Supercharger Performance and Engine Performance Parts

Can I supercharge [this] car?

Is a question commonly asked on car forums, yahoo answers, and in the speed shops. Of course [this] stands for your specifically owned car, be it a work truck, a track dedicated racer, or a dual purpose vehicle that sees both daily use as well as occasional track time.

Any car can be supercharged …

… that is, if you know what you’re doing. Supercharging is by no means a new technology; it has been around since world-war II and is now a fairly straight forward process, so much so that you will find aftermarket companies supercharging the most ‘complex’ cars such as:

• The Gintani Supercharged BMW M3
• The Novitec Rosso Supercharged Maserati Gran Turismo S
• The PPI engineering Supercharged Audi R8

… All while maintaining, factory style warranties, drive-train longevity, and luxury car quality and refinement.

Truly, any car can be supercharged with the right ingredients and the right knowledgebase.

Today I’m going to give you the theory and an example of how to pull it off, using a Subaru Impreza 2.5 RS as an example from our friends over at r25.com.

There are three aspects involved in any custom supercharger application which are:

1. Supercharger Kit Design (or supercharger theory).
2. Supercharger Kit Fabrication (or the build).
3. Supercharger tuning (or the harmony).

The following can be considered your ‘to do’ list if you embark on this journey…

1-Supercharger Kit Design

Once you’ve decided to custom supercharge your vehicle and have set a certain target power goal in mind, the next step towards successfully modifying the car will be to design a complete and comprehensive upgrade path from naturally aspirated to supercharged.

This upgrade path will include (but is not limited to):

• The choice of supercharger head unit capable of producing the required flow and boost to reach your target power goal.
• The choice of the type supercharger (centrifugal, roots, or twin-screw) to match your desired driving characteristics.
• The effect the supercharger will have on intake air temps (depending on the supercharger adiabatic efficiency at the specific demand point on supercharger compressor map), and how that translates into the need for an intercooler system or water injection or an octane boost to reach the desired power goal.
• Designing the supercharger drive system with proper belt contact to the pulleys so they don’t slip and the right size pulley gearing to drive the supercharger.
• Modifying the fuel system with larger fuel injectors, elevated operating fuel rail pressure, and high capacity fuel supply including the pump, filter, lines and fuel rail.
• Modifying the oil delivery system to feed oil to the supercharger drive system as required.
• Last but not least, choosing the right auxiliary parts for air induction and exhaust systems t match your new power level.

I know this seems like an overwhelming amount of ‘research’ to do before you supercharge a car, and that is something I know first-hand from my first turbocharger build about 13 years ago.

However, once you go through the process once, you will realize that it’s really not that complex to perform and can even be calculated and automated using excel sheets, simple formulas or even software calculators.

2-Supercharger Kit Fabrication

So after crossing the first supercharger hurdle (which is indecision, paralysis of choice, and the overwhelming first-thought of doing the work required to ‘design’ the right kit); most people then get stuck in the fabrication of their intended kit.

The fabrication of a basic supercharger kit is going to require the following handy skills:

• Basic welding and brazing skills
• Mockup skills including making templates for supercharger and drive pulley mounting plates and support brackets.
• Knowledge of pulley system design including tensioning and idler pulleys as well as press fit pulley systems.
• Sizing and sourcing the required belt system to drive the supercharger.
• Basic hose and line skills for running fuel and oil lines safely, attaching leak proof fittings to the end of these lines, and tapping into the factory fuel and oil feeds and returns to plumb the supercharger into the car’s vital fluids
• Fittings knowledge such as Army Navy (A/N), National Pipe Thread (NPT), Metric …etc

3-Supercharger Tuning

If you have reached this stage, that means that you have picked the right parts and fit them to your car which is a great success. However, turning the sum of parts installed on the car from a collection of instruments into a symphonic harmony requires the right tune.

When the car is supercharged, a few things will change about the car requiring a new tune as follows:

1-Based on our new peak airflow and our target air to fuel ratio, and based on what modifications we have done to our fuel supply system, then we need to optimize our injector pulse width (or injector duty cycle IDC) to achieve our target air fuel ratio which is both safe and power efficient for the new setup.

2-Now that the mixture is more dense with more boost, more air mass, and more fuel mass and higher intake air temps, we know that the combustion even will create a much faster moving flame front, which means we need to create a custom ignition timing curve to match the characteristics of our new engine combination.

3-Finally, the changes we make to the car’s tune have to be done in a way that the tune stays the same no matter how ambient air temps, humidity, and elevation change. You don’t want to tune a car in the summer only to have it run lean and burn a piston in the winter, and you don’t want to tune a car in the winter only to have it run rich and lose power in the summer.

The tune has to be flexible enough in that it can automatically compensate for slight variations in temperature (using your intake air temp sensor), slight variations in flow or boost levels from summer to winter (using your MAF/MAS or MAP sensors), and slight variations in the air quality or air density such as changes of altitude or humidity (most likely compensated for using feedback from your on board oxygen sensors and your short and long term fuel trims).

1.You also want to check and make sure that your tune isn’t load dependant and that your EGT’s are stable weather you are doing a fast pull in a short gear like 1st gear, or you are doing a slow pull in a long gear (like 4th gear) with a fully loaded truck cab.

If you are reading this far… CONGRATULATIONS…

Because you now know pretty much every base you have to cover to supercharge your own car. What is now left to do is to actually go out and implement this knowledgebase.

I know this seems like too much to work on; however, it IS systematic which is great for two reasons:

It can be applied to any car systematically. The challenges will be different on each platform, but the goal and the methodology is the same.
Once you’ve done it once, it can only get easier on your second try.
It really isn’t that hard to do; in fact, it is more lengthy a process than a hard one and the reason most people don’t make it all the way through is not because any one part of it was impossible, but rather because they just got tired before reaching the end of it.

The proof is in the pudding!

To prove to you that this isn’t a process reserved for people that work in world class speed shops such as Brabus or HKS or Magnuson … I bring to you today a true ‘shade tree mechanic” example of a guy named Shane.

Shane is a passionate Subaru owner that uses his 2000 Subaru Impreza RS both as a daily driver and as a weekend autocross car.

The car comes stock with a 2.5 liter SOHC boxer 4 cylinder engine delivering 165hp @5600 rpms through Subaru’s world class all wheel drive system.

The combination of the low weight of the RS, with the stout Impreza chassis, the traction of the intelligent all-wheel-drive system, and the grunt of the SOHC B4 make the car a great platform for the demands and complexities of the autox circuit.

1-Supercharger Kit Design

Supercharger head unit and type of supercharger:

With a target power goal of 220 horsepower, and the known desire for added low end torque, Shane chose to use an eaton M62 supercharger for this build up. The M62 is capable of 300hp @ 10psi depending on the generation of the supercharger, and so meeting our targets of 220hp @ 5psi is within its operating parameters.

Another advantage of using an Eaton supercharger is that the eaton head unit is a ’sealed’ unit using its own oil reserve separate from the engine’s oil. This means that there is no need to tap into the the engine’s oil feed and return systems simplifying the installation process and preventing any future headache with leaks.

Affect on inlet air temps

Having an efficiency between 55 and 60% for the Eaton MP45, the outlet temperatures for the air exiting the supercharger will be about 84 degrees higher than ambient.

Due to this temperature increase, and in order to produce repeatable power figures run after run without heat soaking the engine, an air to air intercooler was chosen to chill the inlet air charge back down to normal temps. Rather than using a typical WRX intercooler, Shane chose to use a Subar Forester XT intercooler which has a smaller profile and provides more clearance between the throttle body and the firewall. This choice of intercooler is beneficial for two reasons:

1- The top mount intercooler provides a very short air path between the supercharger and the throttle body compared to a front mounted intercooler in the bumper. This short air path should result in better throttle response and torque delivery which is important for a daily drive car and an autox setup.

2- The narrow profile intercooler is a ‘drop in’ solution which doesn’t require trimming and fitting of the larger WRX unit and simplifies the install.

Supercharger Drive System

The supercharger drive system is a simple take off of the original accessory belt on the engine. The factory belt is a 5-rib unit which is good enough for the target boost of 5psi (pressure ratio of 1.33). By looking at the M62 compressor map, we find that to supercharger requires 10,300 rpms to produce 220hp worth of air @ 5psi of boost.

With a stock crank pulley having a diameter of 5.25” and a motor with a 6500rpm redline thanks to the iSpeed reflash, then Shane required a supercharger pulley sized to overdrive the supercharger inlet shaft to 10,300 rpms when the motor is at 6500 rpms.

Plugging all these numbers in we get a 3.3” diameter supercharger pulley which is very close to Shane’s original attempt of 3.25”. However these are idealized calculations assuming no pressure loss due to the intercooler system, and with a thin profile top mount intercooler having a significant pressure loss of 2 to 3psi to the incoming air, the actual supercharger snout pulley had to be downsized down to 2.75” to achieve the desired boost level.

For optimum belt to pulley contact, a custom idler pulley and mounting bracket were fabricated forcing the belt to wrap almost entirely around the supercharger snout pulley giving a healthy 200 degrees of belt contact on the pulley.

The belt for this system is a custom length 6-rib belt with 1-rib shaved down using an exacto knife which is a common ‘hot rodder’ trick for making custom belts.

Fuel Delivery

Since the horsepower increase on this setup is a modest 35% increase, the stock fuel injectors are capable of delivering the required increase in fuel supply , albeit at an elevated fuel rail pressure.

Fuel flow is related to the square root of fuel pressure and so a 35% increase in fuel flow requires a 82% increase in fuel rail pressure bringing the target rail pressure up to around 72psi.

In this specific buildup Shane employed a SX adjustable fuel pressure regulator set to 80psi (and resulting in a rich 10:1 AFR at WOT) with an upgraded STI fuel pump delivering 145lph at the newly elevated rail pressure. Shane also performed a parallel fuel rail modification to ensure equal flow and reduced pressure drop at the two cylinder banks. This is a common modification of V and Boxter configured engines to reduce the overall length of the fuel rail (creating two shorter parallel rails rather than 1 longer series rail) to reduce the pressure drop (and flow variation) between the first and last cylinder in the fuel path.

Auxiliary Buildup

To improve the overall volumetric efficiency of the engine, to produce more horsepower at a lower boost level, and too keep up with the intake and exhaust demands of the power increase, Shane has included into the build a custom 3” cold side intake with a fender mounted K&N filter, feeding into the M62 and out into 2” hot side piping feeding into the Subaru Forster XT top mount intercooler.

The air fuel mixture spins the engine to a new peak redline of 6500 rpms thanks to the iSpeed SR10 reflash and Delta 2000 cams with 226* duration and 8.5mm lift @.050”.

Then the exhaust gas exits through OBX equal length headers with fat 1.65” primaries feeding into a 2.25” collector and out a 2.5” OBX catback exhaust system.

The setup is tuned using an e-manage ultimate which can control both ignition timing as well as fuel injection duty cycle and the tune is monitored using a Dakota Digital EGT gauage, a Stewart Warner -30inHg-15psi Vac/Boost gauge and an Innovate LC-1 Wideband AF Controller displaying data through a ProSport Digital Wideband Gauge.

With a conservative 10:1 AFR and a modest 5psi of boost the car is still a blast to drive and a formidable contender at the autox circuit.

Since this is probably one of the lengthiest blog posts I’ve written in a while, I will cover the fabrication portion of this build through the following gallery (a picture is worth 1000 words).

Related posts:

1. Supercharged Performance for OEM Hot Rods
2. Engine Performance Parts for supercharged performance
3. Nitrous to boost your supercharged car
4. Special Feature: Gintani Supercharged E92 M3
5. A Supercharged LS7 crate engine