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There are a number of reasons
though cost being the main reason, a quality Twin Turbo system is more expensive to purchase than a single centrifugal Supercharger, cost is always a factor particularly in the lower price end of the car market.
If you look at the more expensive end of the local US car market (Corvette and Viper) you will find that Twin Turbo Systems do very well at this end of the market, where lower cost is not the primary consideration.
Another reason is drag racing rules, many classes state that only 1 power adder can be utilized, a supercharger is considered to be 1 power adder whereas Twin Turbos are considered to be 2 power adders, obviously this impacts on peoples choice of FI in they wish to race in a certain drag racing class. :cheers:
Why does a SC work better on a big displacement engine.. why not turbo that same engine?
They don't, and many large displacement engines do utilize turbocharging over supercharging, if for instance if you look at the mustang in drag racing I'm sure you will find that single turbo engines dominate this class as the preferred form of FI. :shiftdrive:
Thanks
Peter