Had a discussion with a friend on the weekend about this and I came to the conclusion that:
An engine code refers to the engine itself. NOT the exhaust or intake systems.
With that in mind it would be fitting to understand that Toyota (unlike Nissan) couldn't really give a toss about how many turbos are put behind the engine as long as they have heavier duty internals in the engine (the 'T' in 3S-GTE) etc.
This would also lead onto the idea that there is a lot of head work in an engine to run EFI on a Carby engine, thus we have an 'E' for an EFI head.
Now one of the questions that this idea raises is:
Are there different internals run for a turbo vs a supercharged engine. And if not, then why have a 'T' for turbo and a 'Z' for supercharged.
Which leads me onto thinking that the section A of the Engine code would be for the block and section B of the code would be head design.
Section A-> 4A (hence the '-' as a separator) GE <- Section B
But in this case are there noticeable differences in head design that would warrant a different letter where N/A and Forced Induction are concerned?
AND if this is true then why are there not different codes for VVT, VVT-i, and Dual VVT-i? Surely the head construction is vastly different for each one?
So I guess my questions are?
Why doesn't Toyota refer to the twin turbo in it's engine code?
My thoughts are because single, twin, or quad, it makes no difference to be build of the engine.
Why does Toyota have different codes for 2 different FI systems (turbo vs SC)?
? Because they can? Or there maybe a difference (compression etc maybe)
Why isn't there a different code for VVT-i, VVT, and Dual VVT-i?
Because it doesn't change the block? But then AFAIK neither would EFI so then Why? Compression again?
We also got into if somebody takes a 4A-GE and adds a turbo does this then change the engine code (bearing in mind there have been no other modifications) or is it simply a 4A-GE with a turbo?
I could argue both sides here


Feel free to completely disagree with what I have said as it is purely just my ponderings. But it kinda made sense to me.