Somewhere between the boosted ZC's that roamed the streets long before mags like this one graced the newsstands and the exponentially more powerful B-series drag mills of today, the lost art of turbo sizing had become exactly that--lost. Proven mathematical formulas have been kicked aside as some struggle to inadvertently secure their manhood through oversized compressors strapped to pint-sized small blocks.
Unquestionably, turbochargers are not one-size-fits-all, bolt-on power solutions and, in our case, size does matter. What works well for one engine combination likely won't work for another, despite seemingly equal displacements and engine speed characteristics. Requirements often vary depending on the engine in question. What's important: top-end power or low-speed torque? How about a broad power curve and long-term reliability? What about acceptable trade-offs: some turbo lag, maybe some added heat or low-end torque loss? Before shelling out for that basketball-sized compressor you've been eyeing, be sure and run these questions through your head.
Choosing the right compressor is arguably the most important part of turbo sizing and perhaps the most often bungled. Assuming a basic level of understanding as to what the turbocharger's chilly side actually does, we'll forgo the turbo anatomy lesson and jump right into sizing details.
Ideally, you want the most efficient compressor; not the biggest or shiniest, but the one that can pump the most air into the cylinders without raising temperatures more than complicated thermodynamics' laws say it should. For this, most turbocharger manufacturers supply us with compressor maps, graphs that assist us in compressor sizing by providing efficiency, surge limit, boost potential and shaft speed data. But before we look at these maps or arrive at an ideal efficiency zone, we first need to arm ourselves with two figures: the proposed boost pressure ratio and the given engine's airflow rate.
Determining an engine's boost pressure ratio is the easy part, but it requires a reality check. First, the calculation: The pressure ratio is simply the proposed absolute outlet pressure divided by the absolute inlet pressure. Restraining yourself to a realistic boost figure may prove more difficult than the calculation itself. Start with realistic numbers, say 7-10psi for street cars on pump gas and upwards of 20psi for race cars on more potent petrol. The key is being a realist. Sure, you know you want to show your friends the 40psi-capable pressure cooker under your hood, but the chances of a turbo like this ever seeing that much boost on the street are slim. Stick with reasonable boost figures and your setup will be better for it.
Airflow is a bit more complex and, unlike boost pressure, you can't pull this number out of your ass; it is what it is. Airflow measures how much air enters the engine for a given period of time and can be quantified at a given RPM if you know the engine's displacement and volumetric efficiency (don't feel bad if you don't; see the attached calculations). With honest pressure ratios, calculated airflow figures and a compressor map, you'll have to try real hard to not end up with the right compressor.
Each compressor has a boost pressure and airflow point at which it works best and often times, more than one will seem to work. The key is matching a given compressor's maximum efficiency point to the most useful part of the engine's RPM range, typically its peak torque point. Plotting based on the maximum horsepower point will do little for daily-driver performance. For the best possible turbo response, mid-range and top-end power, compare compressor efficiency at multiple RPM points. It's best to plot a map at redline and another at, say, 70 percent of redline, near where the compressor may potentially hit full boost. Fortunately, we have those nifty aforementioned compressor maps for this. These charts aren't too far off from something you'd see in your high school math class; they display compressor efficiency by expressing the boost pressure ratio along the map's Y axis and airflow ratings along the X axis. A number of oval-shaped islands within the graph represent different efficiency zones. Any given boost/flow point plotted on an island will yield an efficiency point, ideally as close to the center island as possible with efficiency decreasing as points move outward. Where the two points intersect on the map represents the maximum amount the compressor can flow in the proposed situation. Compressor efficiency is a percentage, with most peaking in the 70 percent range. Stay above 60 percent and you're in good shape.