2JZ - shutting off the second turbo (reverse TTC)

[Draven]

my car
2jz, 2nd turbo exhaust wheel disintegrated on the dyno

bent 2 of my spark plugs, but fortunately I've escaped without bent valves or other bad things
in sequential mode

[Mos]

Guys,

The front turbo (I would call that turbo No.1) has a wastegate. This provides pressure control irrespective where the pressure is coming from. Note that this wastegate is puny and in many cases does not provide enough flow to adequately control pressure with a decent exhaust.

The rear turbo (turbo No. 2) has a "bypass valve". This bypass valve bypasses the main exhaust valve on the second turbo allowing the second turbo to start spinning up before the main exhaust valve and intake valves open. The mechanism is identical to the front turbo's wastegate, but the port is bored differently - it's on the turbine outlet instead of the inlet. It is possible to weld up the factory port and open up a port to the turbine inlet to function as a wastegate.

You would at least have to disable the two solenoids to stop the valves from opening. You may also need to disable the third solenoid that controls the bypass valve - I can't remember how that one is hooked up so not sure what would need to be done (and it's cold outside now so perhaps tomorrow ).

As Norbie said - it's risky. You'll be feeding 3L's worth of exhaust into a tiny turbo and tiny wastegate... I would hypothesise with little doubt that it will overboost.

Can't you last even a little while without thrashing your car?

Mos.

[Draven]

this topic was (for me) decided a long time ago.
I'm keeping it in TTC mode, effectively meaning the engine sees no boost, until I can whack that lovely gt35/40 on

[2JZR31]

It will not overboost. The turbo is way too small to overboost. It is getting 3 liters of exhaust, but it also has to supply 3 liters of air and it can't do it. The single wastegate only overboosts when it has to deal with the air from 2 turbos. Trust me. I know because I didn't have power to my VSVs as I didn't wire them up at first when I done the engine conversion. Boost tapers away to a few PSI near redline as the engine and turbo are choked to the max.

Knowing this, the wastegates will be shut (because there will be less boost than their operating pressure) so the turbine will have to pass all the exhaust from the engine at high rpm as it struggles to reach its target boost pressure of 10psi. This has to be VERY bad for the turbo.

The exhaust bypass valve just uses a VSV in exactly the same way as the wastegate. You won't need to disable it. In fact, this valve will stop any possible overboosting because when only the No1 turbo is working it actually does work as a wastegate. So in this condition you will have 2 wastegates working.

[Mos]

Hmmm interesting - makes sense though I stand corrected.
Yeah, bypass valve will bleed exhaust away from the first turbo (and try to spin a turbo that is possibly seized ).
Mos.

[Mr Ed]

Knowing this, the wastegates will be shut (because there will be less boost than their operating pressure) so the turbine will have to pass all the exhaust from the engine at high rpm as it struggles to reach its target boost pressure of 10psi. This has to be VERY bad for the turbo.

This is the bit I was worried about. Stock tiwns are good for 18psi before they start running outside their efficieny range, so 9spi from each turbo. 10psi would be straining the little single quite alot..

Draven, I suggest you get that GT35 stuff together quickly!

[2JZR31]

Hmmm interesting - makes sense though I stand corrected.
Yeah, bypass valve will bleed exhaust away from the first turbo (and try to spin a turbo that is possibly seized ).
Mos.

Yep, this is why fitting a boost controler on the main wastegate does nothing to increase the boost from the first turbo. Even when its totally shut the by pass valve will hold boost at 10psi...

This is the bit I was worried about. Stock tiwns are good for 18psi before they start running outside their efficieny range, so 9spi from each turbo. 10psi would be straining the little single quite alot..

Ed, it doesn't quite work like that, at 18psi each turbo makes 18psi, but only supplies half the CFM into the engine. (think 1.5 liter engine) Remember once the boost drops below 10psi (which it will at high rpm with only one turbo) the wastegates will both be shut and ALL the exhaust gas will flow through the turbo from 3 litres. Even though its less that 10psi, the turbo will be getting a real workout. Imagine 7000rpm of exhaust gas going through one turbo with no wastegates open. This is going to be much worse than when it makes 18psi along with the other turbo.

[Mr Ed]

Ed, it doesn't quite work like that, at 18psi each turbo makes 18psi, but only supplies half the CFM into the engine. (think 1.5 liter engine) Remember once the boost drops below 10psi (which it will at high rpm with only one turbo) the wastegates will both be shut and ALL the exhaust gas will flow through the turbo from 3 litres. Even though its less that 10psi, the turbo will be getting a real workout. Imagine 7000rpm of exhaust gas going through one turbo with no wastegates open. This is going to be much worse than when it makes 18psi along with the other turbo.

Ahhh I see.