Boost pressure can be deceiving

We see these questions all the time. Why am I only making 22psi today? How come others are making more boost? Why did my boost drop after installing an intercooler? How is a bigger turbocharger making more power with the same boost?

Before we get into how the ECU calculates how much boost it wants and why it can vary so much, we need to really understand turbocharging and look beyond that number we all get excited about. A turbocharger adds power because it increases the effective displacement of an engine, which is essentially how much air an engine can hold in its cylinders at atmospheric pressure and roughly corresponds to the potential power output and fuel consumption of that engine (for example, the K20C1 in an FK8 Civic Type R is a 2 liter engine). The turbocharger harnesses the exhaust flow of the engine to spin a compressor and force more air into the engine.

Performance car enthusiasts have long used PSI as a metric for how much power they’re making in comparison to others but in reality it’s a nearly useless number for that purpose, especially when comparing different setups or cars. Saying how much boost you’re making mainly came from turbo kits utilizing springs that would keep the boost at a specific maximum amount of pressure, whereas the Type R and the mass majority of modern turbocharged vehicles use an electronically controlled wastegate that allows exhaust flow to bypass the turbo and precisely controls the boost made for any amount of desired pressure.

Once you add a turbocharger to an engine, you can force more air into the cylinders than they would hold otherwise. The key here is that it’s not the pressure itself that makes more power, rather it’s the additional molecules of oxygen you’re able to put in the cylinders by pressurizing it. As a side note, nitrous effectively does the same thing because it’s highly oxygenated - you’re adding more oxygen molecules by injecting it into the cylinders as fuel.

So why is it then that pressure doesn’t correspond to the amount of oxygen and power? Three reasons: the volume of the container, the restriction of flow, and the temperature of the air can all vary despite the actual amount of oxygen flowing remaining consistent.

For volume, think about an air compressor and how you can fill up its tank with a lot of air. A larger tank will require more air to reach the same pressure. So too will larger parts: a bigger intercooler, larger charge pipes, etc.

For flow, imagine a pressure washer. The pressure washer greatly increases the outgoing pressure of the water, but ultimately your hose is still usually providing the same amount of water. The pressure doesn’t say anything about how much water is actually flowing through it. The same can be said about the air flowing through a turbocharger and being restricted by the various parts from the intake to the exhaust.

For temperature, consider a pressure cooker. By increasing the temperature of the boiling water and resulting steam, it increases the pressure inside of the pressure cooker. But it’s not compressing more liquid into itself, it’s simply heating up what’s already inside of it and evaporating the liquids to achieve a higher pressure. This is an important factor for why larger turbos can do more with less - they’re more efficient and the air is cooler.

With these three examples, you can see that pressure on its own truly says nothing about the actual mass of air you’re getting into the cylinders.

On the Type R, the engine control mechanism takes this fully into account and uses a complex algorithm to calculate how much air is actually inside the cylinders and how much it needs to achieve a requested torque. This metric is known as air charge and it directly corresponds to the effective displacement of the engine. It is a ratio that represents the amount of air inside the cylinders versus how much they would hold at atmospheric pressure where it is neither under boost nor vacuum. 100% is equal to 2 liters, 200% is equal to 4 liters and under boost, and 50% is 1 liter and under vacuum. The way this is calculated can be found in our air charge article.

The ECU uses what’s known as a torque management strategy, and knowing the actual mass of air versus just pressure is necessary to make it work. Your pedal input and current RPM corresponds to a table of requested torque values, one for each gear and drive mode totaling 18 tables. There is a built in mathematical model of the engine that converts this torque into a necessary amount of air charge, and from there the ECU dynamically determines the amount of boost pressure it needs to target to achieve that air charge on the fly. That boost pressure target is further limited by the barometric pressure and pre-turbo intake temperature to protect the turbo. How this is done is also explained in the air charge article.

The end result of this is that your parts, elevation, intake temperatures, pedal position, RPM, gear, and drive mode all heavily influence the amount of boost pressure your Type R tries to achieve.

It also means there is a metric you can use for comparison of boost that shows the true displacement of the engine. Air charge can be compared between vehicles with the same displacement or even on the same vehicle after part/tune changes to see how much boost you’re making. Just keep in mind that your elevation and intake temperatures can put a cap on it too, and leaks or MAF miscalibration will cause the air charge to be incorrect. It’s also important to know that displacement isn’t the only factor for how much power you’re making, it simply shows the capacity for power. Ignition timing and several other things matter too, but that’s a much larger subject.

A typical Type R at sea level and with a stock turbo will see 220-240% aircharge at peak torque around 4000RPM and 170-200% at redline. With a larger turbocharger, you can see over 250% aircharge for the entire power band. You can brag to your naturally aspirated friends that your engine is basically a 5 liter too.

The only catch is that you’ll need Hondata or KTuner with a jailbroken ECU to access the air charge reading as it cannot be seen in the cluster nor on any OBD2 scanners.