Why have two turbos on a car?... Is one smaller than the other to make up for the turbo lag of the normal size one??

 

Err, no... that is a rather complicated thing to do (a two-stage setup where there's a small turbo for 'starting things off' and a large one to continue...) there are variable vane jobbies that have been thought up of which I have no idea what stage they are at, but the idea behid them is to change the aspect of the vanes to adjust to the rpm/load...

Anyway , a 2-turbo car has each turbo serving half the engine, so a smaller turbo can be used. The engine gets the shorter spool-up etc. of the smaller turbo but a high volume of air movement because there are two of the things!

 

IIRC twin turbo Legacys are sequential, one for low speeds, one for high. There can be a problem at the change over point i believe, hesitation half way up the rev range.
Correct me if I'm wrong, can't remember where I heard it.

 

Also heard that Mitsi 3000 GTO's have sequential turbos. With one spooling up at lower revs, and the other coming onto boost as the other starts dropping off. Cant remember where i heard it, may be bollox.
Sounds like quite a good idea in theory though.