The form is based on traditional engine calculations and so to be useful needs to be fed with realistic data. You can easily juggle the input values to give whatever output you want. For example: Reducing engine VE will increase boost, just the same as increasing blower efficiency will. But that will not help you if you are trying to work out what size pulley you need for your build.

The notes below along with the efficiency suggestions in the drop-down menus should give you a realistic output based on our experience. 

The AMR500 is a relatively modern blower but is based on an old design so it falls somewhere between 70-80% efficiency.

By comparison most Eatons quote a volumetric efficiency of around 80% - that's for a four lobe high helix scroll rotor design. AMR500's only have two lobe straight rotors.

The efficiency of a Rootes blower also drops off at higher pressures and higher temperatures. So the closer you are operating to the adiabatic limit of the blower, the more that the efficiency drops off. Generally this limit is approximately at a 2:1 pressure ratio or 1 bar (14.7psi).

So if you are planning to run at higher boost levels, reduce the efficiency accordingly. 

Normally if you are moving towards this limit, you should really be considering a larger blower.

Aircooled engines are not exactly the most efficient. For a stock 1600cc engine you can expect somewhere around 70% volumetric efficiency. Older engines are less efficient due to smaller valves and ports. The VE guide values shown are typical list values for stock engines.

As you increase capacity, you also typically increase the VE as a result of other sympathetic modifications, for example valves, cam, compression ratio and exhaust improvements. These improvements generate a relative increase in volumetric efficiency. Typical figures for mild states of tune for increased capacities are noted in the drop down as a guide. They may be more or less than this for YOUR engine.

If you are trying to calculate pulley ratios and blower pulley dimensions, it is important that you keep an eye on the Max Blower Speed. For an AMR500 you do not want to exceed 16k rpms at max engine rpms. 

You also need to consider that adding boost will also increase your maximum engine rpms. So for a stock 1600 engine with a redline of 4-4500rpm you will see an increase of between 500-1000rpm, taking your max rpms up to 5500. This means that you need to use this figure in your calculations. 

So working backwards the largest drive ratio you can use in this case is 16000/5500 = 2.9

If you are using a performance camshaft, your max rpms may be even higher.

Engines are essentially air pumps. The engines efficiency is a measure of it's ability to move air.  The higher the volumetric efficiency, the more air it can move / consume per engine revolution. 

Boost pressure is created when the blower air flow exceeds the engines air consumption. The excess air creates pressure at the engine inlet (boost). The amount of boost is a relative to the ratio between the engines consumption and the blowers output.

So, if the engine efficiency increases, the engines air consumption increases, and if the blower is still moving the same amount of air, the net result will be that the boost level drops. 

If you live in a region at high altitude or want to use this calculator for a specific event or elevation (maybe Bonneville which has an elevation of 1200metres and a typical barometric pressure of 12.5psi) you can set this value to your local Barometric pressure.

Typically at sea level the pressure is 14.7 psi - this is also the standard for 1 bar / 1 ATM

If you are unsure, then just leave it set to 14.7 as this is also considered standardised pressure (1 atm)