Has anyone set up there own twins.
does the primary need to be larger or smaller.
what would happen if you used two of the same turbos?

 

Two turbos of the same size wouldnt do you much good unless you were running one on 1-3 and one on 4-6. The smaller top spins the larger bottom in 99% of twin trucks out there.

 

The primary is the bigger of the 2 also known as the bottom charger. The secondary is the smaller or top charger hope this helps.
Joe

 

Lets say you used your stock turbo as the top and a big rig s 400 as the bottom. Your truck runs great with the stock turbo but does not have enough exhaust flow to spin the the bigger one. But, when your top one starts to force feed the engine, your exhaust output doubles or triples and then has enough exhaust flow to spin the bigger s 400 which then feeds the stock turbo with boosted air. The stock turbo then becomes extremely efficient because it now is starting with 20 psi or more and just has to bump that 20 up to 40-60psi. The biggest problem with twins is that in order for the system to work well, the stock type smaller turbo must be plumbed into the exhaust manifold/exhaust flow first so that it can start the whole spool up process at the lowest possible engine speed. This, however, hurts the top end of the truck due to the fact that the stock turbo must have a restrictive exhaust housing in order to spoolup and start building boost at low engine rpm. Since the bigger turbo needs lots of exhaust pressure to spin, but is located downstream "in exhaust system", only so much exhaust flow gets to it and the top turbo in essence "chokes up the engine at higher rpm/ higher exhaust flow due to it's restrictive housing. The best solution for this is some sort of a waste gate that opens and bypasses the exhaust flow around the smaller top turbo after it is up to speed so the bottom turbo can get maximum drive pressure.