Establish Starting Point

As with any project, the first thing you should do is establish your goals.  This is going to require a bit of math, but we’re going to determine the ideal target crank hp gain for your vehicle.  A good rule of thumb is that every 10hp you make with a gas engine requires a pound of air per minute delievered into the intake manifold. For example, to transform a 100peak horsepower (php) naturally aspirated engine into a 200php turbo engine, you’re going to need a turbocharger capable of flowing 20lbs of air per minute at a realistic boost pressure. You’ll need to compute the basic engine airflow rate in CFM as follows:

Airflow=(cubic inches displacement * rpm * 0.5 * volumetric efficiency(V.E))/1728

The *0.5 is there because a four-stroke engine only breathe every other revolution, while 1728 converts cubic inches to cubic feet per minute

If you were to insert 83 percent for V.E for a typical 2.0L Honda engine’s 122CID * 7000 * 0.5 * .83/1728=247 CFM

At 83 degrees ambient temperature at sea level 247CFM converts to pounds/minute follows: lbs/min=CFM * .07

Therefore 247CFM * .07 = 17.29 lbs of air per minute.

Using this rule of thumb this 2.0l engine should produce approximately 173 NA HP.  Now let’s see what we could do with that number using a compressor.

Verify Target Boost Pressure

On a street car, target boost pressure will be the lesser of the boost needed to make target power or (more likely) the engine’s boost detonation limit. Increase boost enough and A/F mixture in the combustion chambers will begin to explode instead of burning smoothly, which is very bad for the engine. There is no point of having a turbo that can push abnormal amounts of air beyond the point of the engines boost detonation limit if your fuel system cannot deliever the fuel required to safely run your engine at that boost. Additional boost limiting factors include fuel supply constraints and engine calibration limitations. With premium street gas (typically 93 octane for most and 91 for us Cali folk), assume a maximum of 10 psi of boost without an intercooler and 15 psi of boost with an intercooler. Highest boost pressure will be feasible with pent roof combustion chambers, really efficient intercooling, higher octane gas, excellent engine management with a high quality calibration using the best anti-detonation countermeasures.

Convert Target Boost PSI to pressure ratio

You need a pressure ratio to work with compressor maps that plot air flow at various pressure ratios and compressor speeds. If you’re working with target boost pressure, you’ll need to convert boost pressure to a pressure ratio, which is the percentage of one atmosphere above nothing at all delievered at the compressor’s outlet. For example: 10 psi of boost + 14.7(atmospheric pressure)/14.7=1.68 boost pressure ratio. Which means BPR of 1.68 equals 68 percent higher pressure than NA.

Convert Pressure Ratio to Density Ratio

Unfortunately, a turbo engine system will not usually flow as many CFM as you’d predict just by multiplying the pressure ratio by the NA airflow of the engine in CFM.  Bear in mind that density ratios vary because of turbo’s thermal efficiency and efficiency of the intercooling system. Most street turbos operate in the thermal efficiency range of 55-85 percent.

Let’s go back to the 2.0l Honda engine. If we were to boost that engine to 7.5 psi without an intercooler, the pressure ratio for that engine will be (7.5+14.7)/14.7, which yields a pressure ratio of 1.5. Using the stock airflow of that engine of 17.29lb/min(247 CFM), at a BPR of 1.5, you might expect the airflow under boost would increase to roughly 28lb/min. However, if this particular turbo with no IC is just 75% efficient, the density ratio at sea level is really about 1.3 which equates to 24lb/min of airflow. Bottom line: when turbocharging your engine, you’ll need to start with a slightly larger turbo to compensate for turbo inefficiency.

Select the RPM range of your engine for maximum compressor efficiency

Contact various manufacturers for a turbo compressor efficiency map for every turbo you’re interested in. You’ll typically want to match your turbo’s maximum compressor efficiency with your engine’s natural powerband. This will require a lot of knowledge about turbos, so you may want to contact the experts at Garret, Precision Turbo, etc.

Rather than aiming to achieve maximum peak power, think of the areas that will greatly help your vehicle acceleration. 500HP at 10,00rpm is useless compared to a 300hp at 4000-6000rpms in a street application. But if you are trying to maximize your peak power, remember that a forced induction system will always shift peak power to a higher part of the rev range. But with the days of electronic boost controllers, users will be able to pinpoint when the peak boost will be created anywhere on the rev range based on rpm, gear, and other factors. You might place peak boost at lower rev range to amplify torque (helpful with acceleration) but you’ll need to reduce pressure at higher speed to limit peak hp to stay within certain fuel constraints or  to protect your drivetrain. For example, dragsters limit boost at lower gears as a form of traction control.

Select a compressor

You’ll notice that a compressor map will resemble a 3d topgraphical contour map of a hill. This map describes the compressor’s efficiency at various combination of airflow rates and boost pressure. To the left of the map also are danger zones at the east and west of the map. This zones illustrates the surge and choke zones of the compressor. Big turbos tend to surge at lower revs with huge turbo lags and usually no streetable power gains. Hybrid turbo is a solution if you are looking for a big compressor but a quicker spool-up time.  The t3/4 hybrid is a perfect example which utilizes a smaller turbine but with a bigger compressor to virtually eliminate turbo lag.  If your engine has a powerful bottom end, you might choose a big turbo to create big peak power but to accentuate your bottom end as it creates full boost. But with a engine with a weak bottom end and a strong top end (such as a b16 or 13b), you might want a smaller turbo that spools quickly and build boost as soon as possible to maximize bottom end power and let your engine’s top end take the rest of the way. Either way a small turbo will run out of steam at the upper reach of the rev range and a big turbo will spool slowly but their airflow at higher rev range will eclipse the smaller turbo counterparts.

Some engines also incorporate a second turbine.  These can be twin turbos (in which both turbochargers are the same, and increase the gains in their particular rev range) or sequential turbos (in which one turbo is smaller to help the engine wind up, and another to carry it through the upper powerband).  Some smaller engines may actually see performance decrease if a second turbo is added because the engine does not produce enough exhaust gas to spool both turbos.

Photo courtesy of Mingo.nl