Turbocharger: How to choose a Turbocharger for your Turbo Subaru:
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.