Turbo Selection Guide
Understanding what turbo to pick.
Selecting a Turbo: What You Need To Know
Over the last 15 years in business I have received countless emails from clients asking what turbo they should use. Looking at the amount of choices on the market for 2018, it's not surprising this question comes up so frequently. Selecting a turbo can be overwhelming, but it does not have to be. While you can crunch numbers, plot data on a compressor map, and speculate on what will work, I have come up with a pretty simple procedure that will get you in the ball park and narrow down your choices fairly quickly. I use this method when I look at turbo choices for a project and it's worked pretty well over the years.
The first thing to keep in mind is that a turbo is a fancy air pump. It uses the exhaust gas to spin the turbine wheel and, in turn, spin the compressor wheel. The compressor wheel has a specific shape and size and that translates to a certain amount of airflow capacity. The airflow capacity is what dictates the power level you can make. So what does this all mean? The first thing to consider when selecting a turbo is figuring out what kind of power level I want to achieve. Once I know how much power I want to make, how do I correlate power level to turbo size? VERY SIMPLE! Compressor wheel flow rating. Most any reputable turbo manufacturer will supply a lb/min flow rating on a compressor wheel. This is going to be the key to giving you the knowledge of what the turbo will be capable of producing. For example, if we have a compressor that is rated at 60 lb/min you would simply multiply by 10 to get approximate horsepower. In this example, the turbo could produce a peak of 600 whp. Wasn't that easy? Ok, so now what?
The next thing to consider in turbo selection is how frequently you will be running the turbo in that power range. If you plan on running the turbo at its full peak flow capacity, this will also be operating the turbo close or at its peak shaft speed. What does that mean? Turbos spin fast...really fast. Shaft speeds can get into the 100,000-130,000 rpm range at peak operation. Once you start exceeding that rpm level, you will over spin the turbo and it can fail as a result. This can be in the form of broken turbine/compressor shaft, compressor explosion, or bearing failure, to name a few. So what do you need to consider? If I know I want to make 600 whp and want to run the engine there all the time I am not going to choose a turbo that just makes 600 whp. I want something that can flow a little more and give me 10-15% headroom in flow capacity. That would put me at looking into a turbo that flows closer to 68-70 lb/min. With that slight adjustment to the turbo selection, I can confidently run the new turbo choice at my desired power level long term while keeping the shaft rpm level down and compressor outlet air temp levels lower. This will result in less cooling demand from the intercooler and, in turn, will not heat soak as quickly under extended WOT conditions.
The last thing to consider when choosing a turbo is the turbine wheel and turbine housing size. This is a bit less of an influence on turbo decision because most all reputable turbo companies will make sure there is a proper compressor to turbine wheel pairing. A mismatch of turbine to compressor wheel sizing will make a turbo perform poorly. Turbos work off of exhaust pressure. The pressure is what causes the turbine wheel to spin. Using a large turbine wheel on a small displacement engine will reduce the exhaust pressure, which spins the turbo slower and results in lag. Lag is the time it takes to build your peak torque and power levels. Simply running a smaller turbine wheel to gain faster spool/more exhaust pressure is an option; however, the compressor won't perform as designed since its operating range won't be correct. Remember that the turbine to compressor wheels have to be matched properly; otherwise, the compressor wheel won't flow what it's rated. It's a delicate balancing act. Luckily, this has been all taken into account in most cases and the turbos that are for sale on the aftermarket are designed by engineers that know what they are doing.
What's important to take away from this explanation is that a larger turbo will be laggier compared to a smaller one. You can't have your cake and eat it too. Essentially, you cannot have a turbo that spools instantly AND supports big compressor flow. The more power you want, the more lag you will experience. Depending on your racing application, this may or may not be a deal breaker. If you need a power band that has instant power, you will have to downsize your power goals to have a broader power band. If you don't mind the extra lag and won't lift to shift between gears, the lag is not going to be an issue. Choosing a turbo is all about balance. I have tuned quite a few cars over the years that just simply had the wrong turbo on them. Either falling short of the power goal, or having so much lag with big peak power that driving the car was just plain slow and unresponsive. I would personally sacrifice higher peak power for a more usable power band.