So I am going to run a 4 inch turbo back. I am going to buy a diamond eye but I have a dillema. I am running a stock turbo but hopefully one day I'll be running an hx 35 or 40. My question is would a down pipe for a stock turbo bolt up to a 35 or 40? I would hate to buy one and have to buy another in the next couple years

 

Yes it is the same flange size for the HX-35.
I don't know about the HX-40, but I do know the make couplers to go from one size to another.
You just clamp the adapter on with another V-band clamp.
I have a Diamond eye on my crew and when I swapped from H1C to an HX-35 it bolted righ on.
IIRC I had to move the exhaust back 1/2 an inch.

 

Stock H1C and HX-35 share the same 3" outlet, v-band flange. An HX-40 has a 4" v-band flange. Therefore, the downpipe can be used for both the stock H1C and HX-35 but not the HX-40.

 

Think it would be smart to buy a down pipe for an hx40 and an adapter to h1c

 

When you get the HE351 or HX40, you cut the small flange off and weld on the bigger one.