ST165 and ST185 cars come equipped with CT26. There is alot of mystery surrounding this turbo. Here we will try and cover some of the more commonly asked questions when it comes to the CT26 and what can be done. For more information about turbo upgrades and where to get them done, visit the Turbo Upgrade page.
Q: What's the difference between ST165 CT26 and ST185 CT26??
A: The original CT26 had a 4 bolt Pattern to it. The pattern is the same on the All Trac 88-89 as the 87-92 Toyota Supra. It has a single flap for a wastegate to open once the turbo reaches full boost. The newer CT26 on the 90-93 All Trac and 91+ MR2 uses a dual wastegate design. By looking at the flange patterns you can see that the back side either has a 4 or 6 bolt configuration depending on year. The single flap types are known for cracking.
ST165/Supra 4 bolt Pattern, ST185/MR2 6 bolt Pattern
Q: What is Twin Entry and why doesn't ST165 have it?
A: Quite a bit of the designs for the original All Trac were borrowed from the Supra Turbo. The original turbo design has a single inlet for the turbine. It also has a single flapper door for excess wastegate gas. The single entry types are known for cracking. What happens is alot of heat is created once inside the single inlet as all the gasses fight their way out, causing cracks. The cracks start inside the housing and work their way outward.
Twin Entry - The updated design of the CT26 consists of a stronger 6 stud pattern which prevents the nuts from backing off. The Twin Entry name leads to the fact that the inlet to the turbine now has 2 ports. 2 of the cylinders are dedicated to each port, in a specific pattern. What this does is give the turbine the exhaust energy in a way that would not allow the gases to hit each other. The wastegate even has 2 ports which lets the gas out still in a smoother pattern. This design offers faster spoolup, higher max HP capability, and prevents cracking.
ST165/Supra Single Entry
ST185/MR2 Twin Entry
Q: Can turbos from ST165 and ST185 be swapped??
A: These turbos obviously cannot be interchanged as they are, due to the differences in the exhaust housing. If you were to take the ehxuast housing off, you could interchange the rest as necessary. Here is a picture of a CT26 with it's exhaust housing removed. Also there is a picture of a ST165 Exhaust housing that has been separated from a turbo.
Exhaust Side of Turbo Exhaust Housing
Q: I read your section about upgrading to a Supra Turbo, how is it done?
A: A more detailed explanation will come. I will give you a quick wrap up. If you have an ST165, the process is more simple. You simply have to remove the exhaust housing of the supra turbo as you can see above. You then need to re-orient the water and oil sections to line up properly, then reinstall. The compressor side of the turbo is held in with a large clip. You will need to unclip it and align the compressor side as well. If you have an ST185, then you cannot use the exhaust housing from the supra. You will need to use your original exhaust housing. Another note, your All Trac compressor housing will not fit on the supra compressor side. The supra compressor is slightly bigger. To properly do the installation, you should have your original compressor housing re-machined or bored out to fit the supra compressor. You don't have to do this, but if you use the Supra compressor housing, the wastegate mounting holes will no longer line up. You should be able to use at least one of the mounts, which will work.
Illustrated here is the difference in wastegate mounting points. You can see that the turbo compressor housing is oriented the same way but the bolt hole locations are wrong. Unfortunately a supra wastegate cannot be used without a custom fabricated mount.
Q: Can I use an MR2 aftermarket turbo "kit"?
A: Yes and no. We can say 75% of the kit can be used, if not more. There are usually a couple of problems. First, since they are designed for an MR2, the end part of the exhaust system may not always line up. This is a minor issue. Also, the intake piping that goes into the turbo is meant for the MR2 turbo AFM, which means it will definitely not line up at all. Some of the kits use large manifolds that stick up higher than normal. While there is plenty of room in an MR2, these kits hit the hood of the All Trac. There is a difference in the 2 cars in that we have our alternator in a location that the MR2 does not. This means that some kits will hit or come dangerously close to the alternator. Sometimes this also prevents access to the oil filter as well. Finally, a few of the kits are designed for the latest generation MR2 only. They are labeled as 3S-GTE but have the newer bolt pattern design associated with that generation of 3S-GTE. So in summary, yes it will work. However there is some footwork involved. We will try and bring you the most All Trac friendly turbo kits we can find. We are currently researching the Greddy TD06-L2 complete turbo kit. It offers 400 HP capability and looks to be a good kit.
Q: I raised my boost pressure, now the car stops and shakes when I get the boost going faster
A: You have found fuel cut.
Fuel will stop being pumped into the engine if the Toyota ECU detects 12 psi on ST165/ST185, and 17 psi on
ST185RC/ST205 models. The Toyota ECU is set to activate fuel cut if it detects 4.4 volts from the Turbo pressure
sensor. The Turbo Pressure Sensor is reading boost from your intake manifold and puts out 4.4 volts (roughtly)
at a certain boost level. First, if you haven't purchased/installed a boost gauge, please do so. This is vital
to the tuning you are about to do. Second, to disable fuel cut we just have to stop the computer from seeing
4.4 volts under any conditions. Go the MODS section to find out how.
Have answers to a commonly asked question? Email us so we can share your knowledge with the world!
Thanks to Dan Mac for CT26 pic.