When it comes to turbocharger upgrades, it truly is a wild wild world out there. You can have a stock appearing turbo, an S300, an S400, you can have an S400 over stock compound or any combination of 2-3 of the turbos already listed. So without completely melting down our brains into sludge, lets go through some of the options and talk about how to figure out what is the right choice for you. With some of these options being north of $3000 it's something we only want to do once.
Before we discuss individual turbo options lets talk about some of the components of turbochargers, how they work, what they do and why it matters to you.
Looking at the turbocharger diagram starting from the cool air side. You have the compressor inlet, this is coupled directly to your intake tube. When the compressor wheel spins, it draws in cool ambient air and forces it into the compressor housing. When air begins to fill the compressor housing it exits compressor discharge and travels through your intercooler pipes/intercooler core while being cooled down eventually entering your engine where it turns dinosaur juice into glorious noise and power. As hot exhaust gasses leave your engine they enter the turbine exhaust gas inlet, this is attached to your exhaust manifold. These waste gasses move through the turbine housing spinning the turbine wheel and then exiting the turbine outlet and leaving through your exhaust system. The magic part of all this, the compressor and turbine wheels are connected by a shaft that transfers waste energy from your exhaust to the compressor wheel to produce more airflow. We're basically just a giant recycling plant at this point.
Now that we have an idea of how a standard turbocharger works, lets talk about about how a VGT/VNT turbocharger is different. The VGT/VNT turbocharger works in a similar way to the fixed geometry turbocharger we reviewed earlier. The place where they differ is in the method of control. Fixed geometry turbochargers typically have a boost actuated wastegate that when it reaches it set point, opens the wastegate which diverts exhaust gas away from the turbine to help control speed. A VGT/VNT unit will have typically an electronic controller that will manipulate either the vane assembly or the nozzle slide ring in order to increase or decrease exhaust gas speed to the turbine. This is extremely useful because you can increase low engine speed turbocharger response by closing the vanes down to restrict the amount of flow (increasing speed) in the exhaust housing. Then when boost targets have been achieved, the controller will open the vane positions allowing greater flow (reduced exhaust gas velocity) which controls the turbo rpm. This flexibility allows a 60mm turbocharger to function in a range of speeds from where a fixed 40mm turbocharger and 70mm turbocharger might perform well. The other trick that a VGT/VNT unit can do, is slow you down. During low airflow situations slowing down/coasting, the vanes can be closed to allow back-pressure to build up inside the exhaust housing, manifold and engine creating an exhaust brake. This lack of flow allows you to assist your service brakes in slowing down.
1- Pivoting Vane, 2- Variable Angle Vanes, 3-Adjustment Ring
Now that we've covered basic turbocharger operation, lets talk about what kind of common turbocharger swaps there are, after that we'll move on to talking turbocharger size, A/R (aspect ratio) and what configuration works best for your typical use scenario. Last we'll talk about power goals.
The most common turbocharger swaps include VGT upgrades, S300, S400 and compound turbo upgrades. With a VGT upgrade you are receiving a stock appearing turbo with all the functionality of a stock turbo in terms of transient response and exhaust braking potential. However there are some mad scientists (I'm looking at you Brayden and Chase Fleece) that will take a stock appearing turbocharger, machine out the compressor and turbine sides and replace them with larger wheels that provide improved compressor and exhaust flow. Next on the list is the S300 swap, this is done on all of the big three domestic diesel OEM's Ford, GM and Dodge/Ram. The S300 provides the basis for a street/strip platform allowing more airflow and subsequently more power than a VGT. You do lose some of the transient response and the exhaust brake but you gain flow. Lots of flow. When properly sized many S300 turbochargers can be neck and neck with a VGT in terms of spooling quickly. In the Cummins world you're likely to hear 3rd Gen swap often, this is the practice of installing a 3rd gen (low mount) manifold in conjunction with an S300 turbocharger.
When installing an S400, you're installing one of the more capable and durable turbochargers available for light duty diesels. Where the S300 left off, the S400 picks up and runs. With larger S400's being capable of making 1000hp there is some serious power potential. Again with the S400 fixed turbo you will lose some more low speed drivability but will gain even more top end flow. This is one of the most common swaps for a race truck. As with the S300, the S400 adds reliability by removing electronics from the equation unlike the VGT. In the Cummins community you'll often hear the term 2nd gen swap which entails installing a 2nd generation style manifold in conjunction with an S400 frame turbocharger.
Last on the list and certainly not the least is Compound and Add-A-Turbo options. Compound turbocharging involves using a larger (atmospheric charger) to feed a smaller turbocharger. The goal with this is to utilize the response at low speed of the smaller turbo which may be a VGT or an S300 and then as engine speed begins to rise and the larger turbocharger is coming up to speed it will take over the flow at the higher rpms. This is exceptionally potent when used with a VGT.
Now, the numbers. Blah boring, lets skip it. Not so fast, this is actually one of the most important parts when it comes to understanding what turbocharger size is right for you.
VGT Upgrade is recommend for you if the following sound right for your needs
Good low speed drivability/great daily driver
Confident towing power and good exhaust braking
Retains factory emissions
Does not require a tune to use but will benefit from one
Capable of up to 650hp
The S300 is the right turbo for you if the following is accurate.
Good low speed response (close to VGT with smaller S300)
Losing the exhaust brake isn't a deal breaker
Tuning with emissions removal will be required
Capable of handling additional fueling up to nearly 800hp
The S400 is the right turbo for you if the following is accurate for you:
Loses some low speed response on larger units but gains big top end flow
Losing exhaust brake isn't a deal breaker
Tuning with emissions removal and map rescaling is needed
Desire more than 650hp up to over 1000hp
A compound turbo or add-a-turbo could be right for you:
Keeps low speed response and improves top end flow
Retains Exhaust Brake for confident towing/load control
Tuning is required but may be emissions on/off
More than 500hp but less than 750hp
The last piece of this turbo puzzle we'll talk about now is A/R (Aspect Ratio). In basic terms the Aspect Ratio of the turbocharger relates to turbine housing size as measured from the inlet to the radius from the center of the turbine wheel. The small the number the tighter the exhaust housing (faster spool but less top end) and the larger the number (slower spool but more top end flow). Common A/R's are .83, .91, 1.00, 1.10, 1.15 and 1.32.
I hope this guide helped you identify which style of turbocharger and what size of turbocharger is right for you. If you have any questions you're welcome to comment them below the blog article, contact us on facebook or even contact us via email.
- Overland Industries