I Don't agree :laugh: There is no real rule of thumb. Nissan are supercharging their new 3cyl 1.2 Micra. I agree that turbos are generally more efficient but that is also not always the case. They 'rob' a whole lot more power than 6% as well.
Turbos take a very small amount of WASTED energy to run, wereas a supercharger takes enery that the car is developing to run.
Theres a big difference, as if you stuck a turbo onto an n/a engine but dont plumb it into the intake there would be very little difference in performance if you did the same with a supercharger it would leach over half the cars power and would be massively undrivable (let me add i have done this to my integra jackson racing super charger roots style charger and t3/t4 hybrid turbo kit)
mini cooper s origionally supercharged then later changed to turbo, why?
more efficient!
the small loss of power through back pressure compared to the the power loss in supercharging makes end power output higher, putting less strain on the engine to reach that out put also reducing fuel consumption and emmissions and thats from BMW
''turbos rob around 6%'' yeah ill admit is from memory, of when i was learning about forced induction in college (was a while back) but even so the amount is quite insignificent compared to the over half of the engines power the s/c leaches, and as i said taking useable power is wastefull were as turbos use wasted energy recycling sort of!
nissans supercharging a 3cylinder 1.2 is frankly upto them i cant stop them but the facts are there they cannot be argued supercharger will take over half your output yes you get more than that back but it makes more sense to me to go the turbo route
my way of thinknig:
lets say you have 100bhp engine a supercharger which offers 100bhp and a turbo which offers 100bhp and lets go with supercharger loss 60% turbo loss 12% (which is about as high as it can be)
supercharged engine 140bhp not bad
turbocharged engine 188bhp better
using the same amount of boost turbo will produce higher outputs as it takes less to run them(Y)