Guys,
Installation of yesterday was a breeze. My mechanic started at 10AM when car was cooled down and job was finished at 8PM. An 8 hour job actually if we deduct 2 hours for the dinner and break. Not a single issue, never needed to "figure out how", never had a CEL to clear.
Now the turbo comparison.
See pictures below for GP500 vs PS2 (and vs stock for those who's interested).
Compressor: both PS2 and GP500 cut from forging billet. PS2 is a little bigger, as the opening is bored out to the edge of compressor housing. Also, PS2 seems to use factory blade design. GP500 has its unique in house 7 blade design and is even lighter than stock.
Turbine: Both are clipped type. Size is very close, maybe PS2 is slightly larger. PS2 also has a more aggressive blade pitch that trade spool up for less restriction. GP500 has a smoothly machined edge of blade that reduce restriction and weight at the same time.
To sum it up (through my untrained eyes), PS2 is more aggressive going all out for high boost. While GP500 feels more refined and advanced (for the price it asks it better be) and is screaming efficiency (at power range it's designed for).
Then the impression of GP500:
1) Exhaust note - The tone felt instantly deeper when I light up the car. Could be all just in my mind, or the in door resonance, I thought. Drove out of the garage and reved it up a little bit. My very discerning ear clearly confirmed it's a little louder and noticeably deeper. Oh...I'd be willing to pay 2k for sound alone
So did my mechanic think.
BTW, what freer flow turbine did to exhaust note is - less turbo restricted = more naturally aspirated. And, the more upstream the exhuast component goes, the greater effect it has on sound quality. Normally turbofold or header is the most significant sound changer in the entire exhaust system.
2) Turbo Lag - Let's be honest, 500hp or 550hp makes no difference to me but turbo lag does, to the point it makes or breaks the deal. Which is why I didn't do STG2 turbo much earlier.
First on the road, excited, I tried hard looking for it. Granted stock turbo and STG1 turbo aren't lag free to begin with. Thank god! GP500 lag was barely noticeable just like STG1 and stock. That was nothing remotely like my experience with my friend's 335xi PS2, the then 1/4 mile world champion 11.11@125mph. PS2 lag on pump gas was quite noticeable on the road, below 3k you have nothing more than a NA, though above 3k5 you're rewarded with a 911 turbo.
3) Power - finally we're onto it. that's why it happens at all, right?
I started with OTS STG2 93OCT, which I flashed few days back. That gave me a good perspective how GP500 makes a difference over STG1 turbo. Let me put it this way - it reminds me of the day I drove my jb4ed 328i to pick up my new M2.
Read this - same map. same boost, same timing (Dinan turbo max out everything OTS map targets), GP500 pulls like a whole different car. The revving up was so rewarding that it never stop giving you more until shifting to next gear. On just OTS map, it feels similar to the BM3 STG1 tuned M4 of my friend which I drive all the time, maybe slightly less at mid range but slightly more at top end.
4) Datalog
30 mins road testing, aggressive driving, went back for oil/coolant leakage check - all perfect. I went to highway for datalogging. As I said boost/timing was already capped out by Dinan turbo, GP500 made no difference of course in these area. Sorry it's not a good time to share my full log, but one thing - I compared the spool up to STG1/stock turbo, I gave up 300~350rpm spool up, well...a tradeoff I'd love to do everyday all day please
PS2 would be a 800~1000rpm delay.
I'm flying to USA on 6/7 and will stay till June end. I'll slightly tweak my map and be done with it for now. Next step is to build 2 of my friend the M2 PS2 when I'm back Shanghai. I'll do the comparison no worries.
Serious custom tuning will happen in Q4. Winter is a much better time for testing.