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CHAPTER 3

MATCHING A TURBOCHARGER TO YOUR ENGINE

Selecting the proper turbocharger for your engine involves many considerations. Not only are the facts about your specific engine necessary, but equally important is the intended use for that engine. The most important approach to these considerations is a realistic mindset. In other words, if you’re turbocharging an engine that is presently rated at 200 hp in its naturally aspirated form, you’d probably love to have it produce 600 hp. However, that may be unrealistic inside of the additional collection of modifications you intend to do. If you’re looking for a nice power increase for all-around street driving, a 50-percent increase is more realistic and matching a turbo to this level of increase will produce more satisfactory results. A 300 percent power increase (200 to 600 hp) is possible in many engines, but increases like that are reserved for competition engines that have an array of additional modifications, both internal and external, that all work together to achieve this level of power. One of the most important factors in determining which turbocharger is most appropriate is to have your target horsepower in mind. But you have to be realistic about what you’re shooting for.

The application and intended use of the vehicle is extremely important as well. An autocross car, for example, would require a rapid boost rise for fast acceleration, whereas a Bonneville car running long straights is more concerned with horsepower at higher engine speeds. Indy cars frequently adjust the turbo for short tracks versus long tracks because of how critical the turbo match is to optimize flow at specific engine and vehicle speeds. Tractor pull applications will likely see the highest engine speeds right at the start of competition, and as the pull progresses, the load is progressively increased much like a prony brake until the engine is maximum loaded down by the pulling sled. These different uses require different turbo matches.

Volumetric Efficiency

The term Volumetric Efficiency, or VE, is a very important term and concept to understand. Maximizing engine VE raises it’s potential for horsepower and RPM. With the exception of fuel and ignition modifications, most of the traditional aftermarket high-performance engine parts essentially raise the engine’s VE. Forced-air induction is all about increasing VE. But what is Volumetric Efficiency exactly?

An engine’s VE is a comparison of an engine’s calculated, or theoretical, volumetric flow rate of air, versus its actual capability. An engine has a fixed displacement, for example, 300 cubic inches. That displacement will theoretically flow 300 ci every two engine revolutions (a four-stroke engine must rotate twice for all cylinders to complete all four cycles). In theory, there would be a linear relation to airflow and engine RPM where doubling the revolutions per minute would double the air displaced by the engine. If an engine were able to flow exactly as much air during operation as the theoretical calculation says is possible, that engine would have a VE of 100 percent. However, in reality that rarely happens.

While there are some engines that achieve 100 percent or higher VE, most do not. There are many factors that impede the engine’s ability to meet 100 percent volumetric efficiency, some intentional, some unavoidable. For example an air cleaner housing and filter will typically impede intake airflow, but you don’t want to operate your engine without air filtration.

The common methods used to increase VE in naturally aspirated engines are things like larger valves, more valves per cylinder, improved intake manifold designs, enhanced valve timing using different camshafts, free flowing exhaust systems and more. But perhaps the most limiting factor to VE is time.

The critical factor here is simply how long the intake valve can be held open. The calculation for 100 percent VE assumes a full cylinder charge of air without regard to time. During actual engine operation, the intake valve is only opened for a short time. The higher the RPM, the less time the intake valve is off its seat. Therefore actual VE is not a constant. It’s typically an efficiency ratio at one spot in the engine’s operating range.

If we did a simple calculation to express, in real time, just how long an intake valve is opened, it places the challenges to make horsepower into perspective. Let’s assume an engine was operating at 3,000 rpm, and it had a camshaft that contained 230 degrees of intake valve duration, measured at the crankshaft. The intake valve would only be opened for 0.0255 second. The reduction in valve open time is inversely proportional to engine speed. This same engine now operating at 6,000 rpm would only have the intake valve opened for half as long, or 0.0127 second! As you can see, volumetric efficiency will limit the upper engine RPM potential of any engine design, as it simply can’t breathe well enough in the short period of time dictated by the mechanical events. So, hot-rodders have been working for years to maximize the time we have.

The reason turbocharging has such a dramatic impact to engine performance can be better understood using this concept of volumetric efficiency. In a turbocharged engine, time still limits how long the intake valve is open, but if the intake pressure is greater than atmospheric pressure (boosted), then we can force more total air volume in during the valve opening. The quality of that air is improved for combustion purposes because its density has also been increased. The combination of boost pressure and air density compensate for the time-limiting aspect of the valve events and allow boosted engines to achieve well over 100-percent VE. But when maximizing total horsepower output, even turbocharged engines will benefit from many of the very same design improvements done to enhance VE on naturally aspirated engines.

As mentioned above, a given engine will have better or worse VE over the RPM band. Every engine will have its sweet spot, which is the point in an engine’s design where, at full throttle, the volumetric efficiency is at its highest. This is typically the point where peak torque will be found on the torque curve. Since VE will be at its highest point, maximum fuel efficiency or BSFC, measured in pounds of fuel per horsepower, per hour, will also be at its peak efficiency.

When calculating the proper turbo match, VE is an important element to consider, as it is an important contributor to determining the airflow demand of a given engine. This will be discussed further in this chapter’s section on compressor matching calculations.

Ballpark Matching

Ballpark matching isn’t an official term, but a rough approach that can be used due to the more sophisticated way in which today’s turbo suppliers are marketing turbochargers. Historically, turbos weren’t marketed in the automotive performance aftermarket as widely as they are today and they were rated only in their mass-flow capability, not in terms of horsepower compatibility. However, a reasonable turbo match can be obtained by using a realistic horsepower target, and simply choosing a turbocharger from a manufacturer where your target horsepower is located right in the middle of the horsepower band it is capable of supporting.

For example, in the above situation where your target is 300 hp (a 50 percent increase over the stock 200 hp) the Garrett brand GT2860RS with a 76 trim, also known as the “Disco Potato,” might be an excellent choice. That turbo is rated to be compatible with horsepower applications of 250 to 360, so 300 is right in the middle of its capacity. This ballpark method allows for both surge margin and broad flow range of operation, as will be discussed in the following paragraphs. This is not the most scientific way to apply a turbo, but it can be done because much of the science of the horsepower match has already been done for you by the manufacturer rating a given turbo in its horsepower compatibility. Also, because many of the turbos offered for sale today have been built with an internally designed wastegate, subtle adjustments in boost pressure optimization can also be performed after the car is test driven. While most turbo experts will raise their eyebrows at this overly simplified approach, it can be very successful for the average turbo car project and it makes the confusion of turbo matching a rather simple process indeed.

The Disco Potato: A Turbo Story

The Disco Potato is a Garrett turbocharger model GT2860RS. Today it’s one of the top selling, if not the top selling, turbocharger in the performance turbocharger retrofit market. Its beginning, however is not unlike the development of the Chevrolet Z28 Camaro. It was born of a type of skunk works effort from dedicated engineers who, in their hearts, are true performance-minded people who love and live their work, as are most that become highly successful.