*NOTE* DO NOT attempt any of this yet. Read further to see what I am working on!


I got my hands on a CT-12b twin turbo assembly, and I know that I want to BPU. My plans are also for the true twin conversion. While many people like the kick in the seat that the second turbo gives when it comes online [at just about 4000rpms], it is said that the rapid start/twisting of the shaft in the second turbo upon activation can cause turbo failure and the dreaded "death whine", and that the true twin conversion can help prevent this.

With the 80+ pound assembly in my hands, I decided to disassemble for inspection and cleaning of my new investment. While looking over the MKiV.com article on permanently wiring the actuators open for TTC, I started thinking.

To do the TTC mod, people remove the C-clip from two actuator rods, slide the rod off the peg, then turn the lever and wire them open this way. This causes butterfly valves to open on the inside and STAY open, taking away the restriction to the rear turbo's compressor and allowing the second turbo to start flowing. One of these butterfly valves is in the exhaust Y-pipe, and the other is just past the compressor outlet on the second/rear turbocharger. Well, I was curious, and since there was a little housing right where the actuator rod hooked up, I opened it.





This housing allows a little bit of air to flow around the butterfly valve when the butterfly valve is closed. The air coming from the compressor pushes a little flap open to a degree, but the flap is regulated by a piece of curved metal; the flap will not open more than that piece allows. This allows the turbo to "pre-spool" so when the second turbocharger comes online it doesn't kill the shaft with the sudden twist.




Here is what I am thinking: you can do two different things.

For a modified sequential mode:
Keep the butterfly valve in place; do not wire THAT actuator open. Instead, wire open the bottom actuator [one that controls the butterfly valve inside the exhaust Y-pipe]. Open up the housing and remove the two screws and curved metal restrictor and flap that they secure. This way, there will be more "pre-spool" so when the second turbo is commanded to come online, it will be spooled more than it would be in sequential mode. Your turbochargers will still operate in sequential mode, but shaft twist will be minimized while still retaining sequential operation.



OR for a new TTC mode:
Open the housing and take the two screws off the butterfly valve. This can be tough because of a blue thread-locking compound on the screw threads, so use a properly sized phillips screwdriver. Remove the plate and two screws. This way, you can hook your actuator rod back up with the C-clip. [You can ALSO pull out the piece that extends across the opening - the bar with two holes that the butterfly valve screws into - and close the holes off with epoxy or something]. You will rig up the bottom actuator the same as in the MKiV tech article, as the flap in the Y-pipe cannot be eliminated so easily.
I believe this mod [TTC] can be done without any problems, but I take no responsibility for any actions you take as a result of reading this "tech article".

whoa! cool. if i had 6spd i would give it a try but i dont want the added lag in the auto. i assume that you have done it. how has it worked out?

I have not done this yet; I need more money, a 2JZ and all of the parts to put it in my car. I'm going to get a cheap-o Aristo front clip, convert the oil sump and convert it to a modified export spec 2JZ [US spec turbines, 550cc injectors, but keep the J-spec MAP sensor and tune it with an AFC].

Wiring the bottom actuator open is not new; that's the way it has been done in the Supra community for some time now.
It's what I've done with the top actuator for the rear turbo that I think is different. I haven't heard of the butterfly valve being removed here. I will be using the TTC method, and I will take out the little bar, then fill with heavy duty epoxy. I may also close off the area with the flaps for my application; it will reduce turbulence of air going to the turbocharger.
As for the sequential mode, taking out the flaps is fine and dandy, but as soon as the actuator is told to open and the valve DOES cause the plate to swing open, there's going to be some turbulence.

-Like I said, this stuff is "use at your own risk". I can't see it breaking anything, but I don't take responsibility for anyone's misfortune.

nice! In theory, I think you may be on to something!

(on my MKIV) I took, and modded the check valve TTC mod, and I am now running this on my MKIV (so that the vaccum lines don't fall/come apart)...

and yes it's an auto, off the line, the lag is no worse than the MK3 with a CT-26, but so far I am liking the way it drives.

I think the lag is just in a different place in the powerband with the turbos running in parallel vs sequential, and I don't want my 2nd turbo ruined, cuz both of mine are still in good shape. I totally hated the boost drop from a roll in sequential mode, but in parallel mode, you don't have that issue that I have noticed.