I just got back from a long dive to adjust the MBC. I had the same experience with different boost in different gears.

1st WOT barely gets above 10 psi (holds until redline)
2nd WOT builds to 14 psi then drops to 11
3rd WOT Builds to 14 psi then drops to 11
4th and 5th WOT (3k-4k) Builds to over 15 then over-boost kicks in

It seems like the compressor can not hold pressure above 4k. Under 4k if you are in WOT for a period of time it will build too much pressure and set off the over boost protection.

 

If you're hitting your overboost protection, turn down the boost. If you're not building pressure at the higher rpm, the simple way to explain it would be the turbo is currently not able to crank out enough air to continue providing positive air pressure in the intake tract while the engine is demanding that air. Could be because its not fully spooling due to a bad wastegate or because the turbo itself is to small.

 

This is typical with an MBC and the main reason I switched to an EBC. Boost is determined by load, in 4th and 5th you have a much greater load than in 1st and 2nd. To dial in a MBC you have to do it in 3rd or 4th but by doing this you wont get the max boost in 1st or 2nd. There really is no way around this with a stock turbo.

I had posted this before but it's nice to see some real numbers to finally put the debate in the past. You are correct that the turbo cannot hold 15psi boost to 6k. You can make it spike over 15psi, say at 4500 it spikes at 20psi, by the time you get to 6k and shift it won't have bleed over enough to go below 15psi. This is why people debate a stock turbo can hold this boost. You do not see this effect because the overboost is kicking in so you never get a chance to see the spike.

 

if you still have the boost line to the KLR, even if you bypass the CV the KLR is still seeing boost pressure, do I have to videotape my k26/6 doing .95 bar to redline for everyone to belive me? I will do it.

 

Holding .95 bar (without spiking over 1 bar) to redline in every gear with a MBC, stock trubo & exhaust? I'd like to see that.

 

I would be greatly interested is seeing this as well.