If the goal is to make the same power, lets say 270hp from 2.0L turbo4, then you can do it with (say) 18psig from a turbocharger, like we do, or twin-charge - so maybe 7.5psig from the turbo and 7.5psig from the S/C.
The engine will see similar intake velocities and pressures and intercooling being equal make the same max power - but you're still flowing the same CFM through the turbo, just compressing it less (assuming you change your A/R for this to optimize the curve!). The advantage is at 2000rpm when you slam on the throttle you're pretty much instantly at 7.5psig already from the S/C, so your powerband is wider and flatter and throttle response is better as the turbo then (quickly... very quickly) catches up.
Volvo just decided their turbo4 would be ~350hp and didn't want it to feel peaky or have tons of lag like a single large turbo would. Ford, when faced with the same problem, increased displacement.
W-Body guys have done this too, trying to turbocharge the 3.8L S/C and the results are consistent that if you want more raw power you're better off with a single large turbo or supercharger than the complications and headaches of twin-charging. Same with twin-turbo engines as well.
For a street driven daily driver, a positive displacement blower is my favorite power adder. Their "instant power" is pretty nice. In the case of the LTG I'd rather stroke the engine an then put on a slightly bigger turbo through.
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Originally Posted by cooper1965
Sounds like a cool ideal. Great boost off the line, quick turbo spool.. I wonder though, and this is all new to me, but would the cost and effort really be worth the gains as far as reducing your ET? I mean, if it's going to cap off around 8psi, how much could you realistically gain MPH or 60' wise ? Again, this is a new concept to me, but would there even be a noticeable gain on the dyno?
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The max psig is the same, but the way it gets there is different. This certainly isn't cost effective, it's done to make a wider and flatter torque curve. Look at LTGs that are boosted higher and harder toward the 350hp mark - they give up power down low but make TONS up top. twin-charging like this robs a relatively small amount of that top peak and adds it several times over in down low power and throttle response.