A new thought for twins - Compound Turbos? Maybe?

[vex]

Originally Posted by dudemaaan

Oh I didn't know you were suggesting the turbine outlet of the SmallT would be dumping into the the LargeT. That would work fine then, the velocity would still be high enough to spool the LargeT. You do realize the compound turbos have wastgates too right? They are plumbed in the same manor you're talking about. Sounds like there is really no difference in operation for the exhaust side of what you're suggesting. And in both cases you still run into the problem of two exhaust housings/wheels in the stream of the exhaust which will increase back pressure/EGT, especially if the first is a small housing to help spool.

I'm sure you know this already, but exhaust gas velocity plays almost no part in turbine spooling. It's temperature and pressure differentials that play the biggest role in it. What I was suggesting was that the small turbo not take up 100% of the exhaust gas flow from the manifold, but that the manifold feeds both the small and the large--which is different than all the compound turbo setups I've been shown thus far. The manifold would need a "wastegate" or a gas diverter to ensure that the large turbo would not "steal" all the temperature/pressure of the exhaust, but have just enough to spool when the small is reaching its efficiency limit.

The only difference I see now, is the cold side. In a compound turbo the LargeT cold side feeds into the inlet (where the air filter goes) of the SmallT. This is where the compounding takes place. The boost is "compounded" or multiplied. While in a sequential system, there is usually a butterfly valve to keep the turbo that's "working", flowing only into the manifold, then the butterfly opens and allows the second turbo to contribute to the total volume of air, but it in itself would not increase boost pressure. The difference is much like the following diagram of 2 pumps in series vs parallel. Series would be compounding, and parallel would be sequential.

I'm aware, but the exhaust flow in the compound setup is different from what I'm suggesting. Couple it with the fact that a small turbo has a very limited efficiency range you really become limited in the turbos one will be able to run. Couple this with a rather large increase in AIT and I see a bad time coming. Even if you held the AIT's low by some super intercooler process the pressure levels one would tune for would not be worth headache the system offers.

EDIT: Just saw your direct comparisons, care to elaborate on those plots?