Boost drops after 4k rpm
 

2300rpm + Boost
4100rpm + 5.2psi
But after 4100rpm boost pressure declines steeply.
What should this newbie look at first?
Please don't say an "S2000" as that is one of my neighbors lines!!

Boost leak?

Check the wastegate rod to make sure that it is not loose.

PCV :dunno: Brake line, some sort of vaccum line off? check the nipple in the back. If it was preturbo leak it would delay spool so look at everything past the compressor, all the way to the Head. Injector grommets etc. GL

how much does it drop off? Sounds like a typically weakass GReddy wastegate issue.

Tighten up the rod, and look into a helper spring.

How much boost are you running? If it's anywhere near 9-10 psi, I'm with Braineack.

No boost controller so boost is greddy spec ??,Manual boost controller arrives very soon.
I did tighten the WG arm as it was way loose when I installed it and the valve rattled.
No discernible vacuum leaks, but at +pressure I am not sure.
Drops about 2 psi.

On a stock greddy tighten the nut to 6 psi. MAX!!!
On 15 psi the boost drops to 12 on my old setup.(no helper spring)

Try to move the arm open with your hands, should be very hard...
If on 6 psi the boost drops to 4 look into any vacuum leaks...(bov is bad??)

timing?

No BOV as yet, (that seems like a complete chapter all to itself)

and retarding timing 1deg. per psi.