Also, the higher the boost ratio, the quicker / faster the cylinder will fill.Your torque is peaking at 3750 and rapidly declining because there is a significant restriction with the stock cam and heads. A properly spec cam should flatten out you torque curve and heads will allow you to make more power in the mid to upper range. You can probably upgrade the lower pulley to a 8.66 or 9.06 while doing the cam upgrade as well.
Once the cylinder fill reaches about 87%, Roots Blowers become very inefficient.
Roughly a 100% cylinder fill on these engines generates about 755 - 775hp.
Let's say is is 765 fwHP.
87% cylinder fill then will produce about 665 fwHP.
That number will vary some on a chassis dyno.
This is where some move over to E85, as it moves oxygen into the
cylinder, around the induction system.
Then many drivable street strip cars, begin to see around 730 rwHP
to 100% cylinder fill, or about 750 - 775 rwHP. This can be done
with good heads and a moderate duration camshaft.
This 87% induction system efficiency issue, is not just with our cars.
A highly refined NA car will see the losses reduced to ~92%.
NHRA 500 CID Pro Stock heads flow around 630 cfm on the bench
at 'Peak Lift'. That would produce about 1688 fwHP.
But on a Dyno, they only flow about 577 cfm, making around 1500 fwHP.
That amounts to 92%. . .
***Anyone that thinks their induction system will flow much at 'Peak Lift'
needs to reconsider that notion. The time spent pulling air into the engine
at peak lift, is not much. Next time, use 87% of the peak valve lift number.
***Total time to fill the cylinder at only 6200 rom, is only 0.0005 seconds.
It comes down to piston speed (Demand) versus induction system (Supply).
And while the engine can be setup to flow a Max 157% VE, an NA IC Engine
has difficulty pulling in more than 135% - 137%, or roughly 87%.