looks kinda like a big snail shell. Unwrap the shell and it resembles a cone. Cutting off the tip of the cone leaves a hole--the cross-sectional area of this hole is the A in A/R. The hole size is important since it determines the velocity at which the exhaust gases exit the turbine scroll and enter the turbine blades. For a given flow rate, the smaller the hole, the higher the velocity--but the greater the restriction to exhaust-gas flow

The R in A/R is the distance from the center of the cone's cross-section to the center of the turbine shaft. A smaller R imparts a higher rotating speed to the turbine; a larger R gives the turbine shaft greater torque to drive the compressor wheel (because the lever arm R is longer).

The compressor housing is a primary factor in determining how much boost (and power) a turbo is capable of supporting. There are at least nine different housings available for standard shaft-diameter TO4s alone! Here are three of 'em.

Why is A/R ratio important? Consider two extremes: Bonneville land-speed racing (LSR) versus quarter-mile drag racing. In an LSR application, the turbo's rate of acceleration is not critical; the setup can be lazy off-the-line, but the overall acceleration rate, once it begins, should be smooth and linear--this application generally calls for a high A/R ratio. At the drags (and on a street car), you need more aggressive, instant response, which tends to lean toward a lower A/R ratio.

Unfortunately there is no easy scientific method for selecting the proper A/R ratio. Seat-of-the-pants feel is important: If boost rise is sluggish, the ratio is too large. In extreme cases, the ratio gets so big the turbo can't turn fast enough to produce the required boost. But if the ratio is too small, the turbo gets into boost so quickly that the vehicle becomes almost undriveable--and on top, it will feel like a choked-up normally aspirated engine that's under-carbureted. Also, what equates to a low or high A/R ratio varies by turbine series and engine displacement. Assuming the ubiquitous TO4-style turbo on a typical 350ci engine, Innovative offers these A/R guidelines as a starting point, based on where you want the turbo to work best:

Operating Range; A/R Ratio
Low-end; 0.58
Midrange; 0.69-0.81
High-rpm; 0.96

The accompanying Turbonetics table lists its baseline recommendations for a variety of engine displacements.


Turbonetics latest Inconel Super T turbine wheels (right) feature 10 improved high-efficiency blades in place of the previous 11-blade design. Super T turbines fill the gap between the TO4 family that tops out about 900 hp and the huge Super Thumper family that starts working at 1,400 hp. Inconel is good up to 1,700 degrees F. Need more? Special Mar-M 247 exhaust wheels survive at 2,000 degrees.

One Turbo or Two?

For racing only, there are super-large single turbo setups that can support over 1,500 hp, but they don't work well down low. Generally, when not restricted by sanctioning body rules, the usual crossover point between single and dual installations is in the 900-1,000hp range. Most under-900hp requirements can be met by one turbo, typically the universal TO4 or a custom derivative based on the TO4 frame. However, some claim that even in the under-900hp regime, two smaller turbos reduce lag over one big turbo; others counter that basic physical laws postulate that the reduction in inertia and flow caused by splitting the exhaust energy in half more than outweighs the supposed advantages of lighter, smaller components--or, in English, one big turbo housing is more efficient than two smaller housings.

But turbos must also be considered as part of the overall induction and exhaust system. There's no doubt that twin turbos have certain advantages on V-type engine layouts. The cross-tube on single-turbo V-8 installations can lose a lot of heat, and heat energy powers the turbine; two turbos permit a greater cross-sectional discharge pipe area, and dual wastegates are more efficient.

Ford 302, Actual displacement: 301.59 ci, Peak engine speed: 6,000 rpm, Airflow: 1,057.67 cfm (74.04 lb/min), HP potential: 740 hp, Compressor: GT76, Approx. efficiency: 66%

Finally, there is a special type of dual-turbo setup called compounding, where multiple turbos are mounted in series instead of in parallel, as is normally the case on a multi-turbo setup. Compounding is for extremely high boost pressures (on the order of 50-100 psi!) and is usually only encountered on tractor pullers, big diesels, and aircraft. With one turbo alone making 50 psi under extended operation, the high boost causes shaft overspeed and eventual unit failure. With compounding, a larger unit mounts ahead of a smaller unit. Since it's able to work harder and draw in more air, the larger unit generates an initial 15 psi or so, which the smaller unit then multiplies by three or four times to generate high boost without overspeed. With the air already condensed, the second, smaller turbo is not a restriction.

Honda B18, Actual displacement: 1,834 cc (111.95 ci), Peak engine speed: 9,000 rpm, Airflow: 588.88 cfm (41.22 lb/min), HP potential: 412 hp, Compressor: TO4E-50 trim, Approx. efficiency: 72%

We've said that heat is good on the turbine side, but bad on the inlet side. When an engine makes over 10 psi of boost, heat buildup on the inlet side requires cooling the incoming air down using a charge-air cooler (aka "intercooler"). We'll get into 'coolers, wastegates, system layout (including turbo location), and turbo engine-building stuff next month. Stay tuned!

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Sample Compressor Maps

These Innovative Turbo maps are just a few examples of some of the many available compressor variations. We've selected them because they meet the needs of common high-performance engine combinations in terms of efficiency and airflow. The selection is based on 15 psi of boost pressure (approximately a 2.0:1 pressure ratio), the absolute maximum for an electronically managed and efficiently intercooled engine running on pump gas.

To select a compressor by means of an airflow map, use the engine airflow in lb/min to establish an operating line on the compressor map for the turbo combo in question. Choose a map so that the intersection point of lines drawn from the desired engine airflow in lb/hr (the green vertical line in these examples) and the boost pressure-ratio axis (the blue horizontal line in these examples) ideally falls within the 70-75 percent efficiency region. On an intercooled application, you can scrape by with as low as 60 percent, but higher is better.

Chevy 350 (+0.030), Actual displacement: 355.11 ci, Peak engine speed: 6,500 rpm, Airflow: 1,349.15 cfm (94.44 lb/min), HP potential: 944 hp, Compressors: Two GT61 (47.44 lb/min per turbo), Approx. efficiency: 73%

If several different maps seemingly meet your efficiency goal, choose one that has the intersection point farthest to the right side of the 70-75 percent island. This results in quicker turbo response. You want to play in right field.

In a dual turbo installation, divide the total airflow requirement in half, then select a map that satisfies those conditions. Note that as engine displacement increases, a given turbo still passes the same amount of air, but observed gauge boost pressure will be lower.

Chevy 454, Actual displacement: 453.96 ci, Peak engine speed: 6,000 rpm, Airflow: 1,592.01 (111.44 lb/min), HP potential: 1,114 hp, Compressors: Two GT70 (55.75 lb/min per turbo), Approx. efficiency: 70%

[Neutrino]

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.

[Neutrino]

Twin Turbo Tech
Advantages Over Superchargers, Plus a Few Pointers on Tire Width and Dragstrip Dynamics

By Bill Watson, Jeff Koch
Photography: Jeff Koch

Online Editor’s note: The November ’01 issue of Hot Rod featured an extraordinary twin-turbo ’66 Mustang owned by Bill Watson, a jet engineer with Garrett/Honeywell. He generously supplied us with way more detail on his project car that we could fit in that issue, so we decided to include this question-and-answer interview that explains why he considers turbocharging superior to either supercharging or stroking an engine for more power. You may not agree with all of his points, but he knows his stuff. (After all, his Fastback runs in the 11s on—get this—SlimFast 215-section tires!) Blower buffs are invited to email us their alternative views on the best way to obtain boost.

Q: Other than the fact that the company you work for makes turbos, why go the turbo route? Why not use a supercharger or build a stroker? Are turbos simply the most efficient way to go?

A:There are four main reasons why I think turbos are superior to a blower, but of course I can’t argue that the supercharger isn’t easier. It is. I can appreciate why supercharger kits are more popular for the bolt-on crowd. But here are the reasons that I prefer turbos:


1) Turbos always will outperform supercharged cars when it comes to power production. It’s simple—you don’t have to spend (for instance) 70 crankshaft horsepower to drive compressors by taking engine horsepower right back off the crank. True, you back-pressure the engine on a turbo car, but much of the compressor horsepower comes from heat—and that is "free." The bottom line is that you don’t sap as much power out of the engine to get your pressurized air.

2) Easily adjustable boost! It’s a breeze to set up a knob inside the passenger compartment to adjust the wastegate setting up or down. When I drive in the mountains, I can raise boost to get back the power lost at higher altitudes. On the other hand, if I buy cheap gas I can turn it down, or change boost to compensate for summer or winter air temperatures (or for that matter, morning or evening temps too!). With a blower car you have to change pulleys—not that hard, mind you, but very few people are going to change pulleys as they’re climbing mountains, or at a gas station after buying a tank of cheap gas.

3) Midrange torque!! Boost vs. rpm is far superior on a turbo car. With a belt-driven centrifugal blower, you have to choose a pulley size so you won’t overboost at redline. But that means that at every rpm below that, you aren’t running as much boost as you can. The nature of centrifugal blowers is parabolic, meaning at half the redline rpm, you make less than half of the boost. So on a typical 5.0L, you might only make 3 psi of boost at 3,000 rpm, where a turbo car can easily be on the wastegate for 11 psi. That’s why I make 550 lb-ft at 3,000 rpm. Hey, if you cut my boost to 3 psi at 3,000, my torque would be down to 340—that’s 200 lb-ft less! To me, the best way to optimize a belt-driven centrifugal blower car would be to loosen the torque converter. Then you can hit the gas, immediately be at 4,000 rpm for example, make boost in an instant, and have a bolt-on car that just hauls. The only downside there is the economy will take a hit unless you can design in a lockup converter when you want to just cruise down the freeway. The turbo doesn’t have any of these compromises.

4) Finally, turbos run proportional to demand. What I’m getting at is that compressor speed is dependent on airflow, which comes from two main variables: engine rpm and throttle position, if you will. Now, a belt-driven blower car’s compressor speed is dependent on only one variable: rpm. So why care? Because when you’re just maintaining speed on flat and level ground, trying to make some mileage, a turbo car’s compressors are going very slowly and hence don’t increase pressure before the throttle body. But a belt-driven blower car has no idea what the throttle position is; the compressor is simply geared to the crankshaft, so it’s spinning much faster and making, say 2 psi in our example here. (And if it’s a Roots-type blower, it’s making 6, 8, or even 10 psi!)

So what do you do? You have to back out of the throttle even more. That means throttling losses are up (bad for mileage), and you’re pumping hot compressed air through your intercooler all day if you have one, and you’re using crankshaft horsepower to compress the air—then throttling it back anyway! At least put the throttle body before a blower; you can see why positive displacement (Roots or screw compressors) blowers have the throttle plate before them to avoid this problem. Then it’s much more of a non-issue.

You’ve also asked why I didn’t go to more cubic inches. Of course that works, but as you note the mileage wouldn’t be as good, plus I’d still have to build a 7000rpm motor to make this kind of power on something under 400 cubes. Then that requires all the money for rpm-related hardware to accommodate moving the operating range up, which I have spent zero dollars on. The turbo approach was different, in the sense that you spend the same money but on different hardware. I’ve done the cam/carb/intake/heads/headers/loose converter thing before. It was fun, but this time I wanted something that was a real sleeper until you lean on it.


Q: Surely you could have in theory used a stock intake, or not? Why not? What was the practical reasoning behind the intake design? Longer tubes equals greater torque?

A: Yes, I could have used a stock intake, but there were two big reasons I didn’t. One, the upper intake is extremely visible and typically is where most people’s eyes go when a hood is raised. This is where it’s easy to set the car apart from the others and do something different.

Two, when a stock intake is used on early Mustangs, the throttle body and the chassis bracing (the "export brace") can’t co-exist without cutting things up. I valued the export brace’s contribution to chassis stiffness enough to leave it intact. (I still need to work in the other option, a "Monte Carlo bar" that is common on early Mustangs which goes straight across from shock tower to shock tower. I used to run one but the latest mods required me to yank it for a while.

In addition, I was trying to maximize tube length for good bottom end torque, hence the runner shape going all the way to the valve covers. By most hot rod standards, 302 cubic inches isn’t a ton of displacement, so I wanted to maximize torque down where it would be driven. Plus, as you’ll note below, it allows you to run larger turbine nozzles so you can make more top end, too.

Q: Any idea how long intake runners are currently versus stock intake?

A: They’re about the same length, maybe an inch longer. Even a stock car with stock cam has a torque peak around 3,250 rpm, so I figured the headwork and larger diameter intake tubes would allow torque to hold on longer before starting to drop off, and I expected it to have peak torque around 3,600 to 3,800 or so. Hey, torque at 3,000 is 265 lb-ft normally aspirated (550 with 11 psi) and it was a pleasant surprise to find the torque just kept climbing between 3,000 and 4,100, for normally aspirated and blown peaks of 314 and 620 lb-ft, respectively. What great mid-range! I’m not Edelbrock, so I only had one attempt on this intake. I got lucky—it works better than I expected.

Q: How much boost are you running?

A: 11 psi. It was only the second night out at the track. This winter I’m sure we’ll go back with just a little more—it’s addictive.

Q: Does each turbo run 11 psi or is that total? Does that work out to 5 or 6 lbs per side, or does the pressure add or square itself?

A: Pressure doesn’t add. Each turbo runs 11 psi. Essentially I’ve got two 2.5L, four-cylinder engines here with a common crankshaft. The two turbo systems do interface together in the common intercooler and throttle body, but it is still important for me to set up both wastegates to open at the same pressure so one side of the engine isn’t back pressured more than the other by doing more than 50 percent of the compressor work.

Q: Why one throttle body, versus twin 57s as on the Ford Lightning or Mustang Bullitt, where each could feed its own half of the engine?

A: For the reasons above, I didn’t want two totally divorced turbo systems. Besides, when I started this project four years ago, the aftermarket was clearly more oriented to the single TB.


Q: As we photographed the car, you mentioned choosing (among other things) gear ratio, intercooler size, and the backpressure-to-boost-ratio work to select turbine nozzles. How did you decide on these? Are any of them slightly overkill with some extra built in, or are they just right for the job and will need to be changed if something else changes?

A: Intercooler size is somewhat like fuel-line size or air conditioning condenser size—you almost can’t make them too big. The entire goal for an intercooler is maximum cooling and minimum pressure drop. As you make them bigger the cooling always gets better, but just making an air-to-air intercooler taller eventually starts increasing pressure drop, which of course is bad. There’s some juggling there where it actually gets worse. The water intercooler I selected is the biggest one I could squeeze under the fender without mods, and the beauty is that it’s exactly twice the size of many 5.0L and 5.7 Chevy aftermarket water-air intercoolers, but they are just dying for more available room. At twice that size, it calculated out pretty well. Once you decide to intercool, it’s a bad place to cut corners to try and save a buck.

Gear ratio is, on the other hand, just like you said; you size it for the job and it might need to be changed if something else changes. I make plots of this all the time. If you are a drag racer the trap speed is very much tied to the power production in the last quarter of the track. If you’ve just shifted into the next gear at the 900 foot point and are groaning the car at 4000 rpm (not peak power in this example!!) your trap speed will clearly suffer. Optimum gear for this car today is 3.60 if drag racing were my only criteria. As soon as I add 50 horsepower, this figure drops (like maybe to 3.40) because you still want to cross the traps at the power peak, and obviously the increase in power will manifest itself as more trap speed—hence the gear-change requirement.

Also, in a turbo car, boost vs. rpm is a transient event, so with a 5-speed, you really have five different torque curves to design for. By gearing the car steeper (4.11, 4.56, etc), engine rpm grows faster than boost can keep up if you will, so in a rpm-sense, this is slowing down boost response. Add on top of this the fact that the effective mass of the rotating group of the engine goes up with gear ratio squared. So there are two downsides to more gear, while the obvious up-side is the increase in torque multiplication (which is why people do it in the first place). This is definitely a juggling act, and turbo cars for this reason tend to do better than expected with slightly taller gears because they have two downsides fighting the up-side, not just one vs. one when you’re normally aspirated.


Turbine nozzles are also a compromise. Much like selecting a camshaft, you’re essentially trading top-end for bottom-end power. This one is very hard to do analytically before you build your turbo system, so you shoot from the hip as best you can, then test the system once it’s together. I use the back-pressure/boost ratio as one of the evaluation tools.

Here’s a little background to appreciate the exchange of bottom end for top end: If you run little nozzles the boost response is early and you’ll make more power at 2000 rpm than you would have with a bigger nozzle because of the increased boost. However, the downside is that your back pressure will be higher with the small nozzles on the top end because the wastegates are open and less and less of your exhaust has to do "all the compressor work." What do I mean? If memory serves me correctly, it takes around 60 to 70 compressor horsepower to supply my engine with 11 psi compressed air at 6000 rpm from the turbo’s compressors. Where does this energy come from? Exhaust heat (which is "free") and exhaust back pressure (not free). With really big turbine nozzles, (no wastegates), 100 percent of my exhaust generates this work and back pressures the motor minimally, less than 11 psi, in fact. With really small turbine nozzles, the majority of exhaust is going through the wastegate (not through the turbine) to keep the compressor from overspeeding. Thus, if you will, "40 percent" of my exhaust has to do all the compressor work. If there’s less flow through the turbine but you need the same power, it must be accompanied by a higher pressure—you guessed it—back pressure.

Anyway, the point is that I do measure the back pressure and compare it to boost because turbo cars are notorious for having exhaust go up the intake during overlap. Lots of cars have back pressure that is two, even three times boost pressure. This is why turbo cars have the reputation of loving cams with tons of lobe separation, which minimizes the time when both valves are open and this problem occurs. If you’re Joe Bonneville, you’ll run huge nozzles, have low back pressure to boost, and traditionally run closer lobe centers just like anyone in the normally aspirated racing world, since your back pressure to boost ratio is much like theirs. Back pressure testing is important on n/a cars, but even more important on turbo cars as it helps you size the turbine nozzles.

Q: For us non-physics guys, please explain F=uN.

A: F, the forward force that accelerates your car, is the product of "u", which is the traction coefficient for a given pair of materials (in this case rubber on asphalt) times the "downforce" ("N") of the drive wheels on the road. The width of the tire is never in this calculation. The only reason wider tires can help is they can increase the "effective u" just a little, not a lot.


Wide tires are far more "show" than "go," no matter how much our egos want to disagree with that. The bottom line is g’s, and we’ve done a lot of testing in this area. The data should speak for itself, and correspond to tests on the street. Track conditions are typically better. The lowest traction limits we’ve measured are all on front-wheel-drive cars. This is because of the weight transfer offthe drive wheels when accelerating. They’re always between .40g and .50g.

Rear-wheel drive vehicles fare better. Most pickup trucks generate between .48 and .52g, thanks to in-optimized weight distribution. Non-posi cars are again a little better. They typically generate a maximum of .50 to .55g almost every time. We tested a Ford Crown Victoria with 215mm rubber, and it would spin the right rear at .50g. Next test was a ‘66 Mustang Coupe with 195mm rubber, which spun ‘em at .53g. A Volvo wagon (195mm tires) spins the right rear at .55g. This Mustang Fastback, with a posi and BFG radial T/A tires (215 mm) pulled .54g before the changes, and with the battery in the trunk now pulls .55 to .56g. Grand National Buicks (with posi) usually got .55g, 5.0L Mustangs (225mm, with posi) get .56 to .59g. My friend Rich has a ‘69 Super Bee, good posi, 215mm BFG’s, and pulls .57g before they spin. Note that these are not big differences from the best to the worst!

Now for some wide tire data: my friend John has a ’66 Nova with 275mm rubber and a good posi, yet it pulls .53 g max, right in there with my skinny-tired ’66 Mustang. Rich also owns a Hemi Charger with 275mm rubber, which can’t generate more than .55g, which you’ll note is less than his Super Bee does on 215s. That’s because he has played the weight distribution game on the Super Bee.

Highest street tire numbers ever? Weight distribution is a player. New Z28s (245 mm) commonly pull .62g max. My friend Shirl has a ‘79 Corvette (245 mm) pulls .65g on street tires.

See where I’m going here? There’s no magic "factor of two" yet. Honestly, even a posi only seems worth .05g or so (10 percent). How many guys do we run into that think a posi will double their traction?

Want to see big improvements? Change the tire compound. My friend Dave bought some BFG Drag Radials for his 5.0L Mustang. His 225mm street tires spin at .59g every time, yet his 235mm Drag Radials consistently pulled .68g. That’s 15 percent! A co-worker brought in his NSX with 245mm race tires, and thanks to the combined help of its mid-engine layout, we were measuring .75 g launches, over and over again!


I’m not saying that wider tires would hurt, I’m just arguing that they’re far more for show than go. They wouldn’t double the traction. Or add 50 percent, or even 25 percent. The max we’re talking is probably under 10 percent. So without tubbing the car, I can probably squeak in some 245mm tires if I had to. By trying to play the testosterone factor low on this car, my best money will go towards a set of drag radials, and selectively moving weight to the rear, not tubbing the car and running 315mm street tires.

Q: What’s the big deal about g’s on launch?

G’s are inversely proportional to velocity. What that means is that peak g’s occur at very low speeds and continue to drop as speed goes up. When we’re talking traction limits, we’re talking peak g’s, and by definition, they happen at launch or soon after when hitting the torque peak in low, if gearing is not optimized. So when you want to talk tires, or traction limits, you talk g’s on launch.

Of course g’s on launch are everything to guys who only think about e.t. Blowing the launch kills e.t. Blowing the last 300 feet of the track kills trap speed. In a related sense when you want to discuss power, you tend to discuss g’s at some higher speed, because, let’s face it, even a Corolla can pull .50 g at 2 mph and chirp the tires. And a Hemi Charger can pull .55g at 2 mph—which doesn’t look like much of an advantage! But, by 30, 40, 50 mph, the Corolla’s low power/weight ratio has g’s plummeting down, so it’s only pulling .20 g at 30, while the Charger can still spin ’em at .55g at 30. Why? It had the power to pull 1.0 or more g at 2 mph, but thanks to tires was limited to the .55 g level. At the track, the Mustang consistently pulls .70g at 60 mph. This makes it clear why, on the street where the traction limit is more like .55g, the car will spin the rear tires at speeds below 70 mph.

PS. Thanks to Thepeug who brought this last article to our attention