Up to this point, we’ve discussed the turbocharger apart from the engine. However, adding a turbocharger to an engine is more than just choosing the turbo for your projected horsepower output. The turbo “system” includes all of the ancillary components that adapt the turbocharger to become “one with the engine.” This is the philosophical approach that you should take when planning to create your own turbo system project. Our discussion will center around components that manage the airflow to and from the turbocharger (often called the “plumbing”). Adding fuel and controlling your fuel injection system is addressed in Chapter 8.
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Today there are turbo kits available designed for your specific application. For most street driven vehicles, where horsepower increases of 50 to 100 percent are desired and internal engine modifications are not planned, these kits tend to work very well. There’s a list of some of the most popular turbo kit makers at the end of this chapter. However, there may not be a kit out there for your application, or you may be looking for a race setup, so the available kits are too mild or too simple for your needs. In this chapter we’ll look at the various turbo system components and the considerations required.
The term “turbo lag” is a broad term that justifies some discussion. In its simplest definition, turbo lag is the response time between when you stomp on the gas and when the turbo actually comes on to boost. There are many turbo experts who’ll suggest that turbo lag shouldn’t exist with a well-matched turbo and well-designed system, and I agree mostly.
Turbo lag exists; it has to. When you stab the throttle, you’re asking the engine to accelerate, drive the turbine, which in turn drives the compressor to build boost. Even the engine has some lag based upon how quickly it will accelerate to speed. So there’s no way to completely eliminate lag, but smooth, strong acceleration will be yours if all is well with your turbo match and system design.
The quality of the system design and tuning can minimize lag to an imperceptible level. Correspondingly, just as a poorly matched turbo will cause “turbo lag,” a poorly designed system can induce “system lag.” The cumulative effect of many small errors in system design will induce system lag and that can be incorrectly interpreted as turbo lag. The difference between turbo lag and system lag can be difficult to sort out.
The primary purpose of this section of the book is to assume that the turbocharger selected for your engine is a good match, and now you need to choose the proper system components so that the sum of their effect helps to make the turbocharger one with the engine.
Understanding the design considerations taken into account on other successful turbo systems will help in designing your specific project. If your project comes down to actually buying an aftermarket turbo system or components already made to fit your engine application, this section will also potentially help you recognize the best designed system and/or components for your money. A highly effective turbocharger system is one that has taken all of the relatively small considerations and variables into account regarding airflow. The sum of the considerations becomes significant when the engine and turbo system are expected to act as one.
The first objective in the design of a turbocharger system is placement. Where is the turbo to go? This answer contains several considerations that really need to be well thought out very early on in the project. Many hours of time and labor, as well as other system components’ design, will hinge upon this decision. In competition vehicles, it’s entirely possible that, space allowing, the best placement may be dictated by how vehicle handling is influenced by the location of the extra weight. But that specific consideration, while potentially important, is beyond the scope of this book.
scope of this book. With an aftermarket turbo kit, the kit builder has already made the decision for you. For most street kits, it’s more a matter of where it’ll all fit. If you’re building your own system, consider the following points to help you determine the optimum location for your turbo:
1) Will it be a twin or single turbo system?
2) What temperature-sensitive engine components or materials may be close by? (Consider belts, hoses, the alternator, fuels lines, painted body parts, etc.)
3) Will you use an aftercooler?
4) Can the turbo’s oil drain easily be routed to a place in the oil pan for proper drain back and maintain sufficient bearing housing drain angles. (ref. Page 96)
5) Is there a clear path for boost tubes leading from the compressor discharge to either the engine’s intake or to the aftercooler without tight bends that would add restriction?
6) Is there a clear path for the exhaust manifolding into and out of the turbine that once run, won’t introduce excessive heat into materials or components that cause premature failure or create safety concerns?
7) If routing of exhaust becomes a potential problem because the best turbo location requires an undesirable exhaust route, is it something that heat shielding can address?
8) From what point will you tap the engine’s oil system to lubricate the turbocharger?
Once the location has been determined, you can begin to design the rest of the turbo system’s components.
Single Versus Twin Turbos
An important decision in your turbo system design is whether to use a single- or twin-turbo arrangement. Cosmetics aside, one of the first concerns is engine size and configuration. A 4-cylinder or straight-6 engine bay will typically contain sufficient room to house a single large turbo. If you have one of these engine configurations, the choice is relatively easy. By contrast, a V-type engine arrangement may require other considerations.
Running a single turbo on a Vengine will require you to route the exhaust from one side to the other unless your vehicle, like Indy cars, has sufficient room to place the turbo aft of the engine. The length of the manifold tubing and the total increase in heat load will likely require the use of expansion joints to eliminate the cracks from thermal expansion and contraction. There may also be a significant problem fitting a single turbo that’s large enough into the engine bay. Applying two smaller units will solve most of these plumbing and fitment problems.
Historically, the major interest in using twins has been to help reduce the turbo lag during engine acceleration. This is especially true for highperformance street engines. Two small turbines have a lower total polar moment of inertia than a single large turbine. Moment of inertia is the resistance of a body to change in speed, up or down. Remember your basic physics: a body in motion tends to stay in motion, and a body at rest tends to stay at rest (also a couch-potato definition).
I = K²M
The moment of inertia is represented by the “I,” the “K” represents the radius of gyration, and the “M” is the mass of the body. The radius of gyration is the distance from the axis of rotation to a point in the body that would have the same I as the body itself. This will not be equal to the radius of the rotational diameter of the turbine wheel because turbines are designed to keep as much of their mass as close to the axis of rotation as possible. The turbine wheel hub is much more massive than the outer areas of the blades. Therefore, K will almost always be less than one-half the rotational diameter.
For good turbine rotor acceleration, it‘s essential to design the lowest possible moment of inertia into the turbine wheel. The formula demonstrates the value of keeping the turbine wheel material near the outer diameter to a minimum to reduce K, because the moment of inertia varies as the square of K. This can be functionally illustrated by applying the formula to see how two turbos will cut the moment of inertia by more than half, which indicates a gain in potential rotor acceleration since two turbos will each have exactly half the exhaust energy as compared to what a single turbo unit would see on the same engine.
For example, let’s assume a pair of turbos where each has a 3.125- inch diameter, 1-pound turbine wheel, where K = 1.1 inch.
K²M =I K²
W/G = I
The “G” is the acceleration of gravity and “W” is the weight.
1.1² x 1/386 = .00313 in lb sec²
If the alternative best match single unit turbine wheel had a diameter of 3.75 inches, a weight of about 1.6 lbs, where K = 1.3 inches, the moment of inertia would be:
K² W/G = I
1.3² x 1.6/386 = .00701 in lb sec²
This would be 2.24 times greater moment of inertia (even two of the smaller turbos means 0.00313 + 0.00313 = 0.00616), which would suggest that twin turbos would accelerate faster and provide better turbo system response.
There are many factors aside from moment of inertia that will affect turbo system response time. Turbine efficiency is another important consideration. An often overlooked and rarely recognized concept is that the turbine wheel running clearance (the space between the wheel and the housing) is a loss feature to turbine efficiency. In the above examples, both turbine wheels would likely have the same turbine wheel contour clearance between the turbine housing and turbine wheel. The total turbine wheel clearance contained in two turbines will therefore be a higher percentage of total turbine flow, thereby potentially lowering the total turbine efficiency in a twin unit arrangement. Today’s newer turbos run higher efficiencies, but reducing the total wheel clearance in the system still helps.
Packaging and abstract efficiency discussions aside, it may make the most sense for the intended use of the vehicle to help you decide between a big single and twins. If it’s primarily a street driven project, twins on a V-engine configuration are likely to be superior, all things considered, simply because they build boost faster, giving you better response. Drag racing vehicles today make good use of tuning features such as anti-lag systems (ALS) discussed in greater depth in Chapter 8. Once the vehicle with a big single unit is launched using such tuning mechanisms, the higher system efficiency takes over and the single unit will pay dividends in lower ETs.
Regardless of whether you plan to make a competition or high-performance street machine, the air intake is an extremely important consideration. In either case, you want to be sure to introduce air that has not first come through the engine’s radiator, aftercooler, or air that has been heated by the radiant heat produced from underhood temperatures. Remember that cooler air is denser, and since you’re concerned with air density by the very fact that you are applying a turbocharger, don’t work against yourself by starting with hotter air than you have to.
If you’re building a competition vehicle, it’s as simple as building a custom intake to pull your cool intake air through the hood. However, if your vehicle is one that will require an air filter such as an offroad racer or street car, you’ve got a few more considerations. The leading edge from where you get your air is the same as in a competition vehicle, but you have two other primary considerations: filtration of small dirt particles and rain. In the case of rain, an impinging surface at the air’s entrance point will help to separate heavy moisture droplets from working their way into your filtration system and blocking the air.
Do not use paper air filter elements in a turbocharged vehicle. They simply don’t flow enough air unless they’re sized way larger than you’ll have room for, and if they get wet, they tend to close off the airflow path. The only filters you should consider are those made from surgical gauze, such as those marketed by K&N and others. While many companies market performance intake systems already designed for your vehicle, be cautious because they have a filter element sized for its stock naturally aspirated state. This will likely be undersized for your turbocharged motor and can cause problems. It’s not just a matter of whether the free-flowing filter will flow enough air when clean, but you actually want the air to flow through the filter slower than it does once introduced into the air intake stream piping. This keeps the pressure drop and resulting pumping loss during intake to a minimum. It also creates excess flow capacity to allow dirt to be more easily separated from the air stream and become trapped, while still having the capacity to flow enough air for the desired performance.
Use this formula to calculate how many square inches of K&Nstyle filter you need. The formula is courtesy of K&N filtration.
Square inches of filter required = (lbs boost / 14.7) + 1 x CID x Max RPM / 20,839
For example: at 10 lbs of boost, a 3-liter engine (183 cubic inches) that’s designed to create max power at 6,000 RPM will require 88.5 square inches of filter.
(10 / 14.7) + 1 x 183 x 6,000 / 20839 = 88.53 in²
The filters are pleated to allow more surface area within a given diameter for packaging purposes.
Now, to help you choose a filter, determine the diameter that will fit your installation, and then use the following formula to determine the filter length (or height, depending on orientation). (Note that this calculation is for round filters. For coneshaped filters, simply estimate the average diameter, which should be about 1/2 of the larger diameter plus the smaller diameter.)
Consequently, in the above example if you had room for a 12-inch diameter filter it would require a filter height of about 3 inches.
88.5/12 x 3.14 + .75 = 3.1 inches
If this seems large to you, then you now understand the value of a properly sized air filter assembly and the value of knowing how to design your own turbo system.
Once you’ve captured the air, it’s time to route it to the compressor inlet. If you have a few feet to navigate, keep your tubing diameter as large as room will allow. This reduces tubing line loss. Unfortunately, air likes to slow down before it’s redirected, which means you’ll want a smooth track with as few bends as possible.
There is some confusion in terminology between aftercooler, intercooler, and charge-air cooler. In the past, aircraft engines would run turbochargers in stages where the first stage compressor would feed the inlet of the second stage compressor, which would further compress the air before it enters the engine. Due to the extremely high boost pressures, an air cooler was positioned between the first and second-stage compressors. That cooler was called the intercooler. Another cooler would be positioned after the second stage, which was the final compressor stage, and that was referred to as the aftercooler. The aftercooler was the cooler whose outlet fed the engine. A charge-air cooler is simply an aftercooler that is typically an air-to-air cooler, meaning that it uses outside ambient air to cool the turbo’s charged (boosted) air before it‘s routed into the engine.
While multi-stage turbocharger systems are still in-use in some tractor pull classes, selected high-performance diesels, and late-model commercial diesels, the term intercooler and aftercooler are used synonymously today. The term intercooler is used today to mean a cooler in-between the turbo and the engine. So feel free to use whichever term you are comfortable with.
The subject of aftercoolers is one that could fill an entire book. The first question normally asked is, “Do I need an aftercooler for my application?” The answer is that it depends. If you’re only running 5–7 lbs of boost, you can probably get away without the expense, but that’s a debatable issue. And does anyone really stick to running just 7 psi? While the air density enhancement is not as dramatic at this mild boost level, a cooler air charge will still raise the fuel detonation threshold and keep you running safe.
However, above that 5 to 7 lbs boost level the benefits are indeed worth the trouble. In addition to dramatically increasing air density, the aftercooler removes a significant amount of thermal stress that would otherwise be seen by the engine. But perhaps the biggest benefit is that an aftercooled charge is less likely to detonate, which will dramatically reduce power and can quickly destroy your engine. Detonation is where the air fuel mixture is so unstable, typically due to heat, that it ignites before it’s properly timed point for ignition, which can cause severe overheating in the cylinder and the explosion tries to drive the piston back down the cylinder the wrong way causing a significant power loss. The cooler keeps the air charge temperature lower without loss in engine thermal efficiency. As a general rule, each one degree F reduction of intake air temperature will also reduce exhaust temperatures by one degree F. This is without a detrimental effect upon BMEP, which is the force that drives the piston down the cylinder to produce power.
Before we get too far, let’s talk about what an aftercooler is and what it does. An aftercooler is nothing more than a heat exchanger. The air leaving the turbocharger is hot. The higher the boost pressure, the more the air is compressed, and the more heat that is compacted into that intake air.
When the air enters the intercooler, it’s routed through a series of tubes that are physically connected to several thin fins, which increase the total surface area to conduct heat away from the boosted air. You can increase the effectiveness of the intercooler by placing it in vehicle frontal airflow, which brings more cool ambient air over the cooling fins. It’s just like your radiator, only you’re flowing compressed air through these tubes instead of water.
Let’s talk more about what the aftercooler is really doing. Its main function is to further increase air density beyond what the turbocharger has produced. Its secondary functions are reduced thermal load and lowered detonation threshold. The goal of your turbocharger system is not to create excessive boost pressure—you want increased air density for enhanced engine performance. Boost pressure is important to the enhancement of VE, but excessive pressure can result from super-heated air if the compressor is operating outside of its range of efficiency. The absence of an intercooler will cause excess thermal stress and detonation. During the act of cooling the air, the aftercooler must actually reduce boost pressure slightly, by about 1 to 2 lbs, because of ideal gas law requirements.
Most well made aftercoolers tend to run between 60 and 75 percent efficiency. The efficiency of an aftercooler basically measures the comparison of the heat removed by the aftercooler as a function of the heat added from compression. In other words if the turbo’s compressor increased the air temperature 200 degrees F over ambient, then the cooler took that 200 degrees back out, it would be 100 percent efficient. If you install an aftercooler and have your engine properly instrumented, you can calculate your aftercooler’s effectiveness (Example 1). If you end up measuring an efficiency of less than 60 percent, it might be time for an upgrade. On the other hand, if you’re confident of your new cooler’s efficiency rating, you can predict your potential T3 value if you have data logged ambient, T1, and compressor discharge temperature, T2 (Example 2).
T2 – T3 / T2 – T1 = Aftercooler Efficiency
T1 = Ambient air temperature
T2 = Compressor discharge temperature
T3 = Aftercooler discharge temperature
Assume an ambient temperature of 75 degrees F (T1), compressor discharge of 275 degrees F (T2), and an aftercooler discharge temperature of 135 degrees F (T3).
275 – 135 / 275 – 75 = .7 or 70 percent efficiency
In example 1, you’re cooler is doing its job well.
Now let’s predict T3 for a nonaftercooled application. Maybe you didn’t have the money or didn’t feel an aftercooler was necessary. But you’re now running higher boost than you originally intended and you’re hearing detonation. This is the formula for predicting T3 (aftercooler discharge temp) when adding an aftercooler with a known efficiency.
T3 = T2 – ([T2 - T1] x .7)
Assume a compressor discharge temperature of 275 degrees F (T2), an aftercooler efficiency of 70 percent, and an ambient temperature of 75 degrees F (T1).
T3 = 275 – ([275 - 75] x .7)
T3 = 275 – (200 x .7)
T3 = 135 degrees F
In this example, your intake manifold temperature drops from 275 to just 135 degrees—a 140- degree improvement. This would decrease your exhaust temperature by about the same amount and likely eliminate your detonation problem. Assuming about a 2:1 pressure ratio, or 15 lbs boost, along with a 70 percent compressor efficiency, you could expect to be able to produce about 15–18 percent more power at the same engine speed while making about one psi less boost.
One important consideration relative to a retrofit of a cooler is that the dramatically lower EGT in Example 2 could lower the available energy driving the turbine. This will slow down the turbine, further reducing boost (the cooler’s efficiency and temperature reduction also lower boost). When this occurs it may become necessary to use a slightly smaller turbine housing to maintain desired boost level. However, if your turbine housing was a bit on the small side and the boost actuator was set to actuate very early, your match may not require a change. Don’t interpret a large drop in manifold pressure as a sign that you’re intercooler is too small, especially if it’s sourced from a reputable source that has rated it well within your horsepower range. This is another reason it’s important to buy your parts from a reputable source.
Now that we’ve run through these examples, let’s return to the question, “Do you need an aftercooler?” If you’re planning to run over 7 lbs of boost, the answer is always yes! Check out the density ratio chart on page 87. First off note the aftercooled and non-aftercooled numbers diverge as a function of boost. The higher the boost the greater the heat produced and the more important an aftercooler becomes. As boost raises, it becomes evident how the cooler begins to add measurable value to air density.
However, note how the two groups of lines, each representing the same compressor efficiencies, have different relative spreads. The lines in the non-aftercooled group are much farther apart than those in the aftercooled lines. One could interpret from this that compressor efficiency is not as important in an aftercooled application, but that would be a mistake. Remember that the turbocharger becomes an integral part of the engine, and a less efficient compressor will require more work from the turbine, which will create more backpressure on the exhaust side of the engine and reduce overall performance. The turbine is driving a compressor that hasn’t yet seen the intercooler. The compressor has no idea there is even an intercooler in the system. So in every situation, the more efficient the compressor is, the easier it is for the turbine stage.
I have heard some say that intercoolers don’t make power, they only increase air density. While this is true in part, it seems like an overly academic argument. Nothing makes power alone, more air doesn’t make power without fuel, and fuel doesn’t make power without air. The point is that individual components, such as an intercooler, support higher horsepower, and that’s the real key. Naturally you’ll need more fuel with a cooler installed, because you’ll have a denser intake charge, so if you burn it correctly, you’ll make more power.
Choosing an Aftercooler
Aftercoolers are an extremely important component in the overall turbocharger system, but they’re not all created equal. There are two primary types of construction used on automotive coolers: tube and fin, and bar and plate. Most commercial diesel applications utilize the tube and fin aftercoolers. That design lends to more cost-effective manufacturing methods, while the bar and plate construction tends to be more labor intensive and contain more material weight.
An aftercooler, by its very nature tends to be somewhat of an engineering dichotomy. It is both a pressure vessel and a heat exchanger. It needs strength to withstand both the boost pressure and stresses of thermal cycling. This means it needs to be robust enough to stand up to the pressure under use, while also being made of material that conducts heat very well and uses thin cross-sectional areas for maximum heat rejection characteristics.
There are even different tube and fin designs to be aware of. The lowcost, low-pressure designs will use tubes formed from flat plates that will be seam welded, while the higherquality, higher-pressure designs will use extruded aluminum tubes. The tube and fin design can be made very robust for high pressures, but header thickness must be increased and extruded tubes are a must. When selecting an aftercooler, it isn’t likely that the supplier will share efficiencies with you, but you can determine whether the type of construction is compatible with your application. If you’re going to run over 20 lbs of boost pressure with a tube and fin construction cooler, you should make sure that the air tubes are made from extruded aluminum.
The bar and plate type construction literally uses a series of bars and plates stacked up to form the air tubes. This design is much more costly because of the labor required, but is capable of higher pressures than even extruded-aluminum tube and fin designs. Extreme boost applications should exclusively use the bar and plate design for dependable performance.
Another advantage of the bar and plate design is the flexibility of cooler thickness. The tube and fin design is limited by the header width and the tube design width. Manufacturing a wider aftercooler for a performance application, while costly, is more possible in a bar and plate design; you simply make the plates wider. This provides increased surface area for more heat rejection capability. If you have the room, it’s an advantage.
Both types of cooler designs should employ the use of turbulators inside the air tubes to help increase cooler heat rejection efficiency. Air flowing through a tube does not move at even speeds throughout the cross-sectional area of the tube. The air toward the surface of the tube tends to move more slowly due to what is called laminar boundary layer flow. The boundary layer in physics and fluid mechanics is the layer of fluid, or air, in the immediate vicinity of a bounding surface. In atmosphere, the boundary layer is the air closest to the ground. This is why the wind speed picks up as you rise in altitude. If we lived in a tube, the air would slow back down as we approached the other boundary at the peak altitude.
Ludwig Prandtl first defined the aerodynamic boundary layer principle in a paper presented in 1904 in Heidelberg, Germany. The understanding of this principle has become extremely important in the areas of turbines, aircraft wing design, meteorology, and heat transfer. Boundary layers are either laminar (layered) or turbulent (disordered). In heat transfer the majority of the heat transfer to and from a body takes place in the boundary layer. Therefore, an aftercooler with completely open air tubes will have a much lower capability to reject heat due to the laminar flow, where the boundary layer would allow heat to be retained in the higher speed outer flow. The outer flow is the specific reference to that portion of airflow farthest away from the bounding layer, which in our case would be toward the middle of the tube. The illustrations below show how air flows through an open tube and how the use of turbulators converts laminar flow into turbulent flow for increased heat rejection.
Horsepower conveniently sizes aftercoolers. Since you already know your horsepower goal from the compressor matching exercises (see how valuable your realistic horsepower objectives are?), then you’ll have a good idea of what you’ll need. However, there is the consideration of available space. Several things go into the calculations of an aftercooler’s horsepower rating, including surface area and thickness. It’s all about cubic inches of heat exchanger capacity, which you can simply calculate by base x width x height. However, you can imagine that the same cubic inch size of cooler with a larger frontal area will be a bit more efficient. With that said, simply choose a welldesigned cooler, rated for your horsepower level, that utilizes all of the frontal area available. Be cautious not to go overboard on thickness.
If you’re project is a street driven vehicle with limited frontal area, adding a big intercooler can create cooling system problems by significantly decreasing the cool airflow to the engine’s coolant radiator. The engine’s radiator wasn’t designed with an aftercooler in mind. The higher thermal load can also affect the viscous fan clutch’s bi-metallic strip or coil, which regulates when it comes on (if you don’t have an electric fan). Typically, the bimetal spring on the fan clutch is calibrated to an air temperature that correlates directly to the coolant temperature in that particular vehicle. An aftercooler can cause a higher heat load and make the fan clutch think the engine is too warm and turn the fan on early. Since an engine-driven fan is typically the largest horsepowerconsuming device on the front of an engine, this can be significant.
If your vehicle has an electric driven fan, it is thermostatically controlled by a sensor located in the water jacket. Watch for adequate cooling with your temperature gauge; there may not be enough cooling reserve to properly cool the vehicle in warmer weather use with your new-found horsepower. If this proves to be a problem you can possibly address the situation by adding electric driven fans in front of the cooler to help address the pressure drop across both coolers, the aftercooler, and radiator.
If you can eliminate the engine driven-fan altogether, that’s even better. A fan consumes a tremendous amount of horsepower, although there’s no free lunch in the natural world. An electric driven fan is still less efficient than an engine mounted fan because of the losses inherent in the alternator to produce the electricity, and the electric motor driving the fan. The most important consideration here is the control. The horsepower required for driving an engine-driven fan increases with the cube of the speed increase. For example, a fan that takes 5 hp at 3,000 rpm will require 40 hp at 6,000 rpm!
The speed increase is a factor of 2 (it doubled, from 3,000 to 6,000), so: (5 x 2³) = 40.
This is but one reason to eliminate the fan, but the other is safety. Fans are typically not rated to spin at the RPM levels competition vehicles run. A fan blade burst is a potentially lethal event, so take precautions in this area.
There are many sources for welldesigned aftercoolers in the performance aftermarket. They come in all sizes and shapes and rated horsepower capacities. Vibrant Performance even offers coolers with polished tanks to complete the highend look that compliments their line of polished boost tubes. Turbonetics likewise carries a wide array of coolers marketed under their Spearco brand. Spearco is a long-time name in cooler technology and with perhaps one of the most complete lines of cooler products for boosted gasoline automobile engines. In addition to ready-made coolers, Spearco also carries a wide array of water-to-air coolers for racing applications. You can also get custom size coolers with a bar and plate design feature. Spearco also offers cooler manifolds for the do-ityourself fabricator.
Mounting your aftercooler is an important aspect relative to maintaining cooler integrity. A cooler can leak and a leak will cause poor performance indeed. Boost leaks are never good. Remember that an aftercooler is chassis mounted and gets very hot during its extreme thermal cycles. Torsion twists in the vehicle frame are part of high-horsepower life, allow for that. The mounting surface for the cooler must allow the cooler to sit square or flush with its mounting points and not be in a bind. If your threaded fasteners pull the cooler onto its mount unevenly and place it in a bind, it will place the entire structure in a torsional twist that once heated, can induce premature failure of the heat exchanger core and allow the brazing of the tubes to the header plate to rupture. It’s also a good idea to mount the cooler in high-density rubber grommets to allow the cooler isolation from frame twisting.
Aftercoolers should, for the most part, not leak air, but in many commercial coolers there is some leakage called bleed. It can be difficult to tell if your cooler is leaking because there isn’t anything running out onto the ground (such as oil or tranny fluid). Further, there’s no boost at idle so measuring a leak with the engine idling isn’t possible either.
The method for testing a cooler is with a proper leak test kit. In commercial vehicles, acceptable bleed is defined as not more than 5 lbs pressure loss in 15 seconds from a static pressure charge using shop air of 30 lbs total pressure. If you have reason to believe your cooler may have a leak, checking cooler integrity is a good idea. But use great caution! Do not make a homemade device! Professional test kits exist that use special rubber plugs and positive plug restraints that mechanically hold the plugs in place.
Most high-performance aftercoolers are made with more care than the typical commercial diesel coolers and will be totally leak free, which means they will not exhibit any level of air bleed. If you find a leak-down can measure a small bleed of say half of a pound pressure in the 15-second test, don’t assume you’ve found a problem. Fix the leak, but if you’ve had a tuning problem, you’ll probably need to look elsewhere.
CAUTION: The air volume contained in an aftercooler and the pressure used in testing can launch a 3-lb missile (the plug and clamp plate) at over 75 mph for over 50 feet! This force is lethal! Only use equipment that is specially designed for this purpose to avoid bodily harm.
Clamps and Hoses
Clamps and hoses must not be overlooked in the assembly of a turbo system. Using proper equipment can shield you from the major headache of a leak down the road. The clamps used in a turbocharger system must be the “constanttorque” type. Most hose clamps in automotive applications are the standard worm-gear type. They can be easily over tightened and break and/or cause a severed hose.
The hose connections in a turbo system are subjected to many cycles of heating and cooling that include constant joint expansion and contraction. Constant torque clamps are designed to automatically adjust their diameter to compensate for the normal expansion and contraction of the connections. Just as important is that the hose ID should closely match the tubing OD. Do not use a clamp to correct a size mismatch between the tubing and hose ID. On extremely high-boost pressure applications, such as over 20 lbs, double clamping is sometimes used in conjunction with boost straps. Boost straps (or boost braces) are simply steel straps that mechanically limit movement between tubing end, and therefore relieve the linear stress on the hose joint and leave them to the job of sealing.
There are a number of types, sizes, and grades of silicone hoses, (rubber hoses should never be used). You can even get them in colors for the cosmetic minded hot rodder. For cost purposes, hoses will commonly have a cold end and hot end designation. Be sure that the hoses you are using are rated for the temperatures you will be seeing. The hottest points will be the compressor discharge connection to the boost tube leading to the aftercooler and the aftercooler inlet connection. To be on the safe side, it may be wise to use hot side rated hoses throughout the entire system. Good hoses will be rated to withstand 400 degrees F or more.
When connecting an enginemounted component like the turbocharger’s boost tube, to a chassis mounted component, like an aftercooler, recognize that these components will move relative to each other. In these cases, hump-hoses are typically used where the hoses are molded with one or more humps in the hose length that allow for an excess of material to be present for a given length, which allows for movement without stressing the hose or the joint connection.
The routing of boost tubes from the turbo to aftercooler and back to the engine may become a plumber’s nightmare. Don’t despair. Many specialty type hoses exist from companies such as Turbonetics and Vibrant Performance for just these purposes.
Building boost tubes can be very simple or complex depending upon the particular application. If you’re making runs of tubing that are fairly straight, the tubes should be only slightly larger than the compressor discharge diameter as you route it toward the aftercooler. If your application is not aftercooled, and you’re routing the compressor discharge to the intake, you may want to enlarge the boost tube before the bend that enters the plenum. This helps to slow the air movement down and assists in the diffusion to convert the air from high-velocity flow into static pressure, which is the object of the turbo’s diffuser to begin with.
While there are pros and cons of ducting and routing of the boosted air to minimize line loss, there will also be reality of just where you have to run the tubes to fit your application. There are many successful applications that route the boost tubes in a manner that may not be the most optimum, but are necessary in that given vehicle and engine combination. There are many sources for pre-bent mandrel tubing, which keeps the fabrication simpler, and even some sources for runs of tubing that are already chrome plated, or polished aluminum if aesthetics are important to you.
Extremely tight bends should be avoided if at all possible, but if you really need one, a cast elbow may be prudent. Tight bends can be cast more successfully than can be formed in tubing. However, you will rarely see this done in the intake routing because there is typically enough room for better options.
The plenum is the part of the system that connects the boost tube leading from the compressor discharge or the aftercooler discharge to the intake manifold. Depending upon your type of engine application and intended use, there are some different design considerations. In a high-performance drag-race engine, the plenum is typically small and serves the primary function of adapting the throttle body to the boost tube. In such applications, the plenum simply provides a smooth air transition to the manifold.
For many street-driven vehicles, the intake manifold was designed as a naturally aspirated engine component. On the street, there are many situations where you’ll be transitioning from one speed to another and will want a smooth transient response. In these circumstances, Gale Banks likes to build in what he calls “gulp capacity.” This is a momentary reserve capacity of a slightly boosted air supply to reduce system lag. During mild accelerations, such as accelerating from 30 mph to 55 mph as you merge onto the highway, the increased volume of boosted air in the plenum will help transition the engine because there is air volume to immediately draw upon. By contrast the drag race vehicle where all-out acceleration is the only concern, that excess plenum capacity could add to the intake system capacity beyond what the aftercooler already adds and increase system response time because it will take more time to fill it in a race car going from zero to all-out acceleration. Therefore, a plenum design needs to take into account your application and usage like so many other factors. If it’s a race vehicle, a small-volume plenum is fine. If you’re building a street driven vehicle where you will expect to have sudden changes in throttle position from part load to full load for passing and on-ramp acceleration, then extra plenum capacity is a consideration as shown in the Banks 6.2 diesel system.
When routing the boost tube into any manifold plenum, the point of entry must insure that air swirl and pressure distribution are considered. The Banks sidewinder turbo system on the early 6.2 diesel is a good example of both gulp capacity and air diffusion that ensures an even pressure feed to the manifold allowing equal air distribution to all cylinders. By contrast, the plenum from Precision Turbo and Engine is a competition piece where gulp capacity isn’t a primary concern, but a smooth transition is. Many types of plenums are ready made for most all types of applications.
Turbo Mounting for Proper Oil Drain
Hopefully you considered oil drain back before you chose your turbocharger’s mounting location. To ensure proper oil draining, the bearing housing must be oriented correctly (see photo). The compressor cover and turbine housing will typically rotate independent of the bearing housing to match up with compressor discharge and exhaust inlet requirements.
When routing the oil drain return line to the oil pan, be sure that its point of entry is well above the oil level in the pan, and that the drain line always runs downhill. You never want the oil to have to climb as it drains out of the turbo. If the oil tries to drain below the oil level in the pan it will back up and flood the drain cavity in the turbo. This will flood the seal ring areas and cause oil leakage out of the compressor or turbine or both.
Exhaust manifolds for street turbo systems are commonly seen in both tubing-type construction and cast-type manifolds. Do not be confused by thinking that manifolds that look like headers are better, just like tubing headers are better than old cast exhaust manifolds. In a turbocharger installation I would rather have a casting for the durability and strength of mount. It’s just that many times there are not enough customers to buy a certain type of manifold for any manufacturer to go to the expense of creating a foundry tool to manufacture a turbo mount exhaust manifold. So don’t be confused between tubing and cast manifolds as to which is superior. If you’re building a street project and there happens to be a cast manifold that fits your engine, you’re lucky!
Many cast exhaust manifolds now exist for popular turbo applications, but competition vehicles will typically use tubular manifolds. If you’re ambitious enough to build your own tubular manifolds, you’ll want to be sure you do not use mild steel tubing. As a minimum material spec, use 304 stainless with a minimum wall thickness of 0.065 inch. Typically, the most difficult part in making your own tubular manifolds is the fabrication of the four-intoone joint for 4-cylinder or V-8 engines or the six-into-one joint for straight-6-cylinder engines. The popularity of turbocharging in today’s market has come to the rescue here, too! Vibrant Performance offers special joints that allow easier custom manifold fabrication. If you use these pre-made joints, your job will become much easier and you can still claim you made them!
In a turbocharged engine, keeping the primary tubes equal length is not as big of a concern as it is in a naturally aspirated engine. There is some thought that providing some length is necessary to allow better cylinder scavenging by giving the gases someplace to go. More important however, is the primary tube diameter. In a naturally aspirated engine, there is typically an optimum primary size that provides sufficient exhaust gas expansion to help lower the pressure behind the exhaust pulse to help scavenge the next pulse, yet not so large as to cause plumbing nightmares or overly dilute the pulse energy where the primaries converge into the collector for adjacent primary scavenge assist. In a turbocharged engine, exhaust gas velocity can exceed 2,000 feet per second. The tip speed of a 3-inch diameter turbine wheel that is rotating at 120,000 rpm equates to about 1,600 feet per second. If the primary tubes are sized the same as the exhaust port diameter, then your system plumbing will not cause the exhaust gas to slow down only to have to speed back up as it approaches the turbine. It’s better to keep the primary tube diameter a constant until it reaches the turbine for improved blending. For this reason, extremely large exhaust manifolds are not recommended.
As the exhaust leaves the turbine exducer it should ideally be flowing axially, but it’s not. The gas will swirl. The swirling gas does not tend to exit as quickly. For this reason, the turbine housing may have a conical diffuser or bell mouth shape as it transitions into its exhaust connection. The diffuser tends to convert the swirl flow to a more turbulent axial flow. This feature built into the turbine housing can consume installation room for tight fit engine compartments. To achieve this diffusion, all that’s needed is about 1-1/2 to 2 feet of length in larger diameter tubing before necking down to transition into an exhaust system (if you’re running one!). In a competition vehicle, that 2-foot length of dump pipe will likely be all that’s needed. If your turbine discharge is a 3-inch diameter, using a conical shaped transition from the 3-inch connection to a 4-inch or 5-inch discharge pipe will do nicely. However, for most performance street driven applications this design feature will have very limited impact.
Heat Bellows & Expansion Joints
The extreme heat of a turbo system can cause tubular type exhaust manifolds to expand and contract, which will lead to cracking and rupture. This is especially true in highhorsepower applications seen on V-6 and V-8 engines where six-into-one or eight-into-one manifolds are used. Placing an expansion joint in the end of the manifold, at the turbine inlet, and on the wastegate duct can help your manifold lead a long and happy life.
The subject of heat shielding is somewhat controversial. Since turbines extract their energy from heat, many believe that wrapping the tubing in a tubular type manifold or crossover pipe will create (or preserve) more heat energy available for the turbine. There have been several tests to try and quantify this effect. While in theory seems sound, there is virtually no measurable gain in performance in doing this. The practical issue here is that the exhaust flow at wide open throttle, where you would be most concerned about efficiency, is traveling so fast, and the fact that the flow will likely be laminar in nature, that virtually no significant heat energy is lost.
The primary reason for adding heat shielding to either the turbine housing or the exhaust manifold tubing is to protect other components from the radiant heat given off. Most OEM turbocharged vehicles feature extensive heat shielding, as it’s up to the manufacturers to protect the rest of the components so they survive the warranty period. To help you out, Turbonetics carries pre-formed turbine housing heat shields, as well as flats of spun ceramic insulation wrapped in corrugated aluminum to form to most any shape for component isolation and heat protection. Other aftermarket companies also offer heat-resistant blankets and sleeves to protect things like starters, rubber hoses, and spark plug wires.
If proximity to temperature sensitive components is not an issue, you are most likely better off not wrapping the tubing. Under the right conditions, it can cause distortion of the bends and accelerated corrosion.
Written by Jay K. Miller and Posted with Permission of CarTechBooks