I get the idea N75 is a total pain to tune, which is fine... but I just want to understand some really basic points first.

Context: I've made a tune, I've increased the boost but I have left the N75 stock. On this particular car peak boost is at 3,000 RPM. I have increased the boost at 3,000 RPM by 130mbar (so not a huge increase). I left the N75 stock to "see where we are at".

I found that the max boost is met, however when booting it in 3rd (to 4,500 RPM) and then changing into 4th (putting the engine to just over 3,000 RPM) and continuing to boot it, the boost spikes.

I am requesting: 2380 mbar, stock was 2250 mbar and the boost spike is 2460 mbar. Outside is 1015 mbar today and about 15c.

I am requesting more fuel too, but well within the axis of all stock maps.

So my questions are:

[*] Generally when tuning the turbo but leaving the N75 stock, surely you'd expect to see underboost or a delay in meeting the requested boost, rather than spikes? All tunes I've ever looked at the N75 is increased and not lowered.
[*] Is upto ~100 mbar boost spikes under the above circumstances actually acceptable? The stock SVBL is 2400 mbar giving 150 mbar of headroom

Edit: Just to add, I have not had limp mode YET but my SVBL is 2480mbar so its going to happen at some point :?

Could N75 be low, PID thinks "ah this is miles out" and then overshoots it?

Just tried again with another better tool and am not meeting requested boost when going through a gear (suggesting not enough n75) but am spiking badly (over 2500 sometimes but no engine light) when changing gear suggesting too much n75. I guess I must have a slightly bad n75 solenoid or turbo vanes.

Will just reduce my turbo request and continue to leave n75 stock. Any comments would still be appreciated.

On WOT, little bit overboost is normal...reducing n75 map opens vanes more which will help with overboost

Thanks for the reply...

After some playing around I started to look at hardware. Swapping the EGR & Turbo vacuum valves over (they are interchangeable on this car) there is already a big improvement on boost control and performance in general so it looks like this was my problem.

I think N75 still needs a bit more work but at least it will be easier with working hardware. I might just put a new vacuum valve on the turbo control as even the EGR one is 12 years old and not "perfect".

Only adding this post incase anyone else comes across this. If you are in a "can't win" situation tuning N75, check hardware, even without engine light :)

105pd engines in my exp have clogged vnt more often than not

I have similar problem with N75/overboosting with VNT17 on AFN, 0,24 nozzles and 11mm plunger. All hardware is ok and checked many times. Another VNT17 with new geometry was installed too.No sense to hardware change. I have overboost till 1,9bar(from 3000rpm to 4500rpm, but target pressure ir 1,45bar. If I reduce PumpVoltage, the overboosting is not that big, but there is no smoke at all. Maybe VNT17 is too small for 11mm and 0,24 nozzles?