compound or twins question
 

sorry maybe a stupid question but have been surfing thru old posts and am finding that some people are running the smaller turbo on top of the larger. why?. would you not want the smaller turbo to help spool the larger one or am i just missing somthing. you would still see boost of the smaller turbo and then the big one would lite. what am i missing. ie he35 under a ht3b. the 35 would help build that LARGE turbo would it not?

There wouldn't be any advantage to running a smaller, (HX-35 etc) turbo under a bigger one, (HT3B etc.) You are correct in thinking the small one will light the bigger one. Funnily enough, the small one is referred to as the secondary, and the larger one as the primary, even though the smaller one gets spooled up first. I never understood that, but whatever. I've never seen a twins set up plumbed the other way around.

Twins are 2 turbos the same, feeding the engine. In an inline engine, cylinders 1, 2 and 3 feed one turbo, and 4,5, and 6 feed the other. On V configured engines, on side feeds one turbo, and the other side the other turbo.
Having the big turbo feed the little turbo is the most efficient. Atmospheric pressure is roughly 14psi, at that is what is available to a single turbo inlet. If a turbo can make 35 PSI with atmospheric pressure, adding 14 psi to the inlet of the small turbo should double the output of the small turbo, in theory. Of course the limitations of the turbos control the over gains, so does the fueling.

thanks guys getting a better of understanding

Because the smaller spools easier which is why its first on the exh side but on the intake the air goes throught the larger first. So basically the lil guy is for lowspeed crusing boost and helps to get the big one moving air the the lil one is regulated on the exh side so it wont over speed and the big boy takes over from there. Clear as mud?