Compound turbos, low rpm boost, high rpm flow
 

I am just trying to get something straight here. In this pic from what I can figure, the small turbo runs like a single turbo would, sucking in clean air from the big turbo (which isn't spooling much yet) and exhausting it to the big turbo which still doesn't spool so it just passes right through. Then when you step on it, the little one maxes out and opens the wastegate so it will hopefully stop at whatever psi, then the big one gets spooling and provides the cfm necessary to give higher boost, along with being cool air since it is a big turbo.

What I want to know is that, the smaller turbo is basically the same size as the hx35 as far as I can tell, and so I am wondering how that doesn't become a restriction when you need it to be flowing? Also, low end boost is also the same as with just a single HX35, right? In other words, it's not like I can get 5psi at 1000rpm. Right?

The big turbo doesn't need to spool much to make a huge difference. Let use numbers at sea level, as they are a constant. At sea level, there is 14.7 PSI of atmospheric pressure, but it is usually measured as 0. If the big turbo can make 14.7 PSI of air pressure, you are doubling the pressure at the inlet of the small turbo.
As you add more fuel, both turbos begin to increase in speed. The big turbo doesn't need the wastegate of the small turbo to open in order to spool up. A wastegate on the small turbo will keep the drive pressure under control though.
Low end boost is increased, but to get 5psi at 1000rpm you would need a great load on the engine.

But is the small turbo a restriction when you step on it..?

It does create some restriction. Depending on how much fuel your pushing, and how much air is flowing, larger exhaust housings on the small turbo help significantly. The big turbo gets more flow, the top turbo doesn't spin as fast, and good boost is still created.

Just as I thought.

You can use an HX35 as a primary, but I'm plannin on using a 62/65/12. You don't want anything bigger than a 12cm or 14cm housing on your primary, otherwise its probably. going to make it spool slow and make you run high EGTs. Now granted if you're running quadruple digits you want something bigger than a 12 or 14, but for a 4-700hp truck, I would stay with a smaller exhaust housing on the primary.

Yes don't run like a hx40 as your top turbo with a 16cm housing. You will have high egts. As I'm finding out. Run a 62/65/12 or even better a 62/68/12. The 68mm flows a little better

just a dumb question.... anyone ever try to run a turbo off a 3406 or 3412 cat as a bottom turbo ?

You were the example I was basing my statement off of! lol

Depending on what it's dimensions are, it might work. My S400 is off a 12.5L Deere engine.