I just got a digital non contact tach to verify the rpm of my supercharger..it cost $25 from this joint.
see the chart below to see the math that I have come up with to stay below the mfg max rpm for the blower. I am not sure if one can calculate boost level linearly or not, I suspect not thus I have not done it. If you have thoughts on this please let me know.
In the chart you can see the blue cell, which is my current state of 12psi at 7500rpm which currently yeilds 212rwhp with a very rich (read SAFE) tune. I am going to change to the 2.8" pulley, which I already have, next year. I just want to enjoy what I have right now
I believe you can very well calculate boost as a linear function for Roots/Twin Screw unit like the Cosworth. The blower will pump the same amount of air into the engine for each rotation inside the supercharger. An easy way of understanding it that I read once was at X engine rpm's the supercharger is pumping Y air into the engine. At 2X engine rpm the blower is pumping 2Y air into the engine. This is why you see a constant boost level throughout the rev range. However, a centrifugal supercharger uses step-up gearing and therefore creates boost pressure non-linearly with respect to RPM.
ReplyDeleteConsider the driving pulley (crank pulley?) to generate the input rpm in the system and the driven pulley (on supercharger head-unit) to notice the output rpm. A change in the supercharger pulley will result in a new output rpm. If you have at least two points with boost values at the same rpm, say 7500, you should be able to interpolate a linear function for boost at that rpm with any given driven pulley size. For that matter, since boost levels don't change through the rpm range you should be able to compare boost levels at different rpm's to find a linear function as long as no other variables were changed.
Yes however there is a Volumetric Efficiency factor that must be considered, no?
ReplyDeletesee the blower chart here
http://thebluemeany.blogspot.com/2010/03/more-boost.html
adam
Yes factors like volumetric efficiency will play a role but I think a linear estimation for boost will still get you close to real world boost levels, within +/- 10% I imagine. The volumetric efficiency curve is close enough to be considered linear without drastically effecting resulting calculations at any given rpm.
ReplyDeleteVolumetric efficiency effects the CFM input into the engine and boost is basically a result of CFM flowing from the supercharger pump. That being said, having x boost at supercharger rotational speed y (dependent on pulley size and engine RPM)will have volumetric efficiency accounted for already; since volumetric efficiency will effect boost seen. I guess what I think I'm getting at is that a function for boost includes all variables including volumetric efficiency, air density, temperature, and a handful of other factors. I say give it a try and see if a linear function gets close :)
I have a few pics of the Blue Meany at the Tire Rack on my blog if you haven't seen them already. I still need to upload an album of all the pictures. I have some video of us running on the course but sadly they don't have sound.
http://clandestinemotorsports.blogspot.com/
Have you gotten any pictures or the article from the magazine shoot? I'd love to see how that turned out!