When it comes to simply replacing or purposely upgrading the turbo on your diesel engine, one of the most important considerations is size. Yes, guys, size does matter. But not how you think it does. Much like other engine parts, bigger is not always better. But, with so many options available, even we can get confused when asked “which turbo” to use on a given application. That’s why we decided to seek some help from the turbo professionals at KC Turbos.
To start the conversation, choosing a turbo depends on a couple of different factors. For a race engine, the answer is simple. Get as big a turbo as you can spool, or the largest turbo the rules will allow. But, when deciding which turbo is best for a street truck, things get a little more complicated. Why? Because your street truck doesn’t spend every waking moment at wide open throttle. This means that everyday drivability has to be a big consideration. Let’s face it, it’s no good to sit at a traffic light waiting for your turbocharger to spool while your buddy pulls away.
It doesn’t matter if a bigger turbo looks better under the hood of your truck. A turbo that is too large will make the truck less responsive and hinder power delivery.
Why Size Matters When Asking Which Turbo
Right off the bat, it’s no secret that certain Cummins and Power Stroke engines, like the 6.0- and the 6.7-liter can support more than 400 rear-wheel horsepower with a stock turbocharger. Duramax engines on the other hand, can be comfortable at around 500 horsepower with a stock turbo. That said, if you’re planning between 500 and 800 horsepower — or if you tow heavy — you should probably consider an upgrade.
When talking turbo size, millimeters is the term to know. This is the most commonly referred to measurement of the compressor wheel’s inducer diameter and the turbine wheel exducer diameter. You will also need to be familiar with the turbine-side housing ratio (its A/R or area over radius). These all need to be taken into consideration as they will affect how the turbo behaves. To give a comparison, stock turbos usually have large exhaust housings, large-diameter turbine wheels, and smaller compressor-side inducers.
Inducer and exducer are terms you need to understand. The inducer is the part of the wheel that brings gasses into the wheel. The exducer is the part of the wheel that expels the gasses.
Let’s use a 5.9-liter Cummins as an example. A stock turbo will have measurements of 54/69. In other words, a 54mm-inducer compressor wheel, and a 69mm-inducer turbine wheel. What many do not typically consider is the exhaust housing size (A/R). This determines how quickly the turbo will spool up. Basically, a housing that is too small can create high drive pressures (backpressure build-up on the exhaust side). On the other hand, an A/R that is too large can increase the time it takes for the turbocharger to spool up, resulting in more turbo lag. This can result in less low-end power, but more top-end power. Both scenarios can be hard on engines, turbos, and exhaust hardware.
According to Cameron Lewis of KC Turbos, “When a customer calls in for a turbo, we typically ask them about their current injector size, turbo size, fuel/oil system upgrades, and if they ever plan to change up sizes in the near future. Next, we will want to know what they do with the truck on a daily basis, how often and how heavy do they tow, where are they located, because elevation plays a huge role in selection, what are their power goals, and what is their budget.”
When it comes to making the decisions about which turbo your truck needs, the engine solely dictates the type of turbo to use. Larger turbos, like those with a 66mm to 71mm compressor wheel inducer, might experience severe turbo lag and not even start to spool until the engine reaches 2,200 to 2,500 rpm. In other words, if you’re putting this turbo on a stock engine that makes peak power at 2,800 rpm, you will not be happy with the results. When your redline comes in at 3,000 rpm, a few hundred rpm matters.
A turbo’s A/R number is a function of the various sizes in the turbine housing. The smaller the A/R number, the faster the turbo will spool. The larger the A/R number, the longer the turbo will take to spool but will have more potential output.
Know What You’re Working With
That said, if you have a few performance upgrades like a high-performance camshaft, ported head(s), and enough fuel to turn your engine to 4,500 rpm, then having your turbocharger spool above 2,500 rpm might not be as big of a deal. An engine that is spinning faster will also require more air, and for that reason, a high-RPM engine will also need a larger turbocharger.
“For us to help the truck owner get the right turbocharger, we need to know the customer’s goals,” says Cameron. “Sometimes, they are set on purchasing a turbo that is too large and we do our best to talk with them so they get the correct turbo for their needs. If they are set on going to a turbo that is too big, we always recommend them to go with our ball-bearing units because they will spool like a smaller journal-bearing turbo but still give them the top end they are looking for.”
Heavy towing will allow the opportunity for a larger turbo. This is because towing puts the engine under load, and that creates higher EGTs. A larger turbo will flow more air which will lower EGTs.
Here at DieselArmy.com, we tow things, heavy things. If you tow heavy loads, 10,000 pounds or more, a bigger turbo might be a good option for you. The reasoning is, towing puts the engine under load, which creates higher Exhaust Gas Temperatures (EGTs), and regulating these needs to be a priority. A larger aftermarket turbo will flow more air even at part throttle, which results in lower EGTs across the board. The EGTs are lowered because of increased airflow by the compressor putting more air into the engine as well as the larger exhaust turbine and housing being less restrictive to exhaust gasses. While spool-up might be slower for the larger turbocharger, the added benefit of EGT reduction means a lot.
It All Affects Performance
When choosing a turbocharger, you need to also keep in mind that a turbocharger of a given size will behave differently on, let’s say, a 7.3-liter Power Stroke as compared to the previously mentioned 12-valve 5.9-liter Cummins. But why? Because the Power Stroke is a physically larger engine. This means that more exhaust gases are produced from the larger displacement, which means more energy is available to drive the turbocharger at lower RPM. This is why the 7.3-liter Power Stroke comes with a larger turbocharger from the factory. This is why such large turbos are often recommended for a 7.3 Power Stroke versus maybe a 6.0-liter Power Stroke or 12-valve 5.9-liter Cummins.
There are many factors that will contribute to how a turbo reacts in a given application. These include rearend gearing, truck weight, torque converter stall speed, and engine speed. But we can’t forget about driving style. If you expect a quick spool-up and good street manners, go smaller rather than larger. If you want to spin the engine faster and are looking for power at the sacrifice of some drivability, go larger rather than smaller.
Upgrading your turbo is a relatively quick and easy way to boost performance — both for towing and non-towing situations. You just need to be sure and choose wisely.
So far, this article has revolved around the size of the turbo itself, but other engine modifications can affect how a turbo reacts. The engine is an air pump, and the amount of air and fuel it consumes must be balanced to run optimally. If you increase the amount of fuel (larger injectors), you also need to increase airflow to match the extra fuel. Fail to do so and you will encounter high EGTs. A perfectly sized turbo for a stock fuel system could be too small if a larger set of injectors and a hot tune are added to the mix. If you can only afford to buy or install one upgrade at a time, consider it a safe option to install the larger turbocharger first and run it with a stock fuel system rather than running a larger set of injectors with a stock turbo.
Finally, we need to mention VGT or Non-VGT (Variable Geometry Turbo) when discussing turbos. Most diesel engines produced during the last decade use a variable geometry turbo as opposed to a non-variable geometry turbocharger. But that is another discussion we had, and you can check that out right here.
Choosing a turbocharger can be confusing, so make sure you talk to your turbo manufacturer when deciding on an upgrade and be honest with them about how you use your truck and what parts are currently in play.
