Turbo sizing 101

Turbo sizing 101

“Intro”

So you’re working on your new project car… whether it’s already factory turbo or NA, you want something that’s going to make good power. You start researching options, and quickly realize this can be a daunting task. If you aren’t familiar with turbos, you can get lost in your head with all the terminology getting thrown around. Turbines, compressors, AR, back pressure, flange type; now you’re lost on where to start. Let’s clear some of this up and help you find the right option for you! This article will address the hot side. Stay tuned for a future article on cold sides and more!

“Things are getting hot”

Lets talk about the turbine side, otherwise known as the “hot side”

The turbine side of your turbo plays a crucial role on how your car is going to perform. The term “hot side” is because this is where the exhaust gas from your engine meets your turbo. There are three main aspects of the turbine half of your turbo; A/R (Area over Radius), wheel size, and wheel design.

“Turn the volume up”

A/R (or Area over Radius) is the size in volume of your turbine housing. This may be one of the most important factors in how your turbo performs on the street or track. The AR is essentially the cross sectional area divided by the radius from the centerline. 

Let’s make that a little easier to understand with an example. You have a short glass and a tall glass both filled with water. The short glass is just enough water to fill a .48AR housing, the tall glass can fill a .86AR housing. It’s quite obvious the .86AR housing can flow a bit more volume out of your engine. Of course this is overly simplified but it gives you the idea. 

To translate this into why it matters when choosing your turbo;

Let’s say you have a 2.0L 4-cylinder and you were considering a 3071R sized turbo. One is a .63AR turbine housing and one is a .86AR housing. 

With the .63AR housing; we have less volume of exhaust able to travel through the turbo, but higher velocity. This in turn will give you very quick spool characteristics and great response, but you will sacrifice some overall power. Smaller housings are great for street car and daily driver setups; giving you great drivability and lower end torque; but are unable to provide the same amount of flow as a larger housing would at high RPM. 

The same exact turbo with an .86AR housing, will make a fairly significant increase in overall horsepower (especially in the upper RPM), but due to the increase in volume and reduction of velocity, you will notice a fair amount of additional lag and less immediate response. 

Engine size does play a role here. While an .86AR housing on a 4-cylinder may be considered on the larger side; on a V8 with a single turbo, it would be considered small. It’s not uncommon to see some V8 builds with 1.01 or larger housings (due to 4 extra cylinders worth of flow, the added volume of the turbine housing is important unless you’re running twins)

“Go for a spin”

Now that we have AR out of the way, let’s talk wheels. 

This concept is fairly simple. When you have two turbine wheels of the same design, but different sizes, the smaller wheel is always going to spool faster. This is due to lower rotational mass, as well as having a little more back pressure. This results in a strong torque curve in the lower RPM. The negative of running a smaller turbine? Increased shaft speed at higher boost levels, and increased back pressure at high RPM. 

With a larger turbine wheel, there is additional mass to spin and reduced back pressure. This causes the user to experience additional lag and less response; though helps increase overall horsepower especially at high RPM. The larger wheel can flow more volume through your turbo. This also allows you to make the same power as smaller turbos with lower shaft speeds. This is helpful in extending your turbochargers lifespan as well. 

Back to using examples. A 2.0 4-cylinder with a GTX2871R vs a GTX3071R.

Both turbos use the same size compressor wheel. The 2871R has a 47/53.8mm turbine wheel (hot side.) The 3071R has a 55/60mm turbine wheel. 

Even though these two units use the same compressor, the driving characteristics and horsepower will be different from each other.

With the 2871R, then user will experience a quicker spool and better on-throttle response, while sacrificing some peak horsepower. The 3071R will have power arrive later, but will flow more air and result in higher peak horsepower numbers. 

The same concept applies here as it did with housing size when it comes to V8’s. With larger engines and more displacement, you will want to run a larger wheel in comparison to a 4-cylinder unless you are planning to use twins. 

In our next article, we will continue this conversation and explain how the compressor side operates; different wheel styles & blade counts; as well as how flange type and wastegates come into play! 

Until then, keep boosting!