What should the diameter be on a twin turbo exhaust manifold? 1 5/8?
1 7/8?

 

I don't think the question is that easy but my guess is 1 5/8 for the most common application.

 

Looking around I think 1-5/8" will do fine too. You see some 1-3/4". There are Mustangs on the turbo Mustang board making 1400HP with 1-5/8" primaries.

 

You want to use the smallest pipe before it becomes a restriction, in most applications 1 1/2" to 1 5/8" primaries are used. You can go too big, if you go with more than you need thats just more surface area on the tubing for heat to be lost.

 

I did 1 1/2" on my single turbo, you will need no bigger then 1 5/8" IMHO

 

BTW, smaller diameter headers will increase velocity and turbo spool......

 

I think it's mostly dependent on the turbo specs and the displacement. 1 5/8 is probably good for a 350-396 motor, and 1 3/4 would probably be best suited to a 400+ CID small block engine. I'm using 1 5/8 tubes on my twin 383 setup.

 

1 7/8 here for my twin project, 355 CI, I think i broke the rules

 

Got any pictures?

 

I am using 1 7/8 tubes for my 388/91mm. The ID of the tube just fits over the port on my heads. Anything smaller and the tube would have been covering the port. Equal length is nice, but not really a necessary in a turbo application. I adopted the "just get there" philosophy from Young's Performance.
Just some thoughts.
Jay

 

I went 1.75 on my twins. I was told to take exhaust valve diameter & add 10%. That puts me right at 1.75"

 

The ones I built are steped 1 5/8 for 3.13 inches the 1 3/4 for 8.8 inches.