I have yet to see this in writing, having reviewed my Vortech manual and looking around in this forum. Is there a mathematical correlation between impeller speed and boost psi increase? Example and guestimation; I have roughly a 6"crank pully and 3" supercharger pully spinning it at 31K rpms. Lets say going to a 2.5" supercharger pully jumps it to say 42K rpm....since it is providing me 5psi currently, will that change equate to a certain psi change?

I want to go from 5 to 6-7psi. How do I determine what pully changes and impeller speed will get me there? Are there other things in play? Exhaust size? fuel flow? Other? I realize that for my model and application limits me to 45K rpm max efficiency and 50k max rpm. Just looking for the math involved.

 

I'm not aware of a complex equation that could answer your question, but sometimes people phrase a certain size pulley to a certain psi. I can tell you the 2.5" pulley equates to 49680rpm @ 6k redline (as that's what i'm currently using). Actual boost can change based on DA, temperature, cubic inches, flow of the heads, etc. When I changed from 2.65 to 2.5, I noticed about 1.5 psi jump if that helps any. Depending on weather, i've seen as low as 7psi (really hot day), and up to 11psi on a brisk winter night.. i'm at 5200ft elevation too.

This calculator is helpful in determining exact rpm. Impeller Speed Calculator | Vortech Superchargers

 

For that small of an increase I'd try a 2.7 or 2.8 blower pulley. Increasing the impeller speed by 11k rpm will likely net you more than 1-2 psi as it's efficiency increases as well, depending on the head/cam combo

 

Boost is partly the restriction of your engine/intake, atm pressure etc. So it is complex to calculate what pulley is going to make 6 pounds of boost with a particular impeller. The reason this sometimes works and you hear about it so much is some guys changing pulleys on stock cars or stock blown cars where they are all pretty much the same systems, then it is a little more predictable.

But from my experience, changing one size smaller blower pulley will usually yield at least a pound of boost, maybe more. So I wouldn't over complicate it too much trying to calculate it and if you go from 3 to 2.5 you could get several pounds of boost over what you have now.

You can look at the compressor maps here and see what the rpms do vs air pressure for a given air flow <%=page_title%> | Vortech Superchargers

 

Your inputs seem to compute with what the local shop is saying. We are dropping to a 2.7 something size pully. He said that should yield the 6ish lbs Im looking for. I will let you know how it turns out to include the dyno results. Your point that nothing is simple math when dealing with modded engines will probably continue with any other mods down the road. But that is part of the process. Thank you both.