Supercharger pulley size calculation

Hi I’m struggling to find a consistent formula to guess my boost levels with a roots charger

its an m45 which I believe is 737cc per revolution - my engine is 1348cc

My crankshaft pulley is 87mm and the charger is 65mm - a 1.34 ratio

I’ve taken it for a spin today - mainly tuning with the bypass locked open - and then briefly to see what boost with it functional - looks as though was around 0.1bar on the boost gauge (I didn’t run a log)

id like to be able to run around 0.5bar boost for now (with a bit scope to increase later) I do have a FM intercooler

I can get a 17% smaller pulley (53mm) for the charger no problem - what size crank pulley should I make ?100mm

Thanks for your help!

Rob

There will be a fair bit of "suck it and see", rather than hard calculations, so my 5c worth of thoughts and comments - and a lot of fudging!

Even them, with so many factors, it's going to be inaccurate, to put it mildly.

There are going to be pumping, and other losses, so it may be easier to look at what you have, using absolute pressure clarify things.

The engine has 1348 cc (Nissan Micra?), so one revolution would be expected to use 674cc of air at 100% efficiency.

The supercharger has a nominal swept volume of 737cc per revolution, and overdriven 1.34 has a nomimal swept volume of 995cc.

With a nominal 0.1 bar of boost, 1.5PSI, that would suggest the engine is using ((14.7+1.5)/14.7) x 674 = 743cc.

So, fudge-factoring in the values, if 995cc at the s/c gives 743cc, and you want ((14.7 + 7.35)/14.7) x 674cc, or 1011cc, in the engine, it suggests the increase of the S/C would be around...

1011/743, or 1.37 times what you have, so 1.84 times engine speed?

With a 53mm driven pulley, that means something like a 97.5mm driving pulley - very close to your 100mm.

That said, I would have expected a bit more boost than that, with your current set-up, and it might be worth going back over the installation looking for choke points that would cause the efficiency to drop - not forgetting checking the actual throttle is opening fully, and the filter isn't restrictive.

Oh, and there's also a good chance the pressure gauge isn't accurate.