Hi Guys
Ive been trying to ponder a simple calculation for determining the starting point for the size of a blower pully.

Can any one tell me if based on 100% VE, does the following look close?

1) Say 8 PSI = 14.7 + 8 = 22.7 / 14.7 = 1.54 (MF - magic figure).

2) Im thinking: MF x 50% of engine cc / blower volume = pully ratio

3) ie for a 1955cc engine at 8 PSI (with 100% VE) using a SC12
1.54 x 977 / 1200 = 1.245x


As a side point say at 6000 revs x pully ratio 1.245 = 7,470 revs ie so is with in blower capacity for revs.


Or am I just totally wrong.

Cheers

Newt

You forgot to take away the number you first thought of.

To elaborate.

If the blower worked at say 80% VE then

For a 1955cc engine at 8 PSI (with 80% VE) using a SC12
1.54 x 977 / 1200 x .8 = 1.567x

So I guess you can then ponder what the VE is likely to be.

But then again - I am only pondering a starting point.


Newt

Quote:

You forgot to take away the number you first thought of.

I can get you some measurements off my pulleys if you like, that was good for 10psi, the crank pulley was 135mm but i fitted a smaller pulley on the blower too.

The stock sc12 pulley was good for 6psi with that crank pulley.

As Joel says you have to know the pully diameters

Thanks Joel, that would be brilliant.

If your 1955cc engine was 100% efficient, it would use 0.9775 lt per revolution right?

So in a perfect world, a blower that pumps 0.9775 per rev,
would create zero gauge boost, (or 14.7 psi absolute).

To create 14.7 psi on the gauge you would need an additional 0.9775 lt per rev.

Problem is, both the engine an blower change in, efficiency,
both volumetric,
and adiabatic.

This means that as far as a gauge number,
if you chase a 10psi gauge number based on maths,
it will end up being quite a bit higher.

My maths gets the same drive ratio that you have calculated,
but you will likely have much more than 8psi in reality,
even just due to temp rise of the pressurised air.

As an example,
my old twin blown 2965cc engine , based on a pulley calculation to get 30psi,
actually had 42psi!

With better flowing heads, and even bigger camshaft duration,
it still had over 37psi.

You can get deeper into the calculated numbers with both adiabatic, and volumetric efficiencies, if you want.

Eventually I got some formulae from a book called "Supercharged", by Corky Bell.

Truth is, your numbers will get you close, although a little high in boost,
and then you can choose something between those pulley sizes,
and the ones that Joel has found to work for him,
to come up with something that suits your engine.