I found this very interesting and exactly as the turbo engineers I have spoken to over the years have said. http://www.turbobygarrett.com/turbobygarrett/news/race_updates/Garrett_Fully_Machined.html
i find that article difficult to believe. The main reason I think they're stressing the similarities is because they don't want admit the GT30 / 35 / 42 are outdated since those are their cash cow products. I would think the billet wheel is lighter than the cast wheel (like Eddie mentioned) mainly because the blades would be thinner and the materials lighter. If that's the case, it HAS to spool faster, it has less centripetal forces on the blade tips. Less mass may mean it moves less air, as the weight may inhibit it from pushing the extra volume the cast wheels can provide. Again, this is all speculative on my part, but look at the numbers 6262 and 6762 turbos are putting up. They're putting Garrett to shame...
You have to remember that you can use very different materials when casting verses forging. Also some of the modern casting technology is crazy.
drew not sure the mass has much to do with how much air it's moving. just less force required to turn
good point, but that in itself should effect spool. I guess i'm thinking when off throttle or whatever, but with egt and eg-velocities, that's minimal to anything of notice. and jeb, i know casting technologies are crazy, but they're still using aluminum...for the most part, forged will be better material for material, they're making $200 turbos, not $200 million dollar top secret gov't projects
forged will be more dense and stronger for less material. less material, less rotating mass - less work required to spool
The billet wheels are more for strength and resistance to fatigue in my understanding. In talking with a some turbo engineers that have done the tests and been in the industry for years, these were mainly developed to have stronger wheels more resistant to fatigue in on/off throttle situations such as garbage trucks and delivery trucks that are constantly going from 0-50 psi over and over again day in and day out.
Ok... I am out of the country and have dookie interweb access but wanted to share my thoughts and experience on this. This test is 100% accurate... If you were to machine two compressor wheels exactly the same except one was billet the performance of the two would be unnoticeable. The only gain the billet one has over the cast piece is that the fin contours will be exact from fin to fin. The down side from it is the billet piece will be heavier. Now the benefit from having billet wheels is that the billet itself is stronger. The stronger material means they can manufacture the fins closer to the center of the shaft with more aggressive profiles. Also since the material is stronger, when the cut closer to the shaft they are able to make the billet wheels lighter. Only catch is that weight on the compressor wheel does not have as huge effect as one may think. Aero profile does.. wheel weight is all relative once it starts to compress and move the air through the turbo.
I would imagine that billet makes it a lot easier to prototype wheels and bring them to market quickly, vs casting.