I'm about to start building my twins and didn't know if it matter if i used 3in or 4in pipe for the cold pipes in between the turbos? pros and cons.

 

Probably best to use 3'' pipe to keep the velocity up to help spool the primary turbo.

 

All the big builders use 4". 3" is probably easier.

 

It depends on what kid of set you are building. 4" is easiest as it's cheapest. However, it is rarely the best choice.

As noted, smaller diameter piping is beneficial for getting that pipe filled as it has a smaller volume and also this will reduce pressure loss slightly. The difference from 3", 3-1/2" and 4" in my truck is huge!

The 3" spools fastest ans works great with the smaller primaries. The 3.5" Works great with the S400 and up primaries and the 4" gained me nothing except slower spool.

The 4" would hold a distinct advantage in nitrous applications IMO.

Now I know the argument being that the inlet of most secondaries is 4" and a straight boot is easier to use. But I use a 3.5" to 4" 5 ply boot or a simple increaser could be welded in and the use of 4" straight boots become possible once again.

Again, just my opinions. Not saying any of this is the right or wrong way to do things.

Dave

 

Whatever size of outlet on the primary would be best is my thinking. On my GT4202, the outlet is roughly 3" ID, the clamp on adapter with o-ring takes it to 3.5" ID. I ended up using 4" since thats what I needed for the exhaust as well.

 

As fishin deep said, the 3 inch will keep the velocity up and allow it to spool quicker. Ive heard nothing but good things about the 3" setup on the cold side, thats what im using to build my twins this winter.

 

The outlet of my S400 was a 4" and the inlet of my S300 is a 4" so I went with 4". I have no problem with lag. If you want to see how the 4" looks, you can check out my gallery.