Quote:
Originally Posted by Raceboy
Ball bearing per se is obviously faster spooling leaving all other things equal....BUT these "other factors" are more important and in most builds many things are left to be desired that affect spool much more than 0.4s.
Twin-scroll turbine vs single scroll means not only faster transient response, but it actually lowers boost treshold.
Use a few cheater bends and you lose another 1.2 psi and few tenth of a second.
And more important: have a crappy tune and lose a second of boost reponse time...or more 
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So true, people tend to forget what factors spools-up a turbo, bearings are not one of them and ballbearings come (IMO) close to bling if put up against the physics that moves air and shaft. Twin scroll is the most over-looked factor (though has been used in the industrial, diesel and racing industry for many many years) and requires some thinking in puls physicto to understand and build correct on WG and cylinder banks. It moves your exaust pulses at a much more higher speed and powerand each puls enter into it "own" half sized housing. Both low end and high end rpm are affected very positive. You control backpressure (ignition advance) to extend where you can use high lift/duration or oem NA cams much easier. Should be noticed that most single wg twin scroll 930 headers from the aftermarket (announced as true TS) are build wrong.
Another important factor is the choise/set-up of wheel size on the compressor inducer side. Let say you want 1 bar of boost at 3000rpm -> Most people ask how quickly a turbo spools...what they should ask for instead is how much air is moving @ what shaft rpm. One small compressor may reguire much more turbo shaft rpm like 80.000rpm for 1 bar (GT35 62mm/62mm), another bigger compressor only 70.000 rpm (65mm)... rough examples ... use the SAME turbine size setup on both examples (or even smaller, like 59mm turbine exducer, on the bigger 65mm comp. version) and you build up boost much quicker ... bearings is not a factor in this game.