Selecting a supercharger: Belt driven power
March 31, 2019(This post contains affiliate links. Check out the affiliate disclosure here)
Selecting a supercharger that fits your needs is key to making good power. What is the first thing that comes to your mind when I say supercharger? Big things sticking out of the hood of an old Dodge Challenger or maybe even a big old train diesel engine. Right here you will learn what types of superchargers there are, how they work and what to choose. Stay tuned.
In this article: Types of superchargers / Which is best / Keep it cool / What to choose / Thanks to
If you have no idea how supercharging works, let’s do a quick recap so you can get the most out of the rest of this article. Superchargers has been around since the 1800s and it was initially created to help ventilate mines.
The way it increases power is to force air into the engine using lobes, screws or impellers. This is what we call boost pressure. To compensate for the added oxygen we need to add more fuel, add a spark and the result is a bigger explosion sending the piston down the bore creating more horsepower.
Back in the days when supercharging became popular in the hotrod community they used to take superchargers from 2-stroke Detroit diesel engines and shoehorn them into their cars. Thus giving the classic out of the hood *blower look. One thing that is similar between all superchargers is that they are all driven by the crankshaft via belt or in some cases chain.
*Superchargers sometimes goes under the name blower.
Types of superchargers
Before selecting a supercharger for your car you need to know what you have to choose from. Lets break it them down into three categories just to simplify things. There are: Roots, Twin-screw and Centrifugal superchargers.
ROOTS SUPERCHARGER
Roots style superchargers is the grandfather at the table, it has been around since the 1800s. It is still used today and is a popular choice by hot rod and muscle car guys/gals. This gives the classic sticking out of the hood look that is so iconic.
Roots design can vary between 2 – 4 lobes depending on the manufacturer. The way it works is that when the lobes spins, air is sucked in and trapped between the lobe and the wall. Then it’s pushed out and pressure builds up in the intake. Roots move a very large amount of air when it spins and it sends it out in bursts rather that a smooth flow.
TWIN-SCREW SUPERCHARGER
Now we are moving into the modern day superchargers. This is the one that you will find on super cars like the Dodge Demon or Corvette. The twin-screw design makes is compact compared to the roots and will therefore not stick out of the hood.
The way it works is that air is drawn into the intake end of the rotors, squeezed between the rotors as it moves towards the outlet and out through the exit. Did you notice that I said it squeezes the air compared to the Roots that just moves it. Twin-screw superchargers achieves this because the air is screwed/forced to the exit. This is why it’s called a positive displacement supercharger, since it compresses the air inside itself before it reaches the manifold.
This in term increases its efficiency but with that also manufacturing costs. Twin-screw superchargers is made with complex shaped screws that requires precision machining to meet the high tolerances.
CENTRIFUGAL SUPERCHARGER
Here we have the latest thing when it comes to supercharging. This has very little similarities to the above, it is instead related to the turbo. In fact it’s a belt driven turbo.
It uses centrifugal energy to force air into the cylinders. A centrifugal supercharger uses a *high speed spinning impeller to draw the air into the compressor housing. The air then travels through the diffuser where it’s converted from low-pressure high-speed to low-speed high-pressure. The air is then fed to the engine via an intercooler (recommended).
It reaches its high spinning speeds thanks to a gearbox located behind the compressor part. This gearbox is the connected to a pulley and via a belt to the crankshaft. The ratio between these two pulleys decides the boost pressure.
*It spins at speed in excess of 30 000 rpm.
Which is best
This is the hard question, why should you choose a roots over a centrifugal or vice versa when selecting a supercharger. This all has to do with what you want out of your engine. Like everything they all have their pros and cons. So let’s take a look at them.
We start strong with the basics. Compressing air still means that it will get hot and hot air is not good for horsepower unfortunately. The efficiency of the supercharger will affect how hot air it will produce, high-efficiency equals cooler air. The winner here is the centrifugal supercharger since it has superior efficiency compared to the other two. At high boost levels is where it really shines as the winner.
The Twin-screw design is still a massive improvement in efficiency compared to the roots blower, but the both fall short when it comes to high boost pressures.
On the other hand both the root and twin-screw take a victory when it comes to boost lag. With the roots and twin-screw you have an instant buildup of boost from low rpm compared to the centrifugal that has to build up the pressure giving it a bit of lag.
Keep it cool
When we are working with compressed hot air we need a way to cool things down. If you use a centrifugal style supercharger it’s easy, just use a front mount intercooler or a water to air intercooler. All you need to do is decide whether you want water or air to cool your charge air.
However with a twin-screw or roots it’s a bit trickier, usually the cooling for this type will be of a liquid type placed in the intake manifold itself. Worth mentioning is that engines with roots-blower usually don’t use intercoolers at all, they rely on the fuel from the carbs/efi to cool the air. One other way to cool the air would be to use water/methanol injection, one nozzle placed before the supercharger spraying into it. This will cool both the charge air and the lobes/screws.
What to choose
What you want under you right foot is the key when selecting a supercharger. A centrifugal supercharger will give you a high boost level and high rpm boost. But since it relies on engine rpm to produce it you won’t see much low rpm boost. In comparison to a root/twin-screw that will deliver good boost from low rpm and keep on delivering a linear boost curve all the way to the rev limiter. This will create the feeling of having a lager displacement engine.
There are some important things to know when selecting a supercharger.
- The pulley ratio will decide the boost pressure. Always check to see if there is different crank pulleys available to your engine. The manufacturer usually specifies a min/max diameter of the supercharger pulley.
- Displacement of the supercharger is important. Choosing a 10 liter capacity supercharger for a 2 liter engine won’t work so good.
Positive displacement superchargers are rated by their capacity per revolution. A 2.4 liter supercharger would deliver 2.4 liters of air every revolution
Roots uses the GMC pattern, they are rated after how many 2-stroke cylinders there are and the capacity of them. A 6-71 would mean 6 cylinder each with a volume of 71 cc.
Centrifugal superchargers uses both compressor map like a turbo and max rpm to calculate size.
Now all you need to do when selecting a supercharger is to decide if you want a brutal low-rpm torque monster that eats tires for breakfast or if you want an engine that just keeps pulling harder and harder the higher into the rpm range you get.
