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5 minute TTC (True Twin Conversion)
Some people love the Supras sequential twin turbo system, some hate it. I suspect that many people buy a Supra, and love the low end torque the sequential system gives, but sooner or later they start to modify the car to make more power. The more they modify the car, the worse the sequential system works, and eventually it starts to detract from the driving experience.

Well, for anyone who's never tried TTC, here's the easiest way in the world to do it!! Again, there is an alternative to this method, ETTC - Electronic True Twin Conversion, described here.

Requirements:

  • 5 min's
  • A short length of vacuum hose
  • Vacuum caps (optional)

Theory:

By bypassing the VSV's that control the sequential operation, the system stays locked in parallel mode due to the fact that the pressure tank stops the pressurised air escaping.

NOTE: This mod WON'T work if your sequential system is stuffed (i.e. leaks). How badly it leaks will determine how erratically the turbo's behave. This mod is also a quick way to check that the sequential system works - if this mod doesn't work, something in the sequential system leaks.

Method:

NOTE: Before you start this, best to point out that if you try this, and kill yourself or your car in anyway, it's not my fault.

There are two VSV's to bypass to complete this mod (there is a third VSV/actuator etc in the system, but it doesn't do jack once you're in parallel mode, so ignore it).

 

This first photo shows the general location of the IACV.

 

First, bypass the IACV (Intake Air Control Valve) VSV. Here, you can see the pressure line (in the background) feeding the IACV actuator via the IACV VSV (the VSV is simply an electronically controlled valve).


NOTE: If you wish to undo this mod, note which way around the hoses are connected to the VSV - this IS important.

Simply pull off the hoses, and connect one directly from the pressure line to the actuator. Done. That should have taken all of about 2 minutes. It would probably be a good idea to put caps on the exposed connectors of the VSV - just to stop engine grime getting in there and blocking it up if you ever want to use it again.
You can see here the general location of the EGCV bits - under the intake piping. To complete this mod, you're going to take the hose from where it says "Pressure line to EGCV VSV", and connect it straight to the EGCV actuator. So follow that hose down until you find the EGCV VSV, and pull it off the VSV.
Here you can see the VSV itself... sort of.. anyway, as stated, remove "Pressure line from above", and follow "Line to EGCV actuator" to find the actuator.

This is the EGCV actuator.
Connect this directly to the pressure line show above. This is where the optional vacuum hose comes in - the standard hose from the pressure line to the VSV only just reaches to the actuator, so if you're having trouble making it reach, get a longer one, and hey presto, Parallel mode!!!

Well, actually, not quite yet. You see until the system is pressurised, it won't be in parallel mode.. So take the car for a drive, and get it to boost. When it first boosts, it'll be funny, because as it comes on boost, it'll cause the second turbo to come on line as soon as you start making boost. After the first time, you should notice three things. Firstly, the car sounds a lot beastier!! That's because you're no longer shutting off half the exhaust when you're under 4000 rpm. Second boost comes on MUCH more smoothly. Third, it doesn't come on fully until about 4000 rpm. You will get some boost from about 3000rpm, but not all of it.

Your immediate impression might be "this is crap, the car seems so sluggish". But give it some time (a few days to a week, depending on how much you drive), a few things will happen. You will get used to the car needing a few more revs to really boogie, the computer needs some time to learn that there's no boost down low, so it can advance ignition timing (you may notice some black smoke from severe over-fueling at first too). It'll also take a while to realise that the car is noticeably more fuel efficient, and once the computer has sorted itself out, you realise that this is not some poxy little 2 litre 4 cylinder, the NA supra makes as much torque as a WRX, so it doesn't need the turbo down low for normal driving. But mostly, I love the linear turbo response - the 3000-4000 rpm range is useful again, and the Supra seems a whole lot more fun to drive.

Enjoy!

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