i know this has to have been covered many many times as probably most people running a 4'' downpipe do not have 4'' exhaust. I have a 3'' downpipe currently on a mini ram manifold. I believe I will stick with this manifold unless i get the sudden urge to make a topmount, or ram but i prefer the fastest spool I can get and this manifold did pretty good on the last setup.

My question is, if I do opt to make a 4'' downpipe for my new setup about how far from the connection to the 3'' exhaust should i start to "merge" the 4'' down to 3'' or does it not really matter?

 

I would think the longer the transition the smother it will be but you can make it as long or as short as possible depending on materials?

 

Its plan stupid to have a 4" DP and a 3" exhaust. Really, just plan stupid. I could see if you had a 4" outlet on your turbo, but Id bet anything that you dont. How much power are you making or trying to make?

 

Wow, alright you dont have to go off there.

Its a sc61 with the S housing. Its no little 18g **** here im trying to slap a 4'' exhaust on. How many people on here actually have 4'' downpipes and 4'' exhausts? Sure I can do a e-cutout but I dont want to. And how is it pointless to have a larger downpipe then what the exhaust outlet is? I believe there was a rather large thread debating that.

This is a 600whp+ plus build.

 

Id do 4" DP/Exhaust or just do 3" all the way. Your outlet on your turbo is 2.5", going from 2.5-4-3 doesnt make much sence IMO. I have done many setups on my car such as:
3" turbo back
3" turbo with 3" e cutout to RSR exhaust
4" turbo back

I dont see any real benifits from doing what you want to do except for bragging rights. 4" the whole way is not that bad on the street if you use a muffler and resignator. my frist 4" exhaust was 4" all the way back with no mufflers or anything - that was loud