After talking about the Holset turbo from my former turbocharger build, I thought I would cover my current turbocharger and why I chose it.
Because I had a larger budget this time around I had more choices in which turbocharger I would choose. That's not to say I couldn't have paid the same as I did prior for another, different, turbo but I really wanted to approach it from another angle. The correct way to approach choosing a turbocharger is to identify how much power you hope to make. If you approach the problem with a "budget" then you will only be able to think about horsepower/dollar, vs straight horsepower. There is a HUGE different between diesel and gasoline turbos (not so much in the build quality but in terms of how they are sized). I identified my intention of making (hopefully!) 900+ rwhp. Yes, this is an absolute best case scenario that relies on many factors, but your ultimate deciding factor is how much air you can move through the motor. Looking at the available units meant I was looking for north of a PT67 (in Precision naming convention, the number is the size of the compressor wheel). If you look at how these units are rated, the amount of CFM they can flow is a direct relation to how much horsepower they support. Motor-type and cylinder count is irrelevant because the assumption is that whatever you put it on will consume the air, so the turbocharger, when the limiting factor, will flow that amount of power. However, these power figures are BHP (obviously) and so looking at various units, you can hone in your options.
In my case, a PT67 makes around 850 hp. A PT70, 72, 74 are all slight increments upward. Really what a 900 rwhp requirement means is with a 18% drivetrain loss (little on the high side), I need to make 1062 BHP. That really means a PT74GTS would be JUST enough, in best case. The next guy up is the PT76GTS featuring a 76mm inducer compressor wheel with a 102mm exducer - a Garrett GT4202R has a 74mm inducer and 102mm exducer for comparison. On the turbine side you have your choice of a .81 A/R or .96 A/R v-band housing. You also have a choice of a T04E housing (3" inlet), and 4 T04S housings. I have the largest T4 housing with a ported inlet. The outlet of this housing is 3" which is what my charge piping is.
Most people will not be interested in this type of unit. They might be looking at something like a Precision 6262 or SC61 or so. Maybe even smaller, like a T3/T04E .57 which spools very fast on a 2.5L M50TU. You just need to identify PEAK horsepower you wish to run. Once you have that, then you can work out the compressor maps on the turbochargers that qualify making that power. Once you have that turbocharger selected, you can decide on turbine A/R and cover options if you have a choice. It is very important that yuo choose the right unit or you will never be happy with it. A lot of times a large turbo can feel laggy but when it does come on, it comes on big and you get a rush of power A smaller turbo may make a car faster even if the peak power is lower simply because the powerband is wider. But, obviously a 450 rwhp car will not be as fast as a 750 rwhp car in almost any case. These are all things you need to consider before spending those hard earned dollars.
