Subaru WRX/STI turbo charger basics

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.

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.

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