How To Choose The Right Turbo's For Your Build
Properly selecting the right size turbocharger for your application is critical in getting optimal results. If you select a turbo that’s too big for your application you will experience turbo lag. Inversely if the turbo is too small you risk not reaching your horsepower target, and increasing your risk of over speeding the turbo, which leads to turbo failure and possible costly engine damage. Don’t just get the big turbo because it looks cool. As technology advances and research continues, engineers continue to innovate the industry by creating more power at higher efficiency from smaller turbos.
Wheel Horsepower Vs Crank Horsepower And Parasitic Loss
Horsepower at the crank is different than horsepower at the wheels due to parasitic loss. Also known as drivetrain loss, this refers to power lost by the engine from the time it travels through the transmission to the driveline, and through the axles to the wheels. Drivetrain loss is affected by transmission type (FWD, RWD, AWD) with the loss being higher in applications with automatic transmissions. You should estimate drivetrain loss before starting your project to avoid any confusion or disappointment when your car hits the dyno.
Finding the right Turbo
For this example we are going to start with a wheel horsepower target of 1000 for a rear wheel drive application. In order to find a turbo that can support our target power we need to calculate for the drivetrain loss so we must multiply 1000 * 1.15 = 1,150
* Front Wheel Drive 10% (multiply HP target by 1.1) Wheel Horsepower * 1.1 = Crank Horsepower
* Rear Wheel Drive 15% (multiply HP target by 1.15) Wheel Horsepower * 1.15 = Crank Horsepower
* All-Wheel Drive 20% (multiply HP target by 1.2) Wheel Horsepower * 1.2 = Crank Horsepower
Now that we have 1,150 as our crank horsepower target we can look to eliminate non-matches. Each turbocharger has an operating range measured by crank horsepower and engine displacement measured in liters. Once you have those two inputs defined it makes it easier to sort your options.
Matching For Vehicle Application
Another important factor for proper turbo matching is vehicle application. An autocross car requires rapid boost response so a smaller turbo or smaller turbine housing would be suitable for this application. This will sacrifice max power at high RPM but boost response will be excellent. A drag car that is seeking max power and top end speed will likely select a larger turbo and large turbine housing in order to maximize air flow going into the turbine stage. These are the very basic steps for turbo matching but important factors to consider when narrowing your search for the right turbo.
Matching a turbo for fuel type is also important. For example if a racer wants to make 500 whp on pump gas vs E85 I’d likely spec different turbos, oversizing the compressor to make the power on pump gas without high compressor outlet temps and to help reduce exhaust temps which helps with margin of safety to knock.
Matching For Single Turbo and Twin Turbo Applications
With target horsepower defined we need to figure out if you want a twin or single turbo setup. Both types can make the power efficiently and with great response. Sometimes engine space is a major factor and all you can do is fit two smaller turbos. Or maybe the manifold you have purchased only comes in a single turbo configuration. The decision is yours to analyze and make based mostly on budgets and availability of components. Here’s an additional calculation to consider for a twin turbo setup. If you have a 6 liter engine and you want to make 1150 crank horsepower with twin turbochargers you need to divide your target horsepower and engine displacement by 2 and find a match based on the sum of half of the target inputs. Our example of a 6 liter engine and 1150hp target power would now become a 3.0L and 575hp match.
Standard and Reverse Rotation Turbo Setups
Turbo companies offer standard and reverse rotation configurations. Sometimes referred to as mirrored or mirror image turbos, these options make it possible to create perfect symmetry to your turbo system. A reverse rotation compressor wheel spins in a counterclockwise rotation and features a left pointing compressor housing outlet. Remember, the compressor wheel rotation dictates the flow path of the air. Standard rotation turbochargers spin clockwise with a right pointing compressor outlet.
Narrowing The Search Results
Now we can narrow down our turbo search from one of the examples above. For this we decided on a twin turbo application. Our next step requires us to look at each turbo product page for different companies and find out what turbine kit options are available, and to review the horsepower and displacement range of each turbo. A basic rule of thumb is if your engine horsepower and displacement targets (575 / 3.0L) are too close to the low or high range of the turbo the match is more likely to have poor efficiency and is not the best option. A good match gives you the power and response you need in an efficient part of the map and that tends to be closer to the middle of the ranges. Remember, our requirements of the turbo are 575 horsepower for 3.0L because this is for a twin turbo application. Our total goal is to make 1,150 with a 6.0L engine.
These are the basic steps for turbo matching. You should now be able to identify and calculate your target crank horsepower target using the calculations for drivetrain loss. Once a single turbo or twin turbo configuration has been figured out you can calculate the proper inputs for your choice. Many companies have websites have pages that will match turbos to your specifications. We can also help you with the process of matching the right turbos and hitting the horsepower goals you have on the dyno. You also know what to look for when looking through each turbo page. This is just one tool that you can use to improve your knowledge of the turbocharger system. There are many more factors that need to be considered when selecting a turbocharger, but these are the basics to get you started in the right direction.