I had the same problem with my 18G. You're getting compressor surge. Basically you're falling out of the effeciency island on the compressor map for that turbo. An 18G is a pretty quick spooling turbo especially under heavy load in 5th gear and your getting high pressure and low flow event. Like others have mentioned you might have to lower waste duty cycle if you can't tune it out in the turbo dynamics table.
Originally Posted by low_&_loud
Thanks for the input but I think you missed what my problem is here. The issue isnt overboosting, since Ive gone to an MBC it hasnt overboosted at all (confirmed w/boost gauge that has memory setting). I will adjust the MBC as needed with temperature decrease in any case. Ive never in my life heard of anyone shattering 5th gear but Ill keep that in mind...
This issue occurs anywhere from 1/4 throttle or higher in 5th gear, depending on the RPMs I am trying to accelerate from. If I need to accelerate hard enough that Id floor it in 5th Id just downshift, I'm definitely not just matting it and waiting for it to spool up and pulling from 50mph or less to 100+, this is just a little nuance that's annoying driving on the highway if you just want to get around someone and give it some gas. I drive ~40 mi of highway daily so I'm just trying to make the car a little more friendly for it (if possible). I also know its modified/turbo swapped and isn't going to drive like a stock WRX which is fine. I'm an engineer and for better or for worse try to keep improving the way car drives, thats all.
This is similar but mine is definitely a higher frequency. Ill try to take a video as well when I can, probably should have done it to begin with. Mine is also occurring with no change in throttle position, i.e. if I'm barely on the gas cruising in 5th, and ease into 1/4-1/2 throttle and stay there, it'll build boost and begin to accelerate then start the chatter after a few seconds. Yours seems to be when you accelerate then back off the throttle a bit?