In case you missed Part One of this series, I recently visited EFI University’s headquarters in Lake Havasu City, Arizona, to attend the school’s five-day-long Accelerated Certification Program (ACP) and eight hour long Advanced Turbo Concepts (ATC) course. The experience I was given through these courses permanently changed the way I look at an engine and EFI tuning. And because the topics covered in these courses are so technical, we knew it wouldn’t be possible to relay the true value of these courses through a generic overview, so instead we decided to pull several of the topics actually covered in these programs and present them in a series of articles.

Just know, we really think you should take these courses yourself, so we can’t give away all of the answers!

As Jeremy Clarkson once put it, “Exhaust gasses go into the turbocharger and spin it, witchcraft happens and you go faster.”

Ben Strader, owner of EFI University, has a much more intimate relationship with the science behind turbochargers and just couldn’t condense this invaluable knowledge into a single tongue-in-cheek sentence. With the support of some of the biggest names in turbocharger technology like Garrett, Turbonetics and Turbosmart, the school’s Advanced Turbo Concepts (ATC) course was born.

Recap

In Part One of this series, we took our theoretical 302 ci small-block Ford and 454 ci LSX — both with a goal of 800 horsepower and an 8,000 rpm rev limit — and determined each engine’s estimated horsepower capability by calculating the Volume Flow (cubic feet per minute), Air Density (pounds/cubic feet), Mass Airflow (pounds/minute) and Brake Specific Air Consumption (BSAC) for each, at sea level.

After working through the equations in our pervious article, we determined that the naturally aspirated 454 ci LSX engine barely squeezed by our power goal with 803 horsepower at 8,000 rpm, while the smaller displacement Ford came out to an estimated 534 horsepower at the same engine speed. So without some divine intervention from the horsepower Gods, our Ford definitely needs a little perceptible help in the form of forced induction.

As Strader noted previously about the internal combustion engine, “The reality is that what you’re looking at is nothing more than an ‘air pump,’ and we have to move X-amount of air to make X-amount of power.”

In this article we are going to dive into the world of turbochargers to see how much boost it will take to get our 302 ci Ford to our goal of 800 horsepower. Then we will finally have all of our airflow requirements out of the way.

Calculating Turbo Compressor Size Requirements

The term boost is merely a measurement of restriction — the volume of air a cylinder can hold is limited by its physical dimensions, not by how air is being fed into it. A turbocharger increases power not by forcing a greater volume of air into each cylinder, but rather increasing the density of that same volume of air.

Properly sizing a turbocharger to most efficiently meet the horsepower goal of your engine involves calculating the required Mass Airflow (pound/min), Volume Flow (CFM) and Air Density (pounds/cu ft) — just like we did with our naturally aspirated engine previously — as well as the required pressure ratio. Once these values have been determined, you can physically plot where your engine would fall on each turbocharger’s compressor map to determine the correct size for your setup.

Determining Required Air Mass

“Similar to what we cover in the ACP course, an engine can only make so much power with a given amount of air mass,” explains Strader. “So, if we want to know what turbocharger our engine needs we should first figure out how much air mass it will require to reach that horsepower goal. Because as a rule of thumb, that air mass requirement will never really change.”

Since our “air pump” is required to move a specific mass of air through its cylinders to achieve our power goal, we need to find the Mass Airflow (pounds/min) required to get there, which is both a volumetric and density measurement combined. To find the required Mass Airflow, simply divide your target horsepower by the engine’s brake specific air consumption (BSAC).

BSAC is most easily measured on an engine dyno and is useful for evaluating how efficiently an engine is able to convert the potential energy from an air mass into captured energy (horsepower). We already determined that our Ford has a BSAC of 10.

Using the formula in the sidebar, if we divide our target horsepower (800) by the engine’s BSAC (10), we can determine the estimated Mass Airflow requirement for our power target.

• 800 horsepower ÷ 10 (BSAC)
• 80 pounds/min of airflow required

This tells us that our small-block Ford — which is capable of moving a maximum of 53.40 pound/min naturally aspirated — needs to move 80 pounds/min to reach our horsepower target.

Calculating Air Density Requirements

If you followed along in our first article you’ll remember that, according to Strader, the Volume Flow of an engine is considered the “Holy Grail” of EFI fundamentals, and it’s important to note that the volume of air it pumps for a given RPM and atmospheric condition will never change without mechanical modifications. Since we already knew that our SBF displaced a total of 302 ci, by converting that value from cubic inches (ci) to cubic feet per minute (CFM), we were able to determine that our engine can flow an estimated 699 CFM.

Now, to find the required Air Density of our surrounding environment needed to achieve this power, we divide the required Mass Airflow of our engine by our previously estimated Volume Flow.

• 80 pounds/min ÷ 699 CFM
• 0.114 pounds/cu ft

Our Ford needs an Air Density of 0.114 pounds/cu ft to achieve 800 horsepower at standard sea level conditions of 14.7 psia (101.3 Kpa) atmospheric pressure and an air temperature of 59 degrees F (15 degrees C).

Pressure Ratio And Absolute Pressure

The Pressure Ratio (P/R) of a turbocharger is defined as the absolute compressor outlet pressure divided by the absolute compressor inlet pressure. And now you might be asking yourself, “What is absolute pressure?”

The molecules that make up the gasses within the Earth’s atmosphere are constantly being pulled towards the Earth’s surface by gravity, this force creates what is known as atmospheric pressure. When most enthusiasts think of measuring air pressure, their first thought is usually a boost gauge or tire pressure gauge. However, a boost gauge reads pressure in units of gauge pressure (psig) — which accounts for atmospheric pressure in its readings — meaning that it only measures air pressure above the atmospheric pressure in your immediate environment. Atmospheric pressure is equal to 0 psig on these gauges.

Absolute pressure (psia) is the atmospheric pressure of your environment added to the amount of gauge pressure being produced. Since our small-block Ford is being operated under standard sea level conditions, our atmospheric pressure will be equal to 14.7 psia.

For example, if your boost gauge reads 10 psig at sea level, that means that the air pressure within the intake manifold is 10 psi above atmospheric pressure, or rather 24.7 psia.

Remember, a turbocharger only increases the density of the air, not the volume of it. This is why turbochargers are measured by Pressure Ratio and not boost, because the P/R required to reach a certain power goal is not affected by barometric pressure or air temperature like gauge pressure would be.

So, to find the required Pressure Ratio to reach our goal of 800 horsepower, we divide the required air density that we calculated above (0.145 pounds/cu ft) by the atmospheric air density of our environment (0.076 pounds/cu ft).

• 0.114 pounds/cu ft ÷ 0.076 pounds/cu ft
• 1.5 P/R required

This tells us that to get our “built” theoretical 302 ci small-block Ford to 800 horsepower we need to find a compressor housing that is capable of efficiently moving 80 pounds/min of air at a P/R of 1.5, or ideally a nice twin turbo setup, but that would be another article all together. So, to find the right turbo for our engine, we need to examine the compressor map for each turbo we may want to use.

Compressor Map

When looking at a compressor map, you’ll notice that what you’re really looking at is a two-dimensional graph, and on the X-axis you have Mass Airflow (as we calculated above) and the Y-axis represents Pressure Ratio. Within those confines you will find efficiency islands, speed lines, a surge line and a choke line that each represent important aspects of a specific turbocharger’s “personality.”

“A compressor map is essentially a chart that graphically displays the behavior of a turbocharger and its ability to supply a specific mass of air based on pressure ratio, or rather the difference in the air pressure going into the compressor and the pressure coming from the compressor outlet,” states Strader. “Once we know how much air mass it will require to reach our power goal and at what pressure ratio, we can pull out a turbocharger catalog and plot out where our engine would occupy for each compressor map.”

For the sake of keeping this article simple, we will be sizing a single turbo to this engine, and using the vast collection of compressor maps on Garrett’s website to find the “best” turbo for our specific needs. We first pulled up the compressor map for Garrett’s second generation GTX3582R and found that this combination would fall on the wrong side of the compressor’s choke line, even during spool up. We then pulled up the map for the second generation GTX5533R, which falls closest to the center of the efficiency islands at peak flow, but will likely surge during spool up, which only proves that a high pressure ratio and power goal would be best or a twin turbo setup at this horsepower goal.

• 
• 

The GTX3582R (left) is far too small to be used as a single turbo setup for this engine, and the GTX5533R (right) falls perfectly across the line of the 73-percent efficiency island and just above a 38,000 rpm shaft speed. Courtesy of Honeywell International, Inc.

Conclusion

We calculated that a Garrett GTX5533R or similar sized turbo would suit this engine “best” for a single turbo setup at this horsepower goal. This is only a very small portion of what is covered in the school’s Advanced Turbo Concepts course, but hopefully this gives you a better idea of what you’re actually looking at the next time you pull up a turbocharger compressor map. So, if you want to be given the tools to become a master in turbocharger theory and sizing, sign up for an upcoming class on EFI University’s website.

Be sure to check out Part Three of our EFI University ACP/ATC crash course. Now that we have all of our airflow requirements out of the way for both engines, we can move on the the final step of calculating our major fueling requirements.