I've been told a billet wheel is lighter than a cast wheel and because of this it spools faster. True? Also does it just spool faster or does it also give more peak boost too?

 

Sometimes they are lighter. Don't fall for the hype though. billet vs cast has a lot to do with a production stand point and bottom line. What I'm trying to say it's cheaper to produce mfs compressor when the order is smaller.

 

Depends on the brand. Most big name turbo companies redesigned their compressor wheels so they spool faster and flow more air. Think bigger turbo flow and smaller turbo spool on the same overall wheel size. The billet material is stronger allowing for thinner sharper blades and smaller center hubs for more area for the wheel to grab and move air. If wheel design is the same there is no real difference in performance other than the billet wheel being stronger. Boost is just a measure of restriction on your setup so as set my the wastegate or boost controller its irrelevant. It was a night and day difference on my setup when I swapped from a 67mm cast wheel to a 67mm billet wheel. Here are my results.

https://www.theturboforums.com/thre...s-Billet-wheel-turbo-results?highlight=billet

 

Billet is more dense than cast... If the dimensions are the same a billet wheel will be heavier and spool slower than a cast wheel. Billet turbos usually have a narrower hub and longer blades that their cast counterpart. This is usually why the "billet" 60mm performs better than the "cast 60mm". Some have more aggressive blade profiles and/or extended tips.

So even though both compressors have a 60mm diameter total in the pic... the Billet "60mm" hs longer blade length because the hub is smaller.

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plus most billet wheels are wider than cast

All of the above posters are correct.....billet or Forged Metal Wheel will not make more power than cast simply because of the material.

 

Okay now I confused! If the billet is heavier but has a narrower hub and wider blades why don't they make the cast with a narrower hub and wider blades? Seems you could have the best of both lightweight and more blade?

 

Billet is stronger and heavier since its denser. Because it's machined smaller/less material it ends up being overall lighter white maintaining strength. The cast material is not strong enough to allow for such thin/less material casting/design.

Like chromoly is heavier and stronger than steel but you can use thinner wall tubing to save weight while maintaining strength.


I added red arrows so you can compare how the vanes are thinner and sharper as well. Which helps it grab more air for better transient response while increasing flow through the wheel as well. Cast material is not strong/dense enough for this design.

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Look up yield strength for 1018 or 1020 and _normalized_ 4130 (normalized is what is used for things like roll cages), their ranges overlap. For the most part a chrome molly cage will not be stronger, and will have brittle HAZ near the welds, it's just people think it is and stupid rules (NHRA and IHRA, Nascar and similar which are more safety conscious outlaw chrome molly anywhere near the driver). Unless you're going to actually weld the cage with chrome molly filler and then stick the whole car in a heat treat oven after you're done with it I can't think of a reason why chrome molly is preferred besides perceptions.

Back to this topic, everyone thinks that "billet" is stronger... that depends on what you start with a billet of. It could be, it could be weaker. What you can do is make finer, more precise shapes with it and modern multi axis machining techniques than casting, and that's where the real advantage lies. More precise shaping means you can remove more of the unneeded weight and can make a more efficient shape (I would bet that most billet wheels are actually heavier, because they also tend to have a deeper shape that would be difficult to cast). Why don't they do it for the production pieces? Because casting is faster and cheaper.

 

The literature I have seen from Borg Warner suggests to that the main reason any production turbo compressors are machined from a solid piece is to start with stronger material (often forged, not simply "billet" hunks off a rolled bar) in order to make the wheels more resistant to fatigue failure. Same reason for using titanium. Similar reason for using boreless wheels like many of Garrett's designs.

Of course the aftermarket saw an opportunity to cash-in...there was a huge market segment they could sell billet wheels to who don't need them, but know any better either. A fool and his money.