The stock turbocharger on the MKII is Toyota’s CT-26. It is water and oil cooled. The turbo also has an internal wastegate. The turbo utilizes a small (possibly 40-trim) compressor wheel, and an equally sized impeller on the exhaust size. Most have found that the exhaust side ultimately limits the turbo’s power capabilities, in that it is not efficient at higher boost. For this reason, owners increasing boost should not exceed 14-15psi with a stock turbo.
Upgraded stock turbos come in various compressor sizes. These include -46, -50, -54, and -60 trim compressors. Most MR2 owners do not recommend going over the –50 trim. The –46 trim is excellent for quick response, while the –50 trim may be the best for creating overall power. Most owners find that with an upgraded stock turbo, horsepower is limited to about 260 rear wheel horsepower (rwhp). With the stock turbo, rwhp is limited to about 220.
The ultimate way to upgrade your turbo is to go with an after-market turbo. The Greddy TD-06 is a favorite among many owners, while true ball-bearing turbos, such as the T-3/4, is the best way to go custom high-horsepower machines. The T-3/4 does require an external wastegate. The Greddy T-67 is another good big turbo, and includes all necessary hardware for installation, including a tubular exhaust manifold. Both Greddy turbos are problematic in that they do not allow the use of the stock primary catalytic converter. This is fine for owners who do not live in areas that require emissions tests, but the rest of us would have to utilize an aftermarket catalytic converter and hope it still meets regulations. HKS produces a TD-05 turbo kit for the MR2, which allows the use of the stock primary catalytic converter. The downfall here is that the turbo is slightly smaller than the Greddy TD-06, so overall power potential will be slightly lower, all things being equal.