Given the fact that you're reading this article, you probably already know that ROUSH is well-known for its Ford F-150 and Mustang supercharger offerings. What you may not know is what type of supercharger design ROUSH employs on these systems and why we have specifically chosen to do so. This brief tech article hopes to shed some light on this topic.
At ROUSH Performance, our line-up of supercharger kits utilizes a specific type of belt-driven, positive displacement, Roots-style superchargers known in the industry as TVS (Twin Vortices Series). Wow... that's a mouthful! You may be wondering, "What does this mean?". Let's take a step back and discuss where this design fits into the larger mix of supercharger types and then discuss why ROUSH has chosen this specific system/technology for its Mustang and F-150 applications.
Primary Supercharger Types: Positive Displacement vs. Dynamic Compressors
There are two main classifications of superchargers: Positive Displacement and Dynamic Compressors. Positive Displacement superchargers work by trapping air and forcing (displacing) it into the engine. The effect of this process is a consistent intake pressure over the RPM range. Dynamic Compressors, on the other hand, force air into the engine by accelerating the air to a high speed and then turning that energy into pressure by diffusing it or slowing it back down. The result of the Dynamic process is that pressure increases over the RPM curve. Put another way, a Dynamic Supercharger needs to "spool-up" to create peak power, making the power less available at lower RPM compared to a similar Positive Displacement system. So, such a system might have a high peak horsepower number, but you won't realize these power (and torque) gains until far end of the RPM curve.
Positive Displacement Superchargers
There are three relatively common Positive Displacement superchargers used for automotive applications. However, only Roots and screw technology have been used in recent years in OEM applications:
- Roots superchargers work by pumping air with a pair of meshing lobes (similar to a set of gears). The air is trapped in pockets around the lobes that shift along the lobes as they rotate. This type is frequently used for engine applications where the system is driven directly from the engine's crankshaft. This system exhibits excellent volumetric efficiency at very low engine speeds. Simply put, it is especially effective at producing boost "right out of the hole" or "off the line".
- Lysholm Twin-screw (a.k.a. Screw) superchargers work by pushing air through a pair of interlocking rotors as the assembly rotates. The rotors on this system literally look like screws. Though very similar to Roots-style superchargers, Screw superchargers have asymmetrical rotors (one male and one female -- and the two in a pair may differ from one another in how many lobes they have), where Roots-style systems have symmetrical rotors (they have the same shape and number of lobes). It is the asymmetrical rotor design that allows a screw compressor to build internal compression. This internal compression can be both an advantage (maintains its efficiency at very high boost pressures) or a disadvantage (additional work and heat created at lower boost pressures) depending on the application. For most high performance street applications, internal compression is not required.
- Scroll-Type (a.k.a. G-Lader) superchargers' moving parts consist of a disc-shaped displacer with matching spirals on each side. A pair of fixed scrolls attached to the end walls of the compressor chamber mesh with the moving scrolls. Instead of rotating, the displacer orbits to create a pumping action between the pairs of scrolls. Air enters the system when the moving scrolls are at their furthest from the fixed scrolls, when the gap is the largest in size. As the system rotates the gap closes, compressing the air more and more until it ultimately reaches the center of the housing. At this point the air is then released into the engine. A key disadvantage to this system is its short life -- many of the systems in existence need to be rebuilt about every 30k miles.
Though there are a number of types of Dynamic Compressors (including Centrifugal, Multi-Stage Axial Flow, and Pressure Wave systems), the Centrifugal supercharger design is the only one that is commonly used in automotive applications:
- Centrifugal superchargers use centripetal force to push air into the engine. This supercharger design is typically driven by the engine crankshaft (either by belt or gear). Turbochargers are technically a type of centrifugal supercharger that is driven by a turbine (hot exhaust gas) instead of mechanical belt or gear. The moving part of this system is an impeller (small rotating wheel) that draws air into a small housing and ultimately into a diffuser, producing high pressure and relatively slow moving air that is released into the engine.
ROUSH F-150 And Mustang Superchargers
Key Roots-Type Design Characteristcs
As stated earlier, ROUSH uses the Roots-type supercharger design. Below are some of the key factors that went into this design choice:
- Roots-type superchargers are able to provide peak boost at low RPM, making for quick availability of power on demand. In other words, the system produces aggressive launches off the line.
- The Roots concept is relatively simple, making it a more reliable option than many other forced induction options.
Key Disadvantages Of Other Design Types
Looking at the other supercharger types, here is a condensed list of disadvantages to the systems that were not chosen. Though, these designs may have their place for a given application, the engineers at ROUSH found these to be problematic for supercharging Ford Mustangs and F-150s:
- Screw Superchargers: Due to the fact that this type has internal compression (the amount of which is varied based on rotor design and inlet and outlet port geometry), these require additional "work" to rotate, especially in "non-boosted" conditions (ie. closed throttle - at idle; and part throttle - steady state cruise or low pedal accelerations). This additional "work", or mechanical energy robs power from the engine and also creates unwanted heat in the engine, especially within the intake manifold and intercooler system.
- Scroll Superchargers: The high complexity of this design of this system makes for short unit life and service issues.
- Centrifugal Superchargers: This system has a low availability of power at low RPMs, making for sluggish starts.
EATON TVS Features
ROUSH's supercharger line-up is more than just another collection of Roots-type blowers. The internals of the current ROUSH supercharger line-up feature EATON's advanced TVS 2300 technology. Here are some of the key features of this technology:
- The unique 160-degre-twist four-lobe rotors and high flow inlet and outlet port design provides enhanced thermal efficiency, higher volumetric capacity, and higher operating speeds. This design provides its enhanced air handling characteristics without increasing the overall size of the unit.
- The system is capable of running with a high thermal efficiency (up to 76%) across a very wide operating range, which enables more compact packaging and greater output.
- EATON TVS superchargers are capable of a 2.4 pressure ratio.
More Than A Summation Of Their Parts
Of course, there is much more that goes into a ROUSH supercharger than just the list of parts that you can see and touch. In addition to the components and design type, we also hold the following as critical to our F-150 and Mustang supercharger offerings:
- The team of people developing the product (engineering)
- Industry-leading warranty and support
- Unparalleled calibration, making for superior drivability and reliability
- Design of the powertrain in context of the larger vehicle system
- Fit and finish (appearance)
- Ease of installation
- Highly-refined assembly process and post-assembly testing to ensure quality
However, we'll have to dive into these topics in future articles.