The Car:

92 G20
SR20DET U13
GT28R ball bearing
444cc injectors
z32 MAF
JDM U13 ECU
SAFC II
AEM UEGO Digital Wideband
10psi boost
Not sure if fuel pump is stock or not (based on other supporting mods I'd expect not)

So here's what's going on...when tuning WOT fuel map with SAFC, from 800rpm-3600rpm Im only pulling out -14 to -18 percent fuel to maintain a a/f ratio in 11.5-12.1, but from 4000rpm-7000rpm I have to pull -26 to -32 percent to maintain that range!

From everything I have looked up online, WOT SAFC settings add MORE fuel as you go into boost to balance the air increase, in my case I have to do the opposite.

Now with the settings currently in place, I can say there is no knocking, and its pulling very smoothly...I guess I just need to understand WHY its behaving the way it is.

Do a thorough check for boost leaks dude! Also whats your fuel pressure regulator set at or are you running a stock one

sounds to me like you have another issue somewhere making it dump fuel and your safc is just a bandaid for it.

so next step would be to make a boost leak tester...if I've got no leak, whats the next possibility?

You said you dont know what you have for fuel pump.... so im sure you dont have a fuel regulator or fuel presure gauge.. so first thing is spend the $100 bukes and get a pump because you are past the limits of a stock pump. You can also go to auto zone they have a rental fuel presure gauge that you can then see if under boost presure drops

ok so here's the rub:

Vacuum at idle is 21inHg

I pressure tested (home depot diy to compressor inlet and cap at the end of hotpipe just before throttle body, soapy water spray to look for bubbles) and found two leaks...one was right at the gasket for the compressor inlet plate (horrible welded mess with uneven plate side), and a pinhole in the rubber line that connects from my manual boost controller to the intake pipe just before the blow off valve and throttle body.

The first leak would effect vacuum not boost, but nevertheless I put a bead of copper rtv to seal that up. Replaced the line to the MBC with new rubber...new pressure test holds 20psi just fine. Take the car out and the only thing different is that it drops into vacuum faster when letting off the pedal...rate of boost climb is the same, a/f ratio is the same with same safc settings.


This is the other interesting point:

initial boost builds at 3200rpm to around 1-2psi, at 4500rpm is 5psi, and at 5500rpm peaks to 10psi and holds to redline...it doesn't taper off once it peaks like you'd expect from a leak before or after my pressure test diagnosis/fix.

Im at my wits end here! Having to lean out on the safc on the top end (as boost starts) would indicate that the engine isn't seeing the air that the maf is metering, but there are no boost leaks? It also doesn't line up with the slow building boost...

What am I missing here guys/gals? From researching a GT28 should hit full boost before around 3600-4000rpm.

Next two things I can think of would be wastegate and MBC...how should I test to see if either are faulty?

Sorry, throwing a lot out there...

discovered from an old post by original owner (92g20Red) that this has a walbro 255 but does not have an afpr! Can it be as simple as that? The pump has a higher output, so not being able to adjust the fuel pressure its by nature dumping more fuel with the air increase?

Talked to a custom bike maker friend who thinks the slow boost build is because the turbo needs to be rebuilt...he based this on not hearing the spool up whine when reving it up in neutral, even though the BOV is pumping air when letting go of throttle. Is there any merit to this? I've got it dialed in pretty good now with SAFC, idle/cruise is 14.0-14.5, WOT is 11.6-12.0 all across...but still not hitting 10psi till 5500rpm is bothering me.

If you don't have one already get an adjustable FPR and gauge. Hearing the turbo noise while revving.....thats a big NO GO and has nothing to do with anything. The the GT28r ball bearing can't be rebuilt you have to buy a new center section. Running crazy rich can cause turbo lag, also check fo exhaust leaks. What FMIC and piping are you running this can effect lag alot more than noted by most ppl.