The low down on Super Chargers

[TurboGem]

Hi Guys,
I hope that you don't mind me starting another thread so that people don't have to wade through the other post to get this information.

Before I start I would like to qulaify what I write with this, I have gained this information from experience as well as from my job as a Mechanical Engineer. I am not saying that everything that I write is gospel but it should be pretty right.

OK. We are talking about positive displacement superchargers.

These superchargers work by displacing a set amount of air on each revolution, hence the name. What this means is that if you have the supercharger from the 2lt toyota engine is designated the SC14 which means that for each revolution of the engine it displaces 1.4l of air. With me so far???

The next thing that you have to remember is that it takes 2 full revolutions of the crank of a 4 stroke engine to cycle through all the cylinders. For example if you have a 2lt engine then you are one displacing 1lt of air each revolution. Still keeping up….good

So if we go back to what Gene was eluding to with the gearing of the pulleys to set the speed of the blower we can alter how much boost is made.

BY CHANGING THE RATIO OF THE PULLIES YOU DO NOT ALTER THE WAY THAT THE BOOST IS DELIVERED ONLY HOW MUCH

To calculate what ratio you need to get a set boost level you need to look at the displacment of the engine and the supercharger.

Example: Using the figures above of a 2l engine fitted with the SC14 blower and a ratio of 1:1

For every revolution of the engine we have 1.4lt of air being pushed into 1lt of engine space. As you can see allready we have a boost situation. To work out how much boost though you need to use this formula

(SC Vol – Eng Vol)
-------------------- X 14.7 = Boost level absolute
Eng Vol

(1.4 – 1)
-------- X 14.7 = 5.88 psi
1

If we want to make more boost then all we have to do is spin the supercharger faster

Example: If we spin the supercharger at twice the speed of the engine then the displacement of the supercharger is now 2.8lt so we put that into the formula above

(SC Vol – Eng Vol)
-------------------- X 14.7 = Boost level absolute
Eng Vol

(2.8 – 1)
-------- X 14.7 = 26.46 psi
1

You have to be aware though that the boost is limited to how fast you can spin the supercharger in relation to the engine the max speed for these superchargers is about 12,000rpm so for example if you have a redline of 6,000rpm then you can make you ration 2:1 for the pulley.

The next consideration is how to work out how fast to spine the supercharger to get a set boost level. All this is is just a variation of the above equation.

(Boost
------- X Eng Vol ) + Eng Vol = SC Vol
(14.7

SC Vol Total
Then to get the drive ratio= -----------------------------
SC Vol per rotation

Example using the above engine and supercharger we want 10 psi

(10
------- X 1) +1 = 1.68
(14.7

Drive ratio = 1.68
------
1.4
This give us a drive ratio of 1.2:1 to give us 10psi on a 2lt engine

I hope that this stuff has been on some help to everyone. The last point to note is that if at all possible run an intercooler because as the supercharger pumps the air it heats it up quite a lot especially when you are get up near 10,000rpm. If you inject a fine stream of water into the supercharger you will get another couple of pounds boost from the added sealing that the water gives to the supercharger lobes.


Cheers
Paul Nelson