08-16-2013, 03:45 PM
Join Date: May 2010
Location: Harrisburg, Pa
Total Cats: 8
Why is my boost so saggy?
Ok I've been fighting boost sag for awhile and figure it'd be time to consult the forum.
So I used to sag with my old set-up which was as follows:
MS2, no VVT control
2.5" straight through exhaust (only one muffler)
Ebay T3 turbo 50 trim compressor, 0.48 A/R turbine
Vac/boost source for the turbo was after the throttle plate
I never got to test this set-up with VVTuner because I blew up the turbo before I had it. But I would sag badly. I could hit boost cut which was 216kpa (16.5ish psi) but was at 8-10psi by redline.
So then I got my Turbonetics T3/To4e with 54 trim compressor wheel and 0.48 A/R turbine. I seemed to hold boost better but wasn't hitting full boost till 5000+ rpm's.
I then got VVTuner working. With everything else the same and DIY's base map I was spooling faster and holding boost perfectly.
Now I've got a new manifold (think S4 but upside down). This got me another 500rpm earlier spool but now I've got boost sag. I've since added a 3" exhaust (also just one muffler) and no change.
I've messed with MBC's, VVTuner, and finally I moved my wastegate source to the cold side of the intercooler.
No I've got a boost curve almost identical to my first setup (log + chinese turbo) I'm currently hitting 12.2psi by 4800rpm but quickly drop off to 8psi.
So my question. Do I suck at tuning that much that I can't hold pressure? Are my homemade parts so crappy they can't flow that much? Or am I just being hampered by a wastegate that can't hold that much pressure?
Right now I'm leaning towards the waste gate. It's just annoying how many things I've changed and that I'm still boost sagging.
Any idea's would be greatly appreciated,