Supercharger pulley ratios

Been doing a bit of work on the supercharger project and decided that I needed to work everything our properly.

The pulley that I have fitted to the supercharger is one that I managed to procure to fit the charger, however, it is normally used in a much different application. My initial napkin calculations ended up at a figure of approx 6psi at max rpm. Tonight I decided to start from scratch and work everything out properly.

To start with I have made a few assumptions:

Engine Displacement - 1600cc

Supercharger Displacement - 500cc

Maximum Engine speed - 4300rpm (peak power figure from VW for stock 1600)

Stock crank pulley diameter - 150mm

Supercharger pulley diameter  - 40mm

Pulley ratio

Crank pulley diameter / Supercharger pulley diameter

150/40 = 3.75

Max supercharger RPM

Max engine speed x Pulley ratio

4300 x 3.75 = 16125 rpm

This is approximately the maximum for the AMR500 charger (16000rpm)

Supercharger output

Supercharger displacement x Pulley ratio

500 x 3.75 = 1875cc per rev

Boost

Supercharger output - engine consumption (1/2 of total capacity)

1875 - 800 = 1075cc

1075 / 800 = 1.34 bar (19.4 psi)

From the above calcs it looks like the supercharger might need to be slowed down a little. The calcs above assume 100% volumetric efficiency, which is very unlikely (impossible), but even given 75% VE (a fairly realistic figure) the result still comes out at about 0.75 bar which equates to about 10psi. 10psi is a little higher then I would like to run but not so high that it would result in an instant engine melt down.

At present I will leave the 40mm pulley in place and get everything set up. Changing out the pulley after the setup is installed in the bus is pretty simple and only involves swapping the pulley and fitting a longer belt. I will wait until I get everything set up and running so that I can take some actual boost readings. Once I have these baseline figures I can then make a call as to how big the pulley needs to be.

It might also be interesting to get a dyno reading at the higher boost pressure, it will obviously be a good sales pitch for the kit but I would not like to guess how long the engine would last running at 10psi.

Dellorto DHLA 40mm

The flow calcs also give me a better overall idea of the carburation requirements. The configuration is draw through which means that there is 1875cc's of intake for every revolution at a maximum engine speed of 4300rpm. We therefore need to flow 8062 litres per minute through the carburettor.

The 40mm Dellorto DHLA carb that I have planned for this conversion has a quoted flow rate of 266.4 cfm per venturi (36mm) which equates to 15087 litres. This is in excess of the required 8062 litres a minute calculated above. The effect that this would have is that with the small air consumption the air speed through the venturi will drop a little resulting in inefficient operation of the venturi. The solution is to reduce the size of the venturi's to increase the airspeed through the carburettor and improve the venturi effect.

I will try to source some Dellorto flow maps to see what the ideal venturi size is.

NOTE: the previous information I posted below was based on Dellorto flow rate of 266.4cfm overall, which I now believe to be incorrect!

The 40mm Dellorto DHLA carb that I have planned for this conversion has a quoted flow rate of 266.4 cfm which equates to7543.61 litres / min - not too far off of the required 8062 litres of air per minute calculated above. The effect that this difference would have is that the top end power would be restricted a little by the carburettor. To find out exactly how this would affect the engine we can work backwards using the new flow figure.

Given 1875cc's of intake charge per revolution:

Maximum flow rate / Supercharger output per revolution

7543.61 / 1875 = 4023rpm.

As can be seen, the effect of the carburettor restricting the flow rate means that the power will potentially start to drop off at 4000rpm. Of course this is all theoretical, and my logic is likely flawed so the real proof will be in the pudding (to coin a phrase).

All in all this means that I can probably work out baseline jetting (more on that later) and will not have to worry too much about my choices for carburation and supercharger ratios. The setup should run okay and if I can get my head around the jetting should only need a little tweaking when on the dyno.