turbo upgradebility?

[pwpanas]

Originally Posted by STRAIGHT-6R

...1. I was under the impression that, it takes specific amounts of exhaust pressure to spool up certain different sizes of turbos, not so?...

Although I agree that it generally takes more exhaust pressure & volume to 'spool up' a larger turbo, that change in pressure & volume isn't generally measured. I'd suggest a key reason that it isn't generally measured is that, for a given engine (eg. 2jz-gte) & turbo set-up, there's no way to change that amount of pressure/volume without changing either the rpm, adding nitrous, or running an anti-lag system. In general, the turbo-selection-related things that most owners of soon-to-be-(re)modified Mkiv Supras do concern themselves with are:
1) What max horsepower you want to achieve
2) Which turbo is recommended to produce that max horsepower level
3) What boost level do you need to run to support that max horsepower
4) What octane of fuel you need to run to support that boost level
5) The rpm point that the turbo spools up on that engine
6) Whether you want to run nitrous and/or anti-lag to make that turbo spool sooner (based on your application (roadrace/dragrace/etc.), torque converter (if auto), and driving style)

Originally Posted by STRAIGHT-6R

...2. If a single turbo set-up has the same horsepower potential as a twin set-up, is it also capable of equal spool-up time? (acceleration off the line)...

1) Yes (assuming we're comparing a single turbo set-up to a non-sequential twin turbo set-up).
2) Spool-up time is not generally the same as acceleration off the line.*

Originally Posted by STRAIGHT-6R

...What are the pros and cons of each?...

Since equivalently-sized non-sequential twin turbo vs. single turbo setups spool at a near-identical rpm point, the comparison is rather straightforward:

Twin turbos (non-sequential) - Pros:
- Sounds cool to say your car has twin turbos
Twin turbos (non-sequential) - Cons:
- More expensive than a single turbo set up
- More complex than a single turbo set up (i.e. increases the number of possible failure points)

Single turbo - Pros:
- Sounds cool to say your Mkiv Supra has a big single turbo
- Less expensive than a twin turbo setup
Single turbo - Cons:
- None?

Spool-up/Lag vs. "Acceleration Off The Line"
* Spool-up is the rpm point that the turbo(s) begin to build boost. Acceleration off the line depends on how you drive (6spd) and/or the stall of the torque converter you choose (auto). In other words, you could hypothetically run a huge laggy turbo, but run it with a high-stall torque converter and brake-boost and still get excellent off-the-line response.