[vex]

Originally Posted by dudemaaan

I'm not sure I really followed what you said vex... The 60-1 is a pretty decent size turbo to be using for fast spool, it works well as a medium size turbo and is a good compromise between power and lag for a single turbo setup. Not ideal for instant response, but that really makes no difference for the theory....

Sorry, I'm on cold meds and my thought process will be fuzzy for a few days until I'm over this. I just used the 60-1 and the other Turbo as examples of their respective comp maps.

If you're saying have a small turbo basically setup like normal, and then a big turbo that's run off of the wastegate exhaust of the smaller turbo... I don't think that would work well. The amount of exhaust energy coming out of the wastegate pales in comparison to that coming out of the turbine outlet. The large turbo would have very little exhaust pulses to get it moving, and would probably contribute very little extra air to the engine.

No, not set up like that. You have both turbos plumbed such that the first turbo (60-1 in the example) received a majority of the exhaust stream to ensure quick boost build and response. The remaining exhaust would be routed in through the large turbo so the exhaust flow would still be causing the larger turbo to spool. As soon as the smaller turbo starts to eek out past its efficiency range or the desired pressure is reached, the exhaust is then diverted via an external wastegate to the large turbo stream. The temperature difference should be negligible to allow the large turbo to spool to the desired pressure.

Next, the large turbo would be a restriction to the wastegate which would almost certainly cause an over-boosting situation for the small turbo.

That really depends on the wastegate and how the pressure in the exhaust system will behave. With nominal exhaust temperatures the large turbine should not be the restriction in the exhaust flow, but rather the small turbo will be and as such the diverted exhaust gases from the manifold, and from the wastegate will feed to the large turbine, causing the large turbo to increase energy conversion.

The other system you mentioned, which seems to be separate from your first idea, is generally called a turbo-compound system. Where a series of gearing is attached to the compressor instead of a compressor wheel. This system adds so much complexity, and would only work well in a diesel application where massive amounts of exhaust gas and pressure is created.

It's actually been used fairly effectively in aircraft applications as well. Though the gearing I don't think would be that complex. A simple unit similar to a transmission should allow for easy application to a car. From the picture I posted it is actually from a Volvo engine.

Compound turbocharging (like we have been discussing), where one turbo flows into the other would make a very flat torque curve with properly sized turbos. With a small primary turbo you could have full boost before 2000 rpms and with a properly matched larger turbo maybe a gt42r you could make well over 700 hp if desired. I think you will lose a little engine efficiency from the added exhaust restriction, but the additional power made by the setup will vastly exceed anything lost. I would be concerned with backpressure creating too much heat and excessive EGT's. Only testing would show if this is an issue.

I think it will be a vary large issue and would need to be evaluated prior to doing any testing. I think it's all in the sizing as you and others have said, though I see it becoming a much larger problem as there is no by-pass for the exhaust. Looking at my friends tuned sequential FD the torque curve was ridiculously flat, and I honestly think that's where the best performance is going to come from; something similar, but with a much more scrutinized design.

Another idea is to use a roots style supercharger for instant boost, and make it a compound setup using a larger turbo. This would actually remove restrictions in the exhaust since you can use a larger A/R housing, and the supercharger will give you instant response and boost. I personally think this is the simplest and most logical solution to having your cake and eating it.

This idea has merit, and I agree.