Turbo FAQ

The primary purpose of an aftermarket turbo is to increase the performance over the stock unit. This can be accomplished by choosing a turbo with better spool, more flow, or a combination suited to the end user’s needs.

What is some good background turbo information?

howstuffworks.com’s turbo walkthrough
Wikipedia on turbos

Common Terms:
Boost threshold- the lowest RPM at which a turbo will generate positive manifold pressure at maximum engine load.
Turbo lag- the time between hitting the throttle and the turbo providing full boost.

Recommended Reading:
Maximum Boost by Corky Bell is considered by many to be THE publication for turbocharger information.

What is the best turbo? There is no best turbo. Generally speaking, aftermarket turbos fall into these generic categories:
a. Turbos with a little more top end power
b. Turbos with a lot more top end power
c. Quicker spooling turbos

What do all the names and numbers of turbos mean? This link sorts many of them out nicely.

What supporting upgrades are required for aftermarket turbos? At a minimum, aftermarket turbos require a fuel pump, injectors, and engine management for safe operation.

What is my stock turbo?

2002-2008 WRX TD04-13T
2004-2005 STI VF-39
2006-2007 STI VF-43
2008 STI VF-48

Is there a turbo upgrade that does not require other upgrades? Yes. A ported and polished (P&P) stock turbo is an easy upgrade over the stock unit. Though there are many turbos that may be used for short periods of time with a boost controller, it is generally unwise to bolt on an aftermarket turbo with a boost controller.

What is the best turbo with a little more top end? The most widely used turbos meeting this criteria are the VF30/VF34 and the 16G.

What is the best turbo with a lot more top end? The most widely used turbos meeting this criteria are the VF22, 18G, 20G, FP Green and it’s clones.

What is the best turbo with quicker spool? The most widely used turbo meeting this criteria is a P&P stock turbo.

What makes a good autocross type event turbo? The big thing to look for in a good performer for autocross use would be quick spool and more than stock flow. The TD04, TD05-16G, VF34, VF22, VF39, 16G, and 18G can all be considered good autocross turbos, but their particular suitability depends on the type of events where the car is generally run. During the consultation with your Vendor, discuss in depth the course length, speeds seen, gears used, and other local venue particulars to assist in determining what best suits your needs. A word of caution….before modifying or changing your turbo, be aware that this will have an effect on what class your vehicle can legally run.

Is there a way to compare Turbo A against Turbo B? Yes and no. This link, this link, and this link show comparative lists of most of the major aftermarket turbos. These listings should NOT be the sole source of turbo upgrade advice though. It should be used as a general indication as to the performance level of various turbos. Matching turbos to your specific needs is where advice from Vendors and tuners comes into play. You will not be happy with the perfect tune on the wrong turbo and likewise, you will not be happy with a poor tune on the perfect turbo.

What is the best way to compare Turbo A against Turbo B? Careful interpretation of a particular turbo’s compression map will give you the best determination as to what will best suit your needs. One reason for this is that Turbo A will flow XXX CFM for a 2.0 liter, and will flow YYY CFM for a 2.5 liter. Displacement can change a “perfect WRX turbo” into one that is less desirable for someone running 2.2, 2.5, or other displacement. Sadly, some turbo manufacturers do not release compressor maps to the public or even their retailers. This means that in the end, the user must consult available compressor maps as well as seek the advice of Vendors and tuning specialists for “best” suitability.

How do I interpret turbo compressor maps?
General guides with examples can be found on this link, this link, this link, this link, this link, and this link.

Where do I find turbo compressor maps?
Website 1
Website 2
Website 3
Garrett maps
Garrett maps (via their .pdf catalog)
Other turbo data

Should I upgrade my wastegate & what advantages would it give? There are a couple of different upgrades for wastegates that will help to solve different problems. Upgrading the wastegate actuator to a higher rated version will allow a higher boost threshold. For example, if a turbo has an 11 PSI actuator, with a good boost controller it can normally be adjusted to about 19-20 PSI, but no lower than 11 PSI. A 15 PSI actuator can easily be adjusted to 23-24 PSI, but again, no lower than its stated static pressure. There are also race actuators available for 19, 24, and 29 PSI. The inverse of this problem/upgrade scenario is a turbo that allows boost to rise beyond its target point. In some cases, very free flowing exhausts combined with the right supporting mods and a hot tune can cause turbos to not be able to vent through the wastegate properly. A condition that allows the car to continue to build boost beyond its target, even with the wastegate flapper in its full open position is known as “boost creep”. Boost creep can be alleviated by several methods; a turbine clip can allow slightly more air to vent through the turbine blades, retuning the car to a slightly richer mix can help in some cases, and in some cases the standard size wastegate flapper can be removed, the hole enlarged, and a larger, more efficient flapper installed in its place. In some cases this will require a retune, especially if boost control is handled by the computer. The larger flapper, in most cases, will help eliminate boost creep events; however, on some of the IHI turbos, this can cause additional work to need to be performed. The IHI’s are more sensitive, so on some of these turbos, the restrictor “pill” in the wastegate actuator vacuum line will need to be changed to a different size to accommodate the changed actuator duty cycle brought on by the larger flapper.

When do I go with an external wastegate? External wastegates and when to use them have become some of the most commonly asked questions in the turbo business over the past year. External wastegates move the wastegate vent from inside the turbine housing of the turbo to a remote location, fed from the same tube (or uppipe) as the turbine inlet for the turbo. There are several advantages as well as drawbacks to this setup. The first, and in many cases, the drawback that discourages most Subaru owners, is the cost. Not only are you adding expense to your turbo upgrade through the wastegate assembly, (normally starting at about $200) but there is also the additional cost of custom piping. Due to the design of the Subaru, it is more difficult than many vehicles to install/fabricate piping for the external wastegate units. This normally adds another $100-$200 in labor costs from a good exhaust shop. There are companies now offering custom uppipes with flanges for the use of external wastegates, however, due to differences in input flange on different wastegates, these are not universal. The biggest advantage to the external gate setup is boost control stability. In very high horsepower cars it becomes necessary to have a large enough port for wastegate venting that it is simply impractical to attempt the use of an internal setup. The larger external units will be able to hold the boost pressure more stable, and require very little actuation to vent fully and stop boost increase. It is this benefit that many times causes a vehicle with an external wastegate to produce additional power over an internally gated turbo, and is how the external has become known to produce more total power. Normally with proper turbo design and proper tuning, externals are not actually needed until power ranges rise into the 600 or greater HP range on a single turbo setup, and 900 or greater HP on twin turbos.

Are there any downsides to turbos?
There have not been significant amounts of problems with turbos. The main downside to turbos in general is uneducated usage. Careful planning, purchasing, supporting modifications, and tuning should allow the end user long and happy usage. Too often, many users take shortcuts, exhibit poor planning, or disregard necessary precautions and end up with disappointing failures. Another downside is poor selection which leaves the user disappointed as the new turbo’s characteristics don’t meet their expectations. The two main disappointments are:
a. Not enough power
b. Incorrect spool characteristics for autocross and daily drivers

Which manufacturer is best? This topic is highly debated. There are just too many factors to consider. The main category breakdowns are:
a. Price
b. Performance characteristics
c. Warranty
d. Compatibility with end user’s desires
e. Bolt on or custom fabrication installation

Who manufactures turbos?
Deadbolt Enterprises
Element Tuning
Forced Performance
Power Enterprise
Slowboy Racing

What turbo construction method is best?
There is no irrefutable evidence that one construction method is better than the other. The real difference in turbo construction is the bearings. There are two main types: floating bearings and ball bearings. Ball bearing turbos were designed for increased reliability and decreased lag. Though both of these elements are true, neither advantage is especially prevalent, so the construction method should not be the main consideration when choosing an aftermarket turbo.

What is a twin-scroll turbo? Generally speaking, it is a turbo with a divided turbo inlet to isolate the pulses coming from each exhaust port to maintain more of the pulse energy from each cylinder all the way down to the turbine wheel. A twin scroll setup will respond faster and produce boost quicker than a equally CFM sized regular turbo. Twin scroll setups are generally costlier and require more components than the average turbo ugrade to work efficiently though due to their requiring a true twin scroll header to operate correctly. Fitting a regular header to a twin scroll turbo basically negates the pros of this type of turbo.

So a twin-scroll turbo spools faster? When properly set-up and compared to an equal CFM regular turbo, yes. However there are many regular turbos that will outspool twin-scroll units. Generally speaking, buy a turbo based on HP/CFM and consider a twin-scroll if there is one that fits your goals and you are looking for decreased spool. Never buy a twin-scroll for the sake of saying you have a twin-scroll. Play into the benefits of a properly thought out twin-scroll set-up, not the marketing hype.

What is a rotated mount turbo? Any turbo that’s physical size prevents mounting in the stock location. They also generally have larger than an OEM inlet size which necessitates TGV deletes and a larger turbo inlet pipe. They also require custom exhaust components and generally use an external wastegate set-up.

How can I decrease turbo lag? There are a number of steps that you can perform to decrease the lag:
a. A silicone Y-pipe IC hose can decrease lag
b. An aftermarket intercooler with decreased pressure drop can decrease lag, though its physical design may negate the benefit
c. An aftermarket uppipe can decrease lag
d. Port and polish turbo services can decrease lag
e. Tuning. Through the tuning of EGTs, wastegate duty cycles, and gains, spool can be accelerated. Properly tuned cars create full boost ~500 rpm sooner through these techniques.
f. Large diameter downpipes and exhaust decrease lag. If the downpipe is catless, lag with be further reduced over a high flow cat model. You also want the cat to be as far from the turbo as possible to promote quick spool.

Can I upgrade my existing turbo? Yes. Most turbo upgrade facilities offer improvement services. State your turbo, its current disappointments, and improvements you would like to see. They might offer services that can save you from buying a new turbo. Though not an all inclusive list, possible services include:
a. Port and polish: Includes heavy porting of exhaust housing, removal of flow obstructions, smoothing of the factory material, and reduction of internal angles to alter flow and evacuation. Entrance to turbo housing (where the exhaust enters the unit) is also heavily ported, removing flow obstructions and smoothing exhaust path. Wastegate pass-through from exhaust housing is also ported, increased slightly in size, and any flow obstructions removed. The most significant improvement with this service is the decreased spool time. Another improvement is the additional torque in the lower RPM band as seen in this graph. Lack of low end torque is a common complaint with 2.0L motors.
b. Internal ported and polish: This opens up the area where the air exits the compressor housing. The turbo has between 1 & 2 mm of material removed, then the rough casting edges are smoothed out. You will pick up a fraction of a HP but get better response.
c. Light clip: Clipping removes part of the turbine wheel. This allows air to pass over it faster. It will help out with high boost (16.5 PSI +) situations but will slow down spoolup.
d. Coating: Ceramic coatings keep heat inside the turbo for increased thermal efficiency.
e. Larger waste gate flapper: New technique where a large wastegate flapper is installed in place of the stock unit. The large piece covers a specially ported, larger waste gate hole in an effort to combat boost creep.
f. Changing internal turbo components: This service depends on the turbo. Generally speaking it means things such as replacing the compressor wheel with a larger unit for increased performance, changing the wastegate spring, and other internal improvements.

What questions should I ask of the Vendor and my tuner before purchasing?
Once you perform your research, you should have your choices narrowed down. Before making your final decision, it is always wise to ask these questions:
1. What fuel injectors should I run?
2. What fuel pump should I run?
3. Is this turbo bolt on or does it need any special fabrication?
4. I plan on _______ racing. Is this a good turbo for my plans?
5. I plan on running the quarter mile in XX.XX or making XXXWHP on XXX dyno. Is this a good turbo for my plans?
6. Other than a fuel pump, bigger injectors, and proper tuning, is there anything else you would recommend for increased safety and reliability?
7. What about a larger intercooler?

Will I need a bigger intercooler with a bigger turbo? This is a very hard question with many opinions and not a great deal of facts. There are quite a few aftermarket turbo users posting impressive numbers with even the stock TMIC. This isn’t to say that they wouldn’t benefit from a larger intercooler, but it shows that it may be considered a supporting mod worth serious discussion and consideration during or just after a turbo upgrade. This decision should be discussed with a trusted Vendor or tuner for the best advice as to what suits your needs. Refer to the FMIC FAQ and the TMIC FAQ for starting information, but in theory, your intercooler’s CFM capacity should equal or exceed your motor/turbo CFM capacity.

What about ceramic coating or other heat wrap? Heat protections options for turbos fall under two categories:
1. Heat shielding. The primary purpose of heat shielding is to decrease heat soaking of the TMIC and other engine components. Examples of heat shielding are listed in the TMIC FAQ.
2. Turbo insulation. The primary purpose of turbo insulation is to keep the heat trapped inside the turbo for better turbo efficiency. The side effect of this is to decrease heat soaking of the TMIC and other engine components. Examples of turbo insulation are ceramic coating, Thermo Tec’s wrap, or DEI’s wrap.

Where do I buy aftermarket turbos? Every Subaru/Import performance store sells turbos. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install an aftermarket turbo? Allow around one hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic. Rotated mount installations are much more difficult and may require professional assitance due to needing custom exhaust work, TGV deletes, etc.

How do I install an aftermarket turbo? Refer to the turbo manufacturer’s instructions. For turbos without instructions, below is a link to one of the better known turbo installation instructions:
TurboXS’s instructions
Useful NASIOC thread
Useful NASIOC thread
Useful NASIOC thread

Is there any additional advice for turbo installation?
Materials Checklist:
Downpipe Gasket (DP to turbo) ~$20
Uppipe Gasket (uppipe to turbo) ~$11
1 qt of oil
1 qt or so of coolant (refer to the Cooling System FAQ for type)

Prior to/during turbo installation:
Pre-oil turbo
Prior to removal of any bolts, spray with a penetrant such as PB Blaster and allow to soak for 5-15 minutes
Uppipe to turbo and turbo to downpipe bolt torque spec is 26-30 ft. lbs.

After turbo installation:
Crank engine for 20 seconds to lubricate turbo (by pulling the ‘IGN’ fuse)
Let idle for about 30 minutes
Check for leaks and fluid levels several times
Break turbo in for a couple hundred miles (for new turbos)
Change oil after break-in (for new turbos)

Turbo Choices Basic

In order to make an informed decision when purchasing an aftermarket turbo, the consumer needs to avail themselves of the different types of turbos first. To this end, we will discuss the various types of turbos on the market. These are just the basics of turbo information though. Please do not confuse this as the main source for turbo information as there are many other factors to an informed turbo choice such as compressor maps, matching the turbo to your displacement, etc. For the best advice, please consult an experienced turbo vendor and/or your tuner.

A regular turbo is, in essence, a pump that forces air into your intake system. The end result is a denser air charge that will produce more power vs. naturally aspired vehicles. The only downside is that more power produces more heat, and the engine’s internal components must be properly suited towards turbo charging. Upgrading this unit to a larger one is the easiest route in terms of time, trouble, and expense. Common upgrades for all turbocharged Subaru models include the VF-30/34/22 and 16/18/20G.

A twin scroll turbo is designed to be used on an equal length exhaust set-up. By internal turbo design and having all the exhaust gases enter the turbo at the same time, this allows the turbo to spool faster vs. an equally sized regular turbo. This is a very important point as many people are confused by the marketing hype of twin scroll set-ups. When comparing a twin scroll turbo that will flow say 500 cfm vs. a normal 500 cfm turbo, the twin scroll should see full boost sooner. So if there are two suitably sized turbochargers, with one being twin scroll and one regular, the twin scroll unit may be your best choice if you do not mind the extra exhaust expense and prefer faster spooling.

This type of turbocharger requires more expense than a simple upgrade though. The biggest concern is the use of an equal length header, proper uppipe design, and the possible use of a different oil pan to accommodate the new twin scroll exhaust piping. Quirt Crawford of Crawford Performance recently did some testing on a GT32 twin scroll turbo Legacy to test the theory about the importance of exhaust flow to a twin scroll unit. When he switched from the correct equal length header and uppipe to a traditional unequal length header and normal uppipe, he saw degradation in turbo response by 750 RPM. This should be word to the wise to anyone who thinks they can avoid the expense of the correct exhaust components and still see the quicker spooling benefits of a twin scroll turbo.

Another consideration is the change in exhaust tone. An equal length header required in a twin scroll set-up sounds entirely different than an OEM or aftermarket unequal length header. To fans of the familiar boxer rumble, equal length headers are just not an option. It may sound silly, but for many, this reason alone is enough to keep them from buying a twin scroll turbo.

A rotated mount turbo is any turbo that’s physical size prevents it from fitting in the stock location and must be mounted at a slightly different angle. Most of the turbos that fall into this category are of the larger variety. Many require custom piping, a front mount intercooler, external wastegates, custom tuning, tumble generator valve deletes, and other technical or expensive upgrades to support it. Most would consider this type of turbo to be outside the scope of the average do it yourself person and should be farmed out to a professional or at least utilize one of the kits supplied by various manufacturers.

As well, many feel that when going this route, strong consideration should be given to fully built motor, or at the very least, forged pistons. Rotated mount turbos produce large amounts of power and though there is no magic horsepower number for switching to forged internals, the larger rotated mount set-ups seem to be commonly used on built motors.

Twin turbos on Subarus are rarely seen. Though there are no official numbers, one would guess that the amount in the United States could be counted on one hand. The reason for the rarity is due to cost, customization, and tuning that is involved with this type of set-up. This is definitely something one would want to leave to a professional. It requires enormous amounts of fabrication, careful planning, and time to execute in a reliable fashion. Aside from the mounting, tuning is also critical, as the turbos tend to fight each other to see which one can push the most air into the engine. This is especially true at higher boost levels and is something that takes a real professional to mechanically isolate and/or electronically tune.

Turbo Choices Advanced

Few Subaru modifications get the heart pumping as much as aftermarket turbos. This article will get down to the brass tacks to allow you to make an educated choice in what turbo specifications are and how to match them to your wishes.

The first step is to learn some terminology as it applies to turbos:

CHRA Center Housing Rotating Assembly. This is the internal part of the turbo from the bearing area between the two halves of the turbo shell to the main shaft and both blades.

Compressor Inducer smaller diameter portion of the compressor blade.

Compressor Exducer larger diameter portion of the compressor blade.

Turbine Inducer larger diameter portion of the exhaust blade.

Turbine Exducer smaller diameter portion of the exhaust blade.


The main halves of the turbo shell draw most of the attention, but sadly, as with many things in life, these halves have many names depending on the source of information. Cold side/inducer/inlet/compressor/intake and hot side/exducer/outlet/turbine/exhaust are all the popular names attributed to each half. For the purposes of this article, we will refer only to the terms turbine and compressor.

Turbine is the complete wheel inside the turbine housing. Its purpose is to spin via exhaust gas, which in turn spins the compressor wheel.

Compressor complete wheel inside the compressor housing. Its purpose is to compress the intake charge on its way to the intercooler.

Compressor map X/Y plot of the compressor’s efficiency range. After mathematical calculations based on your vehicle and desires, you end up with a plot that is used to determine turbo suitability.

Trim relationship of the inducer and exducer of a given wheel. Compressor trim = (inducer²/exducer²) ×100. Turbine trim = (exducer²/inducer²) ×100. All measurements are in millimeters. You will end up with a trim of say .64 which will be expressed as a 64 trim wheel. In almost every case, advertised trim ratios reflect those of the compressor wheel vice the turbine wheel. In a perfect world, the higher trim numbers will produce more power and more lag while lower trim numbers will spool faster but produce less power. This is not always the case though as A/R plays a role in the manipulation of the airflow as well.

A/R Area/Radius Ratio. One way to envision the concept of the A/R ratio is to think of the way the turbine housing wraps around the turbo. Imagine it as cornucopia that wraps around the turbine. The A portion is the small end and its shape and size effects the force which exhaust gases hit the blades. The R portion is the distance from the center of the section area in the turbine housing at the 12 o’clock position to the center of the turbine shaft; the size, shape, and distance of travel all affect the ultimate turbine velocity. The A/R is then the ratio between the volume of the compressor where it discharges to the turbine to the distance to between the turbine shaft to the center of the 12 o’clock position. This is a summary of the definition as provided by Corky Bell.

Interestingly enough, the definition from Garrett is slightly different. The A portion is exactly the same as the below documented MHI THA. The R portion is the distance from the center of the section area in the turbine housing at the 12 o’clock position to the center of the A portion.

As to which definition is most correct, that’s anyone’s guess, but the definition is not so important as the concept of the ratios. The A/R can be applied to both the compressor and turbine housings. In most cases though, the compressor A/R has little effect on turbo performance, so it is not given. As to turbine A/R, the smaller number A/R will generate quicker spool, but less top end power and the higher A/R number will generate more power at the cost of slower spool. This holds true when comparing different A/R figures of the same sized turbine housing.

MHI Turbine Housing Area (THA) is the newly created term for the “A/R” of MHI turbos. While their measurement area differs slightly from the traditional A/R, the end result is similar whereas the smaller numbers spool faster with less top end and the larger numbers spool slower with more top end. This measurement is technically the area of the housing at the 12 o’clock position.

Wastegates internal or external devices designed to control boost. Both control boost levels by allowing exhaust gasses to bypass the turbine, which limits the boost created by the compressor. Internal units are a wastegate built on the turbo. External units are larger and bypass the internal unit to increase flow away from the turbine, though some turbos are externally designed and lack an internal wastegate. Though every turbo can benefit from an external wastegate, they really shine on larger turbos. Wastegates also need tuning to run effectively. The rule of thumb for wastegate tuning is to utilize a spring inside the wastegate that is 50% of the amount of boost you will run. If you exceed the 50% rule, you may run into wastegate creep or other issues.

Wastegate creep where the wastegate will start to open prematurely. As boost gets closer to the set level of the spring, the spring will begin to open the wastegate and slow spool time.

Boost creep when with the wastegate open, boost rises over target levels. This is normally seen at high RPMs or during cold weather. It is usually attributed to the wastegate being too small. Common fixes are tuning to combat the problem or wastegate enlargement. Case in point, the replacement of the VF39 with the VF43 on the STI; identical turbos, but the VF43 has a larger wastegate.

Boost spike or boost surge a brief period of uncontrolled boost, usually encountered in lower gears during the onset of boost. Typically spikes occur when the boost controller can’t keep up with the changing engine conditions. This can often be caused by improper turbo sizing.

Anti-surge ports ports in the compressor housing that permit excess air to pass through the compressor wheel and into the anti-surge ports. One may think of this as a “wastegate” on the compressor side. In essence, this moves the compressor’s surge line to the left as seen on a compressor map. These are generally only used on larger rotated mount turbos.

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

Turbo lag the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed.

Porting and Polishing grinds away excess material within the turbo. Since the housings are cast units, they can leave behind flashing from the cast process and tend to be flat walled. Porting and polishing smoothes the interior portions and knife edges inlets and outlets for more flow.

Clipping process of removing material from the edge of the turbine blades, usually at a 10 or 20 degree angle. This imparts more flow around the blades, increasing flow at the cost of a reduction in spool. Clipping gives you an apparent increase in A/R.

Now that we have the terminology out of the way, we are ready to discuss turbo manufacturers and their products. Most people who think of aftermarket turbos get overwhelmed by the sheer amount of turbos. To remove most of the mystery associated, remember one simple thing, there are only three turbo manufacturers: MHI, IHI, and Garrett. Yes, there are rogue set-ups that certain people have, but 99% of the turbos you see advertised are direct products or variants of the Big Three. We will now discuss each manufacturer and their nomenclature.
Ishikawajima-Harima Heavy Industries (IHI) manufactures around 30 different turbos. Nomenclature is based on VFXX. Example name: VF34. Unlike other manufacturers’ whose naming system refers to capability, IHI turbos are named according to date of origin. Since the origin dates are all purpose built units, their power capabilities are all over the spectrum within the numbering system.

IHI turbos are generally smaller than their aftermarket cousins and cannot be rebuilt. The flipside to this is that they are generally great daily driver turbos and many offer unique features such as twin scroll design or the use titanium components. As well, many of these turbos appeal to Japanese spec minded Subaru owners.

One downside to these turbos is the amazing lack of documentation. About the only information you can get on these turbos, if you are lucky, is the exhaust housing size. P11, P12, P14, P15, P18, P20, and P25 are the known available exhaust housings. As with their turbo number system, the numbers related to their creation date vs. flow capacity. Additionally, no compressor maps exist for IHI turbos.

Mitsubishi Heavy Industries (MHI) is the largest Japanese turbo manufacturer. Their turbos have been used for years with wonderful results on a variety of cars. They are based off of older technology, but are proven performers. They can be rebuilt and upgraded from one size to the next in most cases.

The nomenclature for MHI turbos is somewhat complicated, but is easily broken down into four parts:

Turbine housing: TD04 through TD08 are the most commonly used and these refer to sizes, lower numbers being smaller, higher numbers being bigger. The same rules apply for spool and power, lower numbers spooling faster with less power, higher numbers spooling slower with more power. The turbine housing may also have a letter modifier such as S, SH, H, etc. This modifier refers to the turbine wheel inside and is basically a non-factor.

Compressor housing: Same rules apply as to the turbine housing. Usually are matched to the turbine housing, therefore this data is not in the turbo name.

MHI Turbine Housing Areas: A/R equivalents of THA figures are 6 cm2 = 0.41 A/R, 7 cm2 = 0.49 A/R, 8 cm2 = 0.57 A/R, 9 cm2 = 0.65 A/R, 10 cm2 = 0.73 A/R, 11 cm2 = 0.81 A/R, 12 cm2 = 0.89 A/R. The equivalents given are meant for people to have one understanding of two different concepts. MHI uses THA to denote housing variances and their effect on spool/flow. Garrett uses the term A/R to do the same thing. However each use unique measurement points to do so. The equivalents given are just a way to stick to one standard for people who cannot grasp the two different concepts. Please note this figures are for MHI to MHI comparison and not MHI to Garrett comparison. A/R or THA are comparative measures within each manufacturer’s own line and cannot be used across manufacturer lines or against other turbine housing size lines. Each MHI housing has its own THA variants.

Compressor wheels: 16, 18, and 20, with higher numbers representing more flow. These may also A, B, C, G, or T modifiers. Modifiers can affect the number, height, and pitch of the blades, and whether all blades are full height or some are half blades, like the popular G model.

Example name: TD06H-20G 7 cm2 or housing size/mod.-compressor wheel size/mod. THA

Within reason, most of these components can be changed around so you can have smaller wheels in bigger housings or have a TD04 turbine with a TDO5 compressor. So the possible amount of MHI based turbo combinations is rather high.

The Garrett format for naming/sizing their turbos is the format of GTXaabbcccc. GT is the standard name for all Garrett turbos. The X position denotes any revision to the turbo and is optional. aa is turbine size and bb is compressor wheel size expressed in mm, with higher numbers representing more flow. cccc refers to special codes that are optionally used such as R for ball bearing. Detailed Garrett nomenclature decoding is available on Garrett’s website.

Example name: GT3076R or housing size/compressor wheel size/special code.

As with the MHI units, Garret components can be swapped around as well to the limits of their designs.

The “fourth option” is a hybrid turbo. Most hybrids are MHI or Garrett based, as IHI turbos only accept simple things like clipping, wheel swaps, etc. With regard to the other models though, one can utilize MHI and Garrett parts to create a hybrid within reason.

Now you should have a solid foundation to research which type, brand, and size of turbo will suit your needs. Remember that research is critical and you shouldn’t choose the “turbo of the month” as many seem to do. A 500HP capable turbo can be an absolute dog to drive in stop and go traffic. Likewise, a turbo with Insta-Spool® will not net you an 11 second time slip. Many of the features here are described as smaller = less power, fast boost and bigger = more power, slow boost. The same rules apply to turbos overall. The goal is to find a happy medium which will meet most of your desires.

This article should not your sole source of information though and you should consider the following additional research activities when considering an upgrade:

a. Compressor map plotting and interpretation (MHI and Garrett particularly)
b. Your tuner’s advice and experience
c. Your turbo vendor’s advice and experience
d. Corky Bell’s book “Maximum Boost: Design, Testing, and Installing Turbocharger Systems”. Considered by many to be the Alpha and Omega of turbo information.

Editors Note

This new Turbo FAQ is a culmination of my old Turbo FAQ and two articles I orginally wrote for Subiesport Magazine. While this combination is not in keeping with my traditional FAQ technique and there is some information duplication, I hope the sheer volume of information is enough to give users an intelligent platform from which to make the correct choice on which turbo is the best fit.

I humbly submit that much of the information found in these postings has been verified, sanity checked, or just plain stolen from Mr. Jerry Hagan of www.deadboltspeed.com. Though I am soley responsible for the content of this post, it would be lacking if it was not for his assistance.

This post was created because I wasn’t able to find a good Turbo FAQ. I came up with the text based on LOTS of searching here. It was also created to be intentionally brand neutral so that it serves as a stepping stone for further research. Upon reading this you should have an idea of what type of turbo best suits your needs. The manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, “I have XXX’s turbo and it’s great!” or “XXX’s turbo broke after 1 month” are not appreciated here, that is what the Car Parts Review Forum is for.

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