i think the gds is a 3.5 probly just get it for now
GDS HI-FLOW POWER OUTLET
Need to lose more exhaust heat? The GDS Hi-Flow Power Outlet can easily reduce exhaust temperatures over 100 degrees for less than $200.00! While upgrading your turbo or exhaust system, we recommend always installing a GDS Hi-Flow Power Outlet on the turbo to replace the stock outlet piece. This will eliminate the stock EBP valve on most applications, and further reduce exhaust backpressure even on a non-EBP valve equipped motors. The stock outlet piece has three major flaws, all of which are remedied with this long overdue casting.
1. In most states it contains an electronic backpressure valve to aid engine warm up times. This butterfly valve is located at the worst possible position as far as performance is concerned. It is right where the exhaust is exiting the turbo transitioning from high pressure pre-turbo to lower post turbo pressures. This results in an expansion of the exhaust gas trying to occur right where it hits a huge restriction (the e.b.p valve). This is further amplified by the fact that the exhaust gasses are exiting the turbine in a swirling vortex and hitting the flapper valve within inches of the turbine wheel, further slowing exhaust flow. The GDS casting has no valve or provision for a valve!
2. The stock casting has a divider wall between the wastegate valve outlet chamber and the main exhaust outlet. This is a bad idea when trying to increase exhaust flow because it forces the wastegated exhaust to back up before it can enter the main exhaust stream and exit the casting. Keep in mind that the exhaust exiting the wastegate valve is coming out at a higher pre turbo pressure so when it has to reverse flow and try to enter the exhaust stream just upstream of the e.b.p. valve, it certainly doesn’t help exhaust flow! The GDS casting has removed this dividing wall completely.
3. Next is the inside diameter of the exhaust outlet. Stock is about a 2.8” i.d. with the very restrictive valve making its effective diameter much less. The GDS casting is an unrestricted 3.4” i.d., perfect for, and compatible with any performance exhaust systems with a 3.5” or 4” downpipe. Note; there is also Hi-Flow outlet available for stock turbos with a 3.2" i.d.
This exhaust outlet casting should drop exhaust temperatures about 150 degrees on a stock turbo and will optimize the exhaust flow with the GDS Ball Buster turbo.
The info above is from the GDS site. If you can somehow get the 3.5" outlet for your stock turbo from GDS, make sure share the part number with us all.