What blow-off valve/diverter valve setup is on there? I couldnt find one in your modlist.

 

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Your 98 R.O.N. gasoline is equivalent to U.S. 93 octane pump gasoline, which is rated R+N/2. So, 28# on 93 octane seems too high to me on a dinky turbo with an iffy tune. Probably, 24# tapering to 22# would be more in line with reality taking into account high ambient temps and relative humidity conditions in your area.

 

The Megan O2 housing is problematic at best at or near peak boost levels. The crossover hole on the divider wall is too small. Usually causes boost creep issues and needs that crossover hole enlarged with a grinder.

I realize that you are reporting boost taper and not boost creep issues. But, I still wouldn't rule out the O2 crossover port being too small. At any rate the Megan is worth a look. Incidentally, what DP are you running?

 

I would recheck for pre-turbo exhaust leaks. The Megan header is notoriously prone to cracking at the welds between the primary tubes and the flange. So, rechech with a high intensity spot light. What gaskets were used? Those shiny, silver, aftermarket gaskets tend to fail quickly.

 

Do you do the mechanical work on the car yourself? Specifically, I refer to the turbo swaps, and the installs such as the the boost controllers, etc. I ask because I would like you to run the turbo compressor disharge straight to the WGA and try increasing the boost level via the adjustable WGA.

 

I had a similar problem on my setup it turned out whoever installed the cams had the timing wrong a cpl teeth, costed me 30whp and spool.

 

I have tightned the ARP head studs to 85 lb (115 nm) and it gave me good responce, never expeted this right, it was too loooooosss ?

I have reached 2 bar peak @ 4th and boost is holding better now.

BOV is greddy and its holding very well. Better than HKS,

For now I am considering another tune and i really doubt something wrong with cams/timing as well

1/8 mile (200 m) drag result was very bad 8.2 sec @ 147 km with aggressive anty lag.

I will keep you posted and thaaanks tons for the Valuable feedbacks/advices sparky and the others

 

The Greddy BOV has a known history of losing pressure. Check it for a torn internal diaphragm.