Odd Ball TT Question

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Talking at work with a few guys today, one of them asked why i was going to do two larger turbo's on the car, when i could keep one smaller, and one larger so the small spools up quick, and the larger keeps the top end running strong and happy. As i remember something with this set up doesn't allow the set up to work, either the smaller turbo pushes on the larger one making it spool up much slower, or for some reason it doesn't work. I remember getting into this on another forum a long time ago... but now i can't remember what the verdict was.

Could someone help set me straight so this stops bugging me.