Quote:
Originally Posted by JamesNoBrakes
Except "full boost" for a roots twin screw type is somewhere like 5-7psi,
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?!?
You are not correctly informed.
Here is the performance map of the twinscrew I use (lysholm ax1600):
http://www.lysholm.us/pdf/diagram_lys1600ax.pdf
You can clearly see that the peak efficiency is at 1.8 pressure ratio, that is around 11 psi, and that the widest flow range is about between 1.6 and 2.0 ratio, so between 8.5psi and 15psi.
That is it's ideal operating range for this twinscrew.
Nowhere near the 5-7psi you think of.
It will work on that low range if selected carefully when the cfm requirements of the engine fall within it's range (or you'll loose boost/gain heat in ranges), but it's not designed for it.
Maybe if you want to go in those low boost ranges the little eaton m45 is an option. That's used in a lot of miata sc conversions (mostly sources from small mercedes engines I think), doing it on the cheap so to say. But that is a very small and low yeald supercharger (although I still suspect that it'll still perform at 8psi, but with low cfm/for small engines)
But these are all facts/data which is easily available on the interwebs....
Eaton V series sits between about 5 and 12psi and eaton R series sits even between 7 and 17psi.
So you can pretty much find a roots or twinscrew supercharger for most applications.
Only if you want to go above 17psi (in fact that eaton R will go up to 21...), you have to go turbo, but that is even for gas turbo engines pretty high. For comparison: the n54 runs stock on 8.8psi.
Boost pressures above 17psi are normally only seen in turbodiesels or very very high performance builds where the stock turbo probably also has run out of air (well....gains too much heat) (335i's with 600hp+ and such)
That concludes this weeks' turbo and supercharger lesson
