Even with a relatively large turbo for the street, a turbo powerband will still be decently broad, and after max boost is reached, you'll be living large. We'll say our 400 hp turbo B18 has a torque peak of 5500 rpm a slow steady drop in torque from there on--torque peak, the point at which you will feel the maximum acceleration. If we shifted, from 3-4, at 7500 rpm with the wide ratios of the LS tranny, gear 4 fall right next to torque peak at around 5600 rpm and almost 2000 rpm of forward thrust until the next shift (if our hearts desire to travel at 5000 rpm in 5th). Now if we had opted for the close ratio VTEC gearbox, the shift will still occur at 7500 rpm, but this time the revs drop to ~6000 rpm. 500 revs off of our torque peak, 1500 rpm of forward thrust. And to make matters worse, a turbo sized to flow 400 whp of air will exhibit a small amount of lag. Approx. .1 to .2 of a second at 6000 rpm on a 1.8L--very minimal, but in racing this is an eternity. The true definition of "turbo lag" meaning the amount of time you have to wait for max boost after your foot hits the floor. So with the close ratio, you never really touchdown at torque peak and you never really make full use of the broad powerband produced by the turbo when it's working.
In our other scenario, however, with a narrow powerband of of VTEC motor (yes I know exactly what VTEC is so don't mention anything that sounds self-contradictory about how VTEC gives you a broader powerband). Torque peak is very near redline on mildly built N/A motor. We'll use 6500 rpm as this is where a lot of VTEC motors like to live. Now if we had the same LS gearbox, and shifted at 7500 rpm, revs would drop to ~5600 rpm. Even though the VTEC cams are working hard and acceleration is good, we will still be waiting a long time for the torque peak and the majority of that 1900 revs is not making maximum use of acceleration. With the close-ratio tranny however, revs drop down to 6000 rpm. Right at the engine's "sweet spot" (actually, it'd be a whole lot sweeter if we raised shift point to 8400--then it shift would fall right into the engine's real sweet spot. But now we're stuck with 7500 rpm shift point for comparisons). And no wait to get into the torque band and the shift is accompanied with a surge of power.
Still not convinced? Ever noticed how turbo cars don't come with close ratio gears boxes and big displacement V8's have wide ratios as well? Close raitio transmissions are not the last word in hi-performance.
