A ball bearing turbine shaft
The basic rule of thumb with turbos is that smaller compressor wheels work well at lower pressures whereas a larger compressor wheel reaches peak efficiency at a higher level of boost pressure. This is due to the speed the shaft spins at. A slower shaft speed creates a denser air charge and improved efficiency in the turbo. In simpler terms, a car that isn't raced often will see greater benefit from a smaller sized turbo that responds better than a larger turbo that only performs well on the top end due to turbo lag (less efficient). The experts agree that once you figure out what kind of horsepower goal you want, you can use a compressor map to figure out which compressor will be the right size for the efficiency of your setup.
Ball Bearing Versus Standard Bearing Turbos
Just like your crank, you turbo rides on a thin film of oil sandwiched between a set of bearings, unlike your crank though, the turbine shaft sees upwards of 100,000 RPM. Therefore, any reduction in friction results in a huge increase in efficiency. The introduction of ball bearing center cartridges has given way to smaller frame turbos with capabilities of midsized standard bearing counter parts. Instead of a strip of metal, the ball bearing center cartridge design allows the shaft to rotate along tiny precision round bearings, similar to the ones found in skateboard wheels. This way, you can choose a large compressor wheel that can be matched to a disproportionately smaller turbine wheel.
The second area of interest, when it comes to bearings, is the thrust bearing. Under a high load environment, the exhaust pressure is pushing down on the turbine wheel. The shaft needs something to withstand this pressure. In a ball bearing application, the thrust load is virtually eliminated because of the tight clearance between the shaft and ball bearings. Ball bearing turbos are especially beneficial in an application that requires high response (ie: circuit racing, autocross, drifting and street) because the increase in peak effective boost is not as apparent as the spool up rate. This means that you can reach peak boost faster, increasing the amount of mid range power realized.
If you're thinking that it sounds too good to be true, you may be right. The biggest discerning factor between standard bearing and ball bearing turbos is cost. This is where the rule of "you want to play, then you got to pay" comes in. Expect to pay double (or more) for the price of a ball bearing turbo. On top of that, ball bearing cartridges are so expensive to service, you just don't. You either have to buy a new center assembly or buy a new turbo. As you can see, there's a give and take to everything.Whether you go with a standard type or a ball bearing turbo, maintenance is key having a long lasting turbo. Contrary to popular opinion, it is critical to run a filter on the turbo. At 100,000+RPM, any debris can wreak havoc on the wheels, whether it's on the turbine or compressor side. Rocks, dust, gasket material, pieces of a catalytic converter - they all mean bad news. Properly plumb your pipes and keep a filter on the turbo at all times. Check how much play the center shaft is getting (forwards to backwards, side to side and top to bottom); this will ensure that your turbo is working at its designed efficiency.
Trim Sizing and A/R
When you browse through the various turbo manufacturer's catalogs and websites, they usually list the trim size and A/R. According to Honeywell Garrett, trim is based upon the relationship between the inducer (the area where air/gas enters the turbo) and exducer (the area where air/gas exits the turbo). To be more precise, it's a ratio of area. Generally speaking, the larger the trim size (other things held constant), the more air a turbo can move.
By Phi Phung
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