The real problem with the CT-26 at higher boost levels is that the small turbine housing with twin scrolls to maximize the exhaust energy concentrated on the turbine wheel at low RPMs is actually "chocking" the engine. Once the exhaust system has been opened as far as it will go, the only restriction preventing exhaust from escaping the cylinders is the exhaust valve, the exhaust manifold and the turbine. If two boost gauges are attached to the 3S-GTE, one measuring intake manifold pressure and the other exhaust manifold pressure, what is seen is that at boost levels of around 10psi (what the stock MR2 Turbo was designed to operate at) the boost pressure in the intake manifold and the exhaust manifold are practically equal once the CT-26 spools up. This is a very desirable situation and speaks highly of Toyota's ability to match a turbo to their intended application in the MR2. At 15psi of intake manifold pressure, however, the exhaust manifold pressure climbs to over 20psi. This is not desirable. What is happening is that the exhaust gases are being held back by the turbine and, once that starts to happen, the turbine is unable to extract any additional power from the exhaust to push the compressor harder. The only way to produce more power in this situation is to allow the exhaust gases to move more freely across the turbine. There are several ways to do this: 1) get a larger turbine housing that permits more exhaust to flow across the turbine, 2) get a smaller turbine wheel that allows more exhaust to pass across it or 3) "clip" the existing turbine wheel to allow more exhaust to pass through it.

There are very few options (at least in the USA) for changing the turbine housing or wheel. The single entry CT-26 turbine is easily available, but it does not bolt to the stock MR2 turbo elbow, so serious modifications are required to use it. The most widely available alternative is to clip the turbine wheel. Clipping the wheel does reduce its ability to extract energy from the exhaust, but since there is so much exhaust at the higher RPMs where the stock turbine wheel chokes the engine, clipping is a reasonable choice. Dyno runs with a 12 degree clip have shown ~15 rwhp improvements in peak horsepower and moved the choke point from around 5.5K to 6K RPMs. There is a slight increase in turbo lag, but this can be kept to a minimum if the compressor wheel is kept to a modest size (since there is really no need to go with a large compressor wheel as we determined in the 3SGTE Turbo Sizing primer.

At the current time, my suggestion for a twin entry CT-26 upgrade is to add a TO4E-46 trim compressor wheel and perform a 15 degree clip on the turbine wheel. Those who have ceramic turbine wheels (only used in Japanese spec 3S-GTEs) cannot clip due to the brittleness of the material, but the smaller ceramic turbine is already a little less restrictive than the metal one and naturally supports about 10 rwhp more that its unclipped sibling. For all out performance, there is a CT-26/35R drop-in hybrid available from Extremeboost which couples a low-friction ball bearing central assembly and large GT35R turbine housing with an A/R of 0.85 (the stock CT-26 turbine housing has an A/R of 0.49) and a 56-trim compressor wheel. I have not yet seen a dyno chart of this hybrid turbocharger in action but it should be able to produce well over 300rwhp on a 3S-GTE. In fact, it may even require a complete fuel system upgrade to safely run at higher boost levels on the MR2.