By this I mean a large turbo that's "boosting" a smaller turbo.

In my turbodiesel hobby, we've been doing this for a long time. Typical setup is a 62mm inducer for a "small" turbo and then something in the 95-110mm range for the "big" turbo.

End result is reasonaby efficient boost of 60-100psi with mass flow in the 110lb/min range (again, depending on turbo selection)

Now, there's no need for such ridiculous boost levels on an LSx engine as far as I can tell. The Cummins has pathetic head flow of ~220cfm, hence the high boost numbers.

That said, I've become a big fan of the compound setups as they are a very good way to increase efficiency and give you more air and less lag than either a large or small turbo could do by itself.

Anyone doing compound turbos on an LSx?

 

Not that I've seen, but even if someone figures out the configuration in a typical F-body or Corvette engine bay, the turbos, especially the booster turbo, would have to be TINY as hell... That's my guess.

On that note, it is a wonderful thing to hear compound turbos screaming!!!

 

With 4 head bolts per cylinder, I don't see any of us taking advantage of compounding any time soon.

 

other than boost at lower rpm's, because of the smaller turbo, at the same boost level.

 

I believe this setup was used on stock twin turbo Nissan Skyline GTR with no insane boost numbers.

 

Compound on gas engines doesn't work very well. The boost pressures for a diesel far exceed what is needed on a gas engine. Most gas engines will come apart with more then 30psi and a big single will easily make 50psi without a problem.

Also most stock cars like the skyline, Supra and RX7 were sequential, and not compound.

 

For example, you can use two 2-1 pressure ratio turbos inline, the first would raise to like 15psi (for example) and the second would raise to 45psi of boost with no more effort. Turbos are designed not for a particular boost level but more around a CFM vs pressure ratio base.

By the way... no need for a very small turbo feeding a very big turbo... even equally sized turbos would work well one feeding the other.

 

absolutely wrong.

 

that makes sense

 

I don't see how an identical turbo can be used to compound another. If these identical turbos both have outputs of 35lb/min, how can you see any appreciable gain in air mass flow?

If one turbo is putting out 35lb/min, it's going to have to draw in something more than that, say 40lb/min to account for a less-than-perfect compressor. Now, we have the problem of one turbo trying to draw in 40lb/min from another turbo that only puts out 35 lb/min. Hence, it will be drawing a vacuum from the turbo that's supposed to be "boosting" it.

Moreover, the enthalpy available to the farther away turbo (usually the one that's boosting the main turbo) will be smaller, as the the main turbo has already drawn some energy from the exhaust stream in the form of heat and pressure.

As I see it, you HAVE to have a turbo that's at least double the mass flow of the smaller turbo to have an appreciable compounding effect. For example, if I have a small (primary) turbo that's 35lb/min, then if I'd need to have a 70lb/min compressor to feed it. This will actually pressurize the small turbo's inlet to something around 2 bar (assuming perfect carnot efficiency and adiabatic compression and all that).

If I'm wrong here, I'd like to know how because I've read a lot on this and would have missed a MAJOR point in setting up compound turbos.

Justin

 

Sorry I made a mistake in my writing...

From what I understand (I work on turboed sleds but not for that long) is:

If you want 300cfm at 45 psi , then you will definitely want:

-The first turbo to flow 600 cfm at 2-1 ratio but at same time will need a somewhat smaller A/R turbine housing to make for the exhaust 'energy' lost to spin the the second turbo...
-The second turbo will need to flow 300 cfm at 2-1 ratio...

That is my undertstanding of the thing.. sorry for my mistake I wrote this way too quickly