It is very important to know the basics of how a turbocharger works as well as what you are trying to achieve before you send your turbo in for upgrading.  The most important concept is to understand is that you want to build a turbo that has a compressor and turbine wheel with very similar flow rates. To achievethis, the compressor and turbine wheels should have measurements that are very close in size. The inducer of the compressor wheel and exducer of the turbine wheel are the measurements you want close in size.  The inducer is the measurement of the wheel where the air enters, and the exducer is the measurement of where the air exits. Having less blades on the turbine wheel will help increase flow for a turbine shaft that has a limited measurement. A good example is a 20T compressor upgrade that measures 50 mm x 61 mm, but the biggest turbine upgrade available is the tdo4HL turbine which measures 45.6 mm x 52 mm. The tdo4HL turbine is offered in 12 blade from factory, however we can offer it in 11 blade, and I have also seen it offered in 9 blade too. The 50 mm 20T would have surge issues with the small turbine in 12 blade form, but when the 11 or 9 blade are used, the flow rate of the turbine is more closely match to the 50 mm compressor wheel which being a 45.6mm wheel. Turbine clipping offers the same effect as going with less blades, but instead of going with less blades, the blades are trimmed back to all for more air flow to pass by the turbine wheel. It is always better to go with a turbine that is closer in size to the compressor wheel if it is possible, but it is not always possible. When trying to match a turbine wheel to a compressor wheel that you have already chosen and the inducer measurements of the compressor wheel are inbetween the sizes of two different turbine upgrade sizes and you cant decide which one to go with, always go with the bigger turbine shaft. Turbos work better with an oversized turbine shaft than an oversized compressor wheel. A turbo with a bigger turbine exducer measurement will help prevent surge and support the flow of the smaller compressor rather than using a smaller turbine that will choke it. A good example is if your using a 56mm compressor wheel and the turbine choices are a tdo6h 58mm x 67mm and tdo6 55mm x 61mm, then go with the tdo6h turbine, or you could go with a tdo6 turbine that is clipped or has 11 blades instead of 12.