I was wondering if it was a possibility to do a compound turbo with my stock twin scroll turbo on my n20 bmw without modifying the manifold. So the exhaust off the turbo is 4 inch’s where it would normally go to down pipe but say if you could route the piping to another larger turbo and down sizing the 4inch piping a little would it still be possible to do so?

And would there be a problem with having the stock internal wastegate and an external wastegate for the larger turbo in this process?

I know this process would take a lot of modifications with a fully built motor, so just wondering if this whole thing has ever been accomplished or would I just need to completely get a couple new turbos? I do want to avoid that because I’ve heard even with the proper tuning program (not that you can’t but the codes just keep coming up after time) it is hard to turn off the engine codes for the missing wastegate actuator and diverter valve.

 

Could work, how much boost are you wanting to be at, and do you know the size of the stock turbo wheels?

 

Well the car hits 18 psi stock I’m not completely sure how much the car can handle but the mid 40 psi would probably get the car in the hp range I would be looking for.

Compressor: 56/42

Turbine: 47.3/41.2

 

From what I can find, that is equal to a -14B compressor wheel.

Working the numbers for your 40psi of boost, your manifold turbo is pretty big. It's pretty much on the verge of surging the whole time. Only way around it would be to install a larger than normal atmosphere turbo and let the manifold turbo do most of the work.

Something like a large a/r (1.05) gt35, or a gt37/8 off a diesel truck would work too. A gtp38 off a 7.3L Powerstroke is cheap, and is reverse rotation like your turbo as well. It may or may not package better.