I'm thinking of doing something very similar on my GTO, but using a divided 2.5" flange: https://www.ebay.ca/itm/T4-Divided-T...t/183995122013.

Crossover pipe will be 2.25", staying divided all the way. What are the implications of having an extra 4ft of crossover pipe on the one side? Velocity should be more or less the same side-to-side but I assume backpressure will be higher on the long crossover side. That being said, I wonder if a single wastegate on the longer side would control boost well enough? Any issues to the driver side exhaust ports/valves having higher backpressure? As in premature burnt valves etc.

 

I wouldn't read into it that much, should be fine IMO depending on power goals. All of those things will have an affect on the exhaust gasses, but to a small little degree IMO. The smallest point in the system pre-turbo is the exit of the turbo volute. This and wheel restriction will dictate back pressure for the most part. Not an extra few inches of pipe length. For reference my 2" pipe was 1.88" ID. I was making 900+ crank going by weight and trap with 2" off each stock manifold. Standard "2.25" exh pipe is more like 2.11" ID. An extra .23" of ID per bank will be capable of BIG power numbers. That merge collector is too big IMO. Why do you want dual 2.5" ? Any reason you want a T4 VS a T6?

https://www.ebay.com/itm/T6-Turbo-Tr...AAAOSw2UpfmJM8

 

I have a 7875 with a T4 flange. No room for a T6 with my AC in place. Only merge I could find, I thought it would be better than anything I could make with a flat flange.

 

Not as cheap but there's this one.

https://www.ebay.com/itm/T4-Turbo-Fl...EAAOSwhQhY1~t0

You really don't need it IMO Just shape a 2" or 2.25" pipe and weld it to a standard T4 flange.

 

Well it is just mild steel, I'm sure the pipe ends could be cut shorter and 2.25 pipes welded on if you think it would make a difference.

Any thoughts on a 40mm JGS wastegate in just one pipe? Or should I merge the two somehow at the flange?

 

Merging them is complicated and a pain IMO. but it can work. It also negates the reasoning behind dividing the exhaust in the first place. If you divide them, a single gate for each side is defiantly preferred. I don't know if one gate will give you the control you'd want. I'd think it would creep boost pretty badly depending on how hard you push it. Some have done it, maybe they will chime in. a 38mm gate on each side is MORE Than enough gate. I believe I paid $50 each for my "mini" 38mm gates. They worked very well. You could make space and provisions to run an additional gate and start with one, see how it acts?