I think the real reason for TT is size constraints. It's easier to find space for two smaller turbos than one (equal boost) larger one. At least I think that's why BMW stuffs TT along side there six cylinders -- can't find room under their cute little hoods for a big one. LOL
However, considering that our V6 engine bays also fit V8s... there's more than enough room for largest (properly matched) single turbo you'll need. And to make the most power, you will need a properly mapped turbo...
A positive displacement SC (roots or twin-screw) would be second on the overall power. It biggest advantage is also it's weakest point: since it's in constant boost, people say it's "streetable" = power when you want it. But since it's in constant boost, mpg will suffer.
And then there's centrifugal SCs -- they are similar to turbos (lag, then greater boost), but are limited since they run from the serpentine belt. They are probably the easiest to install, but don't make as much power and also decrease mpg (since they are always pulling on the serp belt).
BTW, a turbo won't affect mpg as long as your not in boost. Since it runs off the exhaust gasses, which are "waste", they don't really tax the engine. The only reason your mpg drops in boost is because you need enough fuel to match all that extra air!
But if you're looking for something unique -- a twin turbo would be a cool project!
FLASH = 98 V6, chrome yellow, black & blue accents, 146k daily driver POWER
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