I just want to ask, The turbo tapering down isnt the fact that the mass of air coming out at say 6000 rpms isnt enough to propel the turbine to make X amount of PSI on compressor blade side, is it simply wastegate control? Also is the wastegate either fully opened or closed? does it fluctuate on how much ehxuast it lets out. Cause if the stock Boost controler with the BLeed off style control controls the wastage, it can keep it shut or only have it open at a partial level to hold X amount of boost at 6000 rpms?

 

the boost taper I observe is not a steady state phenomenon, but is entirely due to the time history of the engine speed.

let's look at how your motor achieves say 21psi of boost as you go WOT.
1) Go WOT
2) Turbo spools, wastegate is fully closed
3) Boost rises monotonically
4) As boost shoots past 21psi, wastegate opens. boost starts dropping
5) As boost drops below 21psi, wastegate closes. boost starts rising.
6) wastegate finds steady state duty cycle (i dunno say 80%) that gives a constant 21psi FOR THAT ENGINE SPEED. This duty cycle is a function of boost AND engine speed. herein lies the problem
7) Engine speed starts rising
8) airflow increases into cylinders at that given duty cycle. Boost starts dropping.
9) Duty cycle starts rising to bring boost up
10) Higher duty cycle means turbo starts spinning faster. This takes some time to
accelerate turbo.
11) Faster turbo speed means more compressed air is flowing per second into intake manifold. This takes some time to get this air going.
12) During the time it took to get steps 10 and 11 going the engine speed has risen even more and the increased turbo flow is no longer entirely adequate. Boost keeps dropping (tapering).
13) Repeat steps 9 through 12 over and over again..boost keeps tapering.

Boost tapering can be countered by either increasing the response of the turbo (lighter CHRA, shorter post-turbo intake piping) or decreasing the rate at which the engine speed is rising (taller gears)

There is actually another way to counter boost tapering...this is by having a feedback scheme that accounts for the time lag in the feedback response and adjusts the duty cycle more aggressively to account for the fast rising engine speed. These sort of intelligent (feed-forward?) feedback algorithms need to be implemented electronically and would be a lot of work to address this relatively minor phenomenon imo.

 

You'll see more boost with different loads. Try uphill. It is interesting to see the differences.

You can do a few things to eliminate the taper but it never goes away completely

 

There are a few reasons why.

1. Turbo eficiency drops at higher rpms, requires more duty cycle for the same boost.

2. weak *** 12 psi wastegate spring.