Home | WRX/STi/XT/GT/Baja Turbo | Impreza 2.5 | Legacy/Forester/Baja | Specials | Gallery | Contact Us | Order Form

Turbochargers 101

The purpose of this page is to answer some general questions about turbocharging. It is not intended to be the be-all, end-all technical resource on turbocharging; rather, this is a primer to help you understand the basics of operation. We hope that you find this information useful.

What is a turbo?

Quite simply, a turbo is merely an exhaust-driven compressor. Imagine a small shaft about the size and length of a new pencil. Now rigidly attach a pinwheel to each end of the pencil. One pinwheel (called the turbine) is placed in the path of the exhaust gases which are exiting the engine. These gasses are 'caught' in the turbine, causing it to spin. This in turn spins the whole shaft, along with the pinwheel on the other end (called the compressor). The compressor is placed in the intake air's path; once it begins spinning, it actually compresses the air on its way into the engine.
Why is this beneficial? Well, normally aspirated engines have to work to draw in their intake air. In other words, as the intake valves open, the piston's downward movement creates a vacuum which 'sucks in' some air through the intake system. Ideally, the piston's movement would suck in 100% of the air that could fill the combustion chamber. In the real world this is not the case; the typical engine will draw in only about 80% of the total volume of the combustion chamber. There are many reasons for this--intake restrictions, valve timing, camshaft design, and much more.

Now imagine that the engine mentioned above has a turbocharger. When the turbo compresses the air, it builds up pressure in the intake manifold. Now when the intake valves open, air is actually forced into the combustion chamber. (This is one reason why turbocharged engines are sometimes referred to as 'forced-induction' engines.) As you might imagine, this allows more air molecules to fill the chamber.

Okay, so now we have more air molecules entering the engine. To benefit from this, we need more fuel to match. On computerized vehicles such as these, various sensors will "see" this amount of boost pressure and increase the amount of fuel accordingly. Now that we also have more fuel entering the engine, more power is made. (When you get right down to it, the only way to make more power--on any engine--is to shove more of the proper air/fuel mixture into the engine.)

How do turbochargers and superchargers differ?

While they perform the same function, turbochargers and superchargers go about it in completely different ways. As has already been mentioned, a turbo is driven by the exhaust gasses which are already being expelled from the engine. So, in effect, turbos add 'free' power since their compression is created by what was already discarded.
Superchargers, however, are different: they are belt-driven. They feature a pulley whose belt is directly attached to the crankshaft, this allowing them to spin in direct proportion to the engine itself. The upside is a near absence of lag (see below); at least some boost is typically available the instant you crack the throttle. The primary drawback to a supercharger, however, is that they take power to make power. The overall result is more power than there would be without the supercharger; it's just that they aren't as efficient as a turbocharger from an energy standpoint. Other drawbacks include lower mid-range power than a turbo, lower thermal efficiency than a turbo, (sometimes) much harder to incorporate intercooling, etc.

What is turbo lag (and how do I avoid it)?

The majority of turbochargers feature a wastegate--a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo's shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a 'stand by' mode. Since the turbo isn't needed during relaxed driving anyway, this effect is harmless...

...until you suddenly want to accelerate. Let's say that you are loafing along, engine spinning 1500 rpm or so. You instantly floor the throttle. The exhaust gas flows through the turbo and cause it to spool (spin up to speed and create boost). However, at this engine speed there isn't very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas flow to spool the turbo. This wait time--the period between hitting the throttle at low engine speed and the creation of appreciable boost--is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you're already at a sufficient engine speed to create boost. For example, let's say your engine can make 12 psi at 4000 RPM. You're cruising along at a steady road speed, engine spinning 4000 RPM, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo's lag time. Between the two, slow boost response usually causes the most complaints.

There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag... although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your RPM range. (Behold the catch-22 in designing one turbo for all uses.)

Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don't get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticeable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you'll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.

What's an intercooler and how does it help?

To answer that question, a discussion of thermodynamics is involved. Turbos, as has been mentioned, compress an engine's intake air. By laws of physics, compressing air also heats it. For an engine, heating the intake air is a bad thing. For one, it raises the combustion chamber temperature and thus increases the chance of detonation (uncontrolled combustion which damages your engine). Another bad thing is that air expands as it is heated. So in other words, it will lose some of the compression effect and the turbo must work harder to maintain the desired level of compression.
Thus enters the intercooler into the equation. An intercooler is a heat exchanger--sort of like a small radiator except that it cools the charge (your intake air) rather than the engine coolant. Now that the charge is being cooled, two benefits appear: combustion temperatures decrease (along with the detonation), and the charge becomes denser which allows even more air molecules to be packed into the combustion chamber. Exactly how much heat is removed varies greatly; some factors include the type of intercooler used, its efficiency, and its mounting location. From what I've seen, getting your intake charge temperature within 20 degrees of ambient is excellent; consider this a practical limit for a street-driven car (meaning you might get closer but probably not without spending tons of money).

There are two types of intercoolers: air-to-air and air-to-water. Air-to-air means that as the charge passes through the intercooler, the intercooler itself is cooled by air flowing through its fins. Picture your car's radiator but substitute the intake air where the coolant goes and you'll have a rough idea of how it works. In an air-to-water intercooler, the intercooler is cooled by a liquid rather than air; this liquid has its own radiator placed where it can receive airflow, hoses connect this radiator to the intercooler itself, and the liquid must be circulated throughout the entire system.

Each type of intercooler has its strength and weakness. Air-to-air units tend to require longer ducting to route the air from the turbo through the intercooler then back to the engine; this extra tubing might increase lag slightly on mass-flow engines and may also present interesting packaging challenges. Air-to-water units, however, can have significantly shorter intake plumbing; the intercooler can be placed in hot underhood areas where no airflow is present since the liquid coolant circulates to its radiator. This allows for simpler installation but at an expense of reduced cooling efficiency. Note that both kinds cool better when air is flowing through the intercooler (air-to-air) or the radiator (air-to-water); both kinds can benefit from the installation of a fan for low-speed operation.

Which type is better? Depends on your goal. From where I sit it seems that air-to-water intercoolers are used either for convenience--to eliminate the possible ducting nightmare of the intake--or for drag-only vehicles where a "one shot" setup uses ice to actually drop charge air temps below ambient... for a very short while.

Can I mount more than one intercooler?

Sure you can; your limits will be defined by the room you have to work with and your budget. (Owners of mass-flow setups will need to keep an eye on the total length of their intake plumbing.) If you try this, should you mount your intercoolers parallel or in series? The correct answer is simple: in parallel. ALWAYS. Mounting intercoolers in series doubles your pressure drop, which is very bad, while mounting in parallel cuts your pressure drop in half while also allowing for more thorough cooling. Twin intercooling can cause great results, just like upgrading to one large intercooler. A racer's rule of thumb states you can never have too much intercooler.
Can I make my air-to-air intercooler more effective?

Certainly! What can be done? For starters, maximize airflow through the intercooler. This means remove anything between the incoming air and the intercooler's fins--the A/C condenser, funky ducting, or anything else that actually impedes airflow. If your intercooler isn't directly in the path of air, relocate it so that it is. If you are unable to move it around, create some sort of shroud/airdam to redirect air through the intercooler (tin or plastic should be great for this).

Another idea for you creative types is to make a mister. Get a windshield fluid reservoir, mount it where it will stay cool, and fill it with water. Now run the output tube to the intercooler. Mount a few spray nozzles aimed at the front of the intercooler's core, then join them to the output line with tees and such. Rig up this reservoir pump to a switch or button inside the car so that you can momentarily enable it when desired. The water evaporation will help draw even more heat off the intercooler, further lowering the temperature of the intake air that flows through it. You can get really fancy here; I had a friend that rigged the on/off switch to the throttle body so that the mister would activate at WOT.


What are the main tuning problems when dealing with Turbos? Engine calibration - fueling and ignition timing. Under boost, it is crucial that there is no engine-killing detonation occurring within the cylinder. This is done by fine tuning the air/fuel ratio a bit rich to help cool the combustion gas, and by tuning the ignition advance curve to ensure that combustion chamber pressures stay below the level that causes unburned fuel to ignite ahead of the advancing flame front.

What are the main differences between a Single and Twin Turbo setup?

1. A single turbo receives exhaust flow from and supplies air to all cylinders.

2. The most common type of twin turbo setup is the parallel system where each turbo is fed by of the engine's cylinders. Here, both compressors supply air to the intake manifold simultaneously.

3. There are also sequential twin turbo systems, which run on one small turbo at low engine speeds and switch to two parallel turbos at a predetermined engine speed and/or load.

4. Furthermore, there are series twin turbo systems where one turbo feeds the other turbo. These are primarily used on diesel engines due to the extremely high boost levels that can be generated.

Choosing between a single or parallel twin turbo setup is primarily based on packaging constraints in the engine bay, or a personal choice by the tuner. In most cases, for top performance, a single turbo is preferable because larger turbos are generally more efficient than smaller turbos. However, often there is not room for one large single, or the tuner wants the visual impact of twin turbos. The notion that two smaller turbos will build boost faster than one large turbo is not always accurate because even though the turbos are smaller, each one is only getting half of the exhaust flow. Sequential systems seem to have the capacity to support big power. In theory, the sequential twin turbo setup is a potent combination. A few O.E.s have produced systems of this type but control issues have proven significant, making them challenging to function seamlessly. One slight draw back to a sequential twin turbo system is that sometimes during daily driving (specifically, in cornering) if the driver is not constantly aware, the second turbo will spool and result in a lot of unpredicted power.

What is Turbo Lag? Turbo lag is the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo lag is determined by many factors, including turbo size relative to engine size, the state of tuning of the engine, the inertia of the turbo's rotating group, turbine efficiency, intake plumbing losses, exhaust backpressure, etc.

What is Boost Threshold? Boost threshold is the engine speed at which there is sufficient exhaust gas flow to generate positive manifold pressure, or boost.

What is a boost leak? A boost leak means that somewhere in the turbo or intake, there is an area where the air (boost) is escaping. Typically a boost leak is caused by a loose or bad seal, cracked housing, etc. When a boost leak is present, the turbo will be able to generate boost, but it may not be able to hold it at a constant level and pressure will drop off proportionally to the size of the leak.

How can I adjust the turbo boost? Adjusting the boost is straightforward. However, it depends on the type of boost controller.

1. For a standard Wastegate actuator, simply recalibrate the actuator to open (more or less) for a given pressure. Changing the length of the rod that attaches to the

2. Wastegate lever accomplishes this adjustment.

For mechanical boost control systems, adjustments may involve changing the setting on a regulator valve(s).

3. For electronic boost control systems, adjustments may need to be made to the vehicle's engine management system.

4. For an external Wastegate, adjusting the boost often requires turning the adjustment screw (when equipped) to increase/decrease spring load, changing Wastegate springs, or shimming Wastegate springs.


What is boost spike? A boost spike is a brief period of uncontrolled boost, usually encountered in lower gears during the onset of boost. Typically spikes occur when the boost controller cannot keep up with the rapidly changing engine conditions.

What is boost creep? Boost creep is a condition of rising boost levels past what the predetermined level has been set at. Boost creep is caused by a fully opened Wastegate not being able to flow enough exhaust to bypass the housing via the Wastegate itself. For example, if your boost is set to 12psi, and you go into full boost, you will see a quick rise to 12 or 13psi, but as the rpm's increase, the boost levels also increase beyond what the boost controller or stock settings were. Boost creep is typically more pronounced at higher rpm's since there is more exhaust flow present for the Wastegate to bypass. Effective methods of avoiding or eliminating boost creep include porting the internal Wastegate opening to allow more airflow out of the turbine, or to use an external Wastegate.

What is compressor surge? The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side. Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself. Common conditions that result in compressor surge on turbocharger gasoline engines are:

1. A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body

2. The outlet plumbing for the bypass valve is too small or restrictive

3. The turbo is too big for the application

How does a Wastegate work? A Wastegate is simply a turbine bypass valve. It works by diverting some portion of the exhaust gas around, instead of through, the turbine. This limits the amount of power that the turbine can deliver to the compressor, thereby limiting the turbo speed and boost level that the compressor provides.

1. The Wastegate valve can be "internal" or "external". For internal Wastegates, the valve itself is integrated into the turbine housing and is opened by a turbo-mounted boost-referenced actuator.

2. An external Wastegate is a self-contained valve and actuator unit that is completely separate from the turbocharger.

3. In either case, the actuator is calibrated (or set electronically with an electronic boost controller) by internal spring pressure to begin opening the Wastegate valve at a predetermined boost level.

4. When this boost level is reached, the valve will open and begin to bypass exhaust gas, preventing boost from increasing.

What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary? A Blow Off Valve (BOV) is a valve that is mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge. Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.

How should I break in a turbo? A properly assembled and balanced turbo requires no specific break-in procedure. However, for new installations a close inspection is recommended to insure proper installation and function. Common problems are generally associated with leaks (oil, water, inlet or exhaust).

What is/causes Shaft Play? Shaft play is caused by the bearings in the center section of the turbo wearing out over time. When a bearing is worn, shaft play, a side to side wiggling motion of the shaft occurs. This in turn causes the shaft to scrape against the inside of the turbo and often produces a high-pitched whine or whizzing noise. This is a potentially serious condition that can lead to internal damage or complete failure of the turbine wheel or the turbo itself.

What is causing my turbo to sound like a sewing machine's whistle? The "sewing machine whistle" is a distinct cyclic noise cause by unstable compressor operating conditions known as compressor surge. This aerodynamic instability is the most noticeable during a rapid lift of the throttle, following operation at full boost.

I want to make x horsepower, which turbo kit should I get? or Which turbo is best? Select a turbocharger to achieve desired performance. Performance includes boost response, peak power and total area under the power curve. Further decision factors will include the intended application. The best turbo kit dictated by how well it meets your needs. Kits that bolt on without any modification are best if you don't have fabrication capabilities. Less refined kits can be cost effective if you access to fabrication capabilities. For more information on the right turbocharger for you, please contact Xcceleration.

Does my turbo require an oil restrictor? Oil requirements depend on the turbo's bearing system type. Some types of turbos have two types of bearing systems; traditional journal bearing; and ball bearing. The journal bearing system in a turbo functions very similarly to the rod or crank bearings in an engine. These bearings require enough oil pressure to keep the components separated by a hydrodynamic film. If the oil pressure is too low, the metal components will come in contact causing premature wear and ultimately failure. If the oil pressure is too high, leakage may occur from the turbocharger seals. With that as background, an oil restrictor is generally not needed for a journal-bearing turbocharger except for those applications with oil-pressure-induced seal leakage. Remember to address all other potential causes of leakage first (e.g., inadequate/improper oil drain out of the turbocharger, excessive crankcase pressure, turbocharger past its useful service life, etc.) and use a restrictor as a last resort. Tuners can tell you the recommended range of acceptable oil pressures for your particular turbo. Restrictor size will always depend on how much oil pressure your engine is generating-there is no single restrictor size suited for all engines. Ball-bearing turbochargers can benefit from the addition of an oil restrictor, as most engines deliver more pressure than a ball bearing turbo requires. The benefit is seen in improved boost response due to less windage of oil in the bearing. In addition, lower oil flow further reduces the risk of oil leakage compared to journal-bearing turbochargers. Oil pressure entering a ball-bearing turbocharger needs to be between 40 psi and 45 psi at the maximum engine operating speed. For many common passenger vehicle engines, this generally translates into a restrictor with a minimum of 0.040" diameter orifice upstream of the oil inlet on the turbocharger center section. Again, it is imperative that the restrictor be sized according to the oil pressure characteristics of the engine to which the turbo is attached. Always verify that the appropriate oil pressure is reaching the turbo. The use of an oil restrictor can (but not always) help ensure that you have the proper oil flow/pressure entering the turbocharger, as well as extract the maximum performance.

How much shaft play should my dual ball bearing turbo have? The full ball-bearing turbo is designed to have clearance between the bearing cartridge and center housing for hydrodynamic damping in addition to the internal clearances of the bearing cartridge itself. Hydrodynamic damping uses the incompressible properties of a liquid (oil in this case) and the space around the bearing cartridge to dampen the shaft motion of the rotating assembly. When the turbo is new, or has not operated for a long period of time allowing most of the oil to drain out, the rotating assembly will move more in the radial direction than a typical journal-bearing turbo because there is no oil in the center housing. This condition is normal. As long as the shaft wheel spins freely and the wheels don't contact their respective housings, the assembly will function properly.

What other systems are affected by turbocharging? (Fuel, Oil, Cooling, Drivetrain, etc)
There are several factors that must be addressed when deciding to turbocharge a previously naturally aspirated engine, such as: Is the current fuel delivery system capable of providing increased, adequate amounts of fuel? Is the cooling/oiling system capable of handling the extra power and consequently, extra heat that is generated by the turbo? Is the clutch/transmission/drivetrain up to the task of handling the extra power? Etc


How is boost measured? (Bar, mmHg, PSI) and How do you convert from one to another?
Boost is measured as the pressure that the turbo creates above atmospheric pressure.
Normal Atmospheric Pressure (1 atm) = 14.7 psi = 760 mm Hg
1 Bar is not actually equal to 14.7 psi, but rather it is equal to 14.5 psi, = 0.9869 atm = 750.062 mm Hg

The turbo gauges measures turbine speed, right?
The "turbo gauge", commonly called a boost gauge, does not measure turbine speed. It measures the intake manifold pressure. Under light loads the boost gauge will indicate a vacuum due to the turbocharger shaft not rotating fast enough to create positive pressure (boost). Once load (throttle position) increases, the boost gauge will indicate a positive pressure.

What is a boost controller?
A boost controller is a device that bleeds or blocks the boost pressure signal entering the Wastegates actuator. The idea is to keep the Wastegates closed to allow higher boost pressures than the actuator would otherwise allow. These can be simple mechanical or sophisticated electronic devices, with price tags to match.

Which boost controller should I get? (Manual or Electronic)
Boost controllers vary widely in performance, price, and functionality. We have found the manual boost controllers simple and easy to setup, as long as your engine is capable of handling constant boost and is tuned properly. Electronic boost controllers are more complicated, able to dial in boost for each gear and handle high boost with proper tuning. As to which one is best for your application, we suggest discussing your application with us.

How much boost can I run on pump gas?
The primary limitation to maximum boost is engine knock. It is also not advisable to run the maximum amount of boost your car can handle on a daily driven basis as a precaution against if the boost spikes. If your car's manual states to use premium fuel, do not use regular!

What is Knock/Detonation?
Knock is a condition caused by abnormal combustion of the air/fuel mixture and can result in damage to an engine.
The three factors that result in engine knock are:

1) knock resistance characteristics (knock limit) of the engine

2) ambient air conditions

3) octane rating of the fuel being used

Since every engine is vastly different when it comes to knock resistance, there is no single answer to "how much." Design features such as combustion chamber shape, spark plug location, bore size and compression ratio affects the knock characteristics of an engine. In addition, engine calibration of fuel and spark plays an enormous role in dictating knock behavior.

For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger.

The octane rating of fuel is a measure of a fuel's ability to resist knock. The octane rating for pump gas ranges from 85 to 94 while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant it is to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for your application.

Generally speaking, the more boost you run, the higher the octane requirement.

Should I run a Turbo Timer?
A turbo timer enables the engine to run at idle for a specified time after the ignition has been turned off. The purpose is to allow the turbo to cool down thus avoiding "coking" ("coking" is burned oil that deposits on surfaces and can lead to blocked passages).
The need for a turbo timer depends on how hard the turbo and engine is used. Running at full speed and full load then immediately shutting down (heat soak) can be extremely hard on a turbo.

June 2001 TechTIPS published by Subaru for Subaru Technicians (http://www.subaruwest.com/PDF_files/Tech_PDF_Folder/june_2001_techtips.pdf) states:


FHI's position regarding this is that it is not necessary to perform a "cool down/idling" procedure, as was recommended with past turbo models. Our current 2.0L turbo engine has a far greater cooling capacity and, coupled with technology advances, makes this practice no longer necessary. This explains why information about cool down is not included in the 2002MY Impreza Owner's Manual or newer.

"The heat contained in the turbo charger will begin to vaporize the coolant at the turbo charger after the engine is stopped. This hot vapor will then enter the coolant reservoir tank which is the highest point of the coolant system. At the same time the vapor exits the turbo charger, coolant supplied from the right bank cylinder head flows into the turbo. This action cools the turbo charger down. This process will continue until the vaporizing action in the turbo charger has stopped or cooled down."

Water-cooling of the turbocharger's center housing has essentially eliminated the need for turbo timers or extended idling periods (which are OEM on Subarus).

Do I really need the cool down procedure on my turbo?
The need for a cool down procedure depends on how hard the turbo and engine is used, and whether or not the turbo is water-cooled. All Garrett turbochargers must pass a heat soak test and the introduction of water-cooling has virtually eliminated the need for a cool down procedure. Garrett is one of the few turbocharger manufactures that subjects their turbos to several OE qualification tests. When you buy a Garrett turbo you can be sure it's a reliable one!

How can I remove and clean the oil condensation box/oil catch can?
The oil condensation box, or catch can, can be cleaned once it is removed with any cleaning solvent. Simply fill the box with a cleaner and slosh it around until oil deposits are gone. Removing the oil condensation box can be a challenge and varies by vehicle.
NOTE: some vehicles are not equipped with an oil condensation box.

What is the purpose of an oil catch can?
An oil catch can's purpose is to catch oil blow-by gasses that can eventually create a carbon and oil sludge build-up in the intake and turbo.

What additional maintenance is required for the turbo?
Good, clean oil is extremely important to the turbocharger. It is best to change the oil and filter at least as often as the automobile manufacturer recommends.

Turbo performance is sensitive to turbo inlet conditions. A clogged air filter can drastically affect the turbo inlet. Air filters should be inspected at every oil change and replaced at 12,000 to 15,000 mile intervals.

NOTE: Never exceed the vehicle manufacturer's recommended filter change intervals.

What compression ratio should I run with my turbo engine?
Allowable compression ratio depends on many factors, and there is no one right answer for every application. Generally, compression ratio should be set as high as feasible without encountering detonation at the maximum load condition. Setting the compression ratio too low will result in an engine that is a bit sluggish in off-boost operation. Setting it too high however, can lead to serious engine problems due to knock.

Factors that influence the compression ratio can include: fuel anti-knock properties (octane rating), boost pressure, intake air temperature, combustion chamber design, ignition timing, and exhaust backpressure. Many modern engines have well designed combustion chambers that will allow modest boost levels with no change to compression ratio, assuming appropriate tuning. For higher power targets with more boost, compression ratios should be adjusted to compensate.

How do I adjust my compression ratio?
The easiest and most effective way to accomplish this is through the use of either higher/lower compression pistons, and/or using a head gasket of a different thickness.

Should my turbo/exhaust manifold glow red after driving?
Yes, the turbo/exhaust manifold can glow red under certain driving conditions. The exhaust gas temperature can reach over 1600F under high load operating conditions; i.e. towing, extended uphill driving, or extended high rpm/boost conditions.

What should I look out for when buying a turbo?

1. Condition of the turbine housing - inspect for cracks on the exterior and inside the inlet of the housing. If the housing has cracks then the housing needs to be replaced.

2. Condition of the turbine and compressor wheels - inspect for cracks and damaged blades. If either of the wheels are damaged then the wheel (s) need to be replaced and the center section balanced.

3. Condition of the bearings - spin the turbocharger shaft and check for roughness. If roughness is detected then the turbocharger needs to be disassembled and the internal components inspected and replaced if necessary.

4. The most important factor is to make sure the turbo is the proper one for your application. A properly matched turbo will provide better performance and more reliable operation. A properly matched turbo includes matched turbine and compressor wheel sizes and appropriate housings.

Are oil deposits indicative of impending turbo failure? There is blue/black smoke, is my turbo going bad?
Blue/black smoke can be caused by numerous conditions, and one of them could be a turbocharger worn past its useful service life. The following are potential reasons that blue/black smoke could occur:

1. Clogged air filter element or obstructed air intake duct. This condition creates a vacuum due to high differential pressure resulting in oil drawn into the compressor and subsequently burned during engine combustion.

2. Engine component problems; i.e. worn piston rings or liners, valve seals, fuel pump, fuel injectors, etc.

3. Obstructed oil drain on turbocharger resulting in pressure building inside the center housing and forcing oil past the turbocharger seals

4. Damaged turbocharger or turbocharger worn past its useful service life

5. Black smoke is also sometimes indicative of too rich an air/fuel mixture.

How fast will my car go with xyz?
This question cannot be answered as how fast any given car will go depends on the unique individual setup, road/weather conditions, and of course, the driver's skill.

What is the Inducer?
Looking at a compressor wheel, the inducer is the "minor" diameter. For a turbine wheel, the inducer is the "major" diameter. The inducer, in either case, is where flow enters the wheel.

What is the Exducer?
Looking at a compressor wheel, the exducer is the "major" diameter. For a turbine wheel, the exducer is the "minor" diameter. The exducer, in either case, is where flow exits the wheel.

SUBARU Service Bulletin for Turbo Cars


Home | WRX/STi/XT/GT/Baja Turbo | Impreza 2.5 | Legacy/Forester/Baja | Specials | Gallery | Contact Us | Order Form