Here's some more info from 68 fastback in another thread:
http://www.teamshelby.com/forums/in...its-available/page__view__findpost__p__291434
Some thoughts on S/C considerations...
Roots types are typically more affected by marginal intake flow more than twin-screws are, and I suspect the TVS is not nearly as affected as the stock Eaton by the intake, but moreso than a twin-screw is affected. If intake flow capacity is well adequate, on a fixed-displacement S/C 10% more = 10% more -- effectively linear since it's a fixed-disp pump.
However, the stock Eaton will return less of that 10% increase as rwHP, imo, for several reasons (mechanical and thermal): clearances are not quite as tight on the stocker as with the twin-screws or the TVS (mechanical disadvantage for the stocker); backwash is more of a factor with traditional (low-helix) Roots S/Cs (mechanical disadvantage for the stocker). That backwash also generates additional heat which further reduces the effectiveness of OD'ing the S/C and heat pulls timing/HP potentially even netting a HP loss under severe heat-soak (thermal disadvantage for the stocker).
All other things being equal, the S/C knows-not whether it's being OD'd by smaller S/C pulley or larger crank pulley -- it's just a linear [in therory] increase in rotor rotation speed either way. But it's that additional spin's ability to actually make HP that separates the candidates. Mechanically and heat-soak-wise, the twin-screws and TVS will well outperform the stock Eaton design. Adiabatically -- thermal efficiency (not soak) -- the TVS and twin-screws should be very close ...with the stock Eaton well behind. Just a guess, now, but I'd bet the net-efficiency (both mechanical, induced thermal and adiabatic thermal, from most-efficient to least) would place them in this order up to +/- 650rwHP, with the first 3 all fairly close): #1 TVS, #2 Whipple (historicaly they're a scratch over the KBs from the analyses I've seen, though this is undoubtedly highly debated by some), #3 KB, and a very distant #4 the Eaton stocker. That would be my best guess at modest boost levels (see below).
This is further affected (and not trivially determined) by the displacement of the S/C in question. Generally, the larger the displacement the less prone to mechanically-induced heat-soak if rotor-edge speed is not disproportionately more than the increase in displacement. So, for a given design, larger-diameter rotors actually can have a slight disadvantage vs rotors of the same diameter that are physically longer (to make the S/C disp larger) ...because rotor-edge backwash-indiced heat is more proportionate to edge-speed at a given operating pressure than to edge length at the same pressure. So,HOW the S/C design makes displacement is also a factor. Of course the physical length of the engine and other packaging considerations also affect which relationships are even possible/suitable. The big Whipple barely has a snout (long rotors) but that's goodness vs disproportionately larger diameter rotors. Further, operational boost levels also affect this -- larger-displacement and lower edge-speed designs having a bigger advantage as boost levels get higher. Over 20-something# boost, the big Whipple woule be very hard to beat.
The big Whipple (3.4) is therefore well-suited to big boost ..a bit less so for the 2.9 and then the KB, but to a slightly lesser extent, imo (also debated for sure). The TVS may beat them all in total net efficiency under 15-18#, but not at higher boost levels where it's smaller physical displacement would have to start outweighing it's inherent design (mechanical and thermal efficiency) advantage. A larger intercooler and/or heat-exchanger can extend the operating range from a thermal perspective but can't change a given design. Similarly, on a larger motor, the TVS would start to drop off in effective efficiency (regardless of its excellent mechanical and thermal) at a proportionately lower boost level -- because it would have to spin faster to maintain a given level of boost on the larger motor -- analygous to ODing it past it's sweet-spot on the 5.4 motor. Still, Eaton claims the TVS is ideally suited to displacements considerably larger than the 5.4, but that will be true for a still lower level of boost, for examle, might be about 15# on a 6.2 and maybe 11# on a 7.0L engine could be the edge of it's sweet spot ...for example only. (How much boost will the ZR-1 get with the TVS? Whatever that is, that's probably a conservative sweet-spot boost level for 7.0L.)
Not everyone will agree with the above and I have no interest in starting a pissing contest -- just calling it as I see it based on what I'm able to bring to bear. My intent is not to give arbitrary answers because, as you can see, it's not a cut-and-dried matter and is affected by usage, size, static (largely mechanical) and dynamic (largely thermal) efficiencies, engine size, boost-level/HP goals, intercooler capacity, heat-exchanger capacity, etc. And that is the whole point of the above discussion.
Dan
We'll see what happens with the new tires and Justin's tweaks on the tune.
