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An interesting twin turbo setup

Old 08-01-2006, 08:23 PM
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For years people have asked if they could do twin turbo setups that used a large turbo for top end and a small one for low end. Repeatedly the answer has always been somewhere along the lines of yes anything is possible but it's not going to be easy to fab up and the transition will be very difficult to do. Even I have given this response on a few occasions. That is of course because we have all been thinking inside the box when the answer has been right in front of us. There is a way to do it with minimal effort as long as you can find the space. I hate to say this but GM engineers have actually thought of a way to do it easily. Of course they are doing it through their Opel partnership in Europe and on a diesel but the idea is still brilliant. I am kicking myself for not thinking of this.

The problem has been that everyone has been thinking in terms of parallel turbo setups like the overly complex 3rd gen RX-7 turbos when they should have been thinking about running twins of different sizes in series! The diagram I am attaching below shows the necessary way to do the piping and where to put the valves needed for the transition. Interestingly enough you only need 2! After thinking about it I can even see a way to make them work properly off of only air pressure of the turbos alone. I can even think of how to size the turbos so that you don't need a separate wastegate to bypass the second turbo although it wouldn't be too hard to make one work if you did need it.

This is actually very cool. All of your exhaust goes through the small turbo only. It spools up quickly. The exhaust leaves this turbo and flows into the 2nd larger turbo but due to it's size isn't traveling fast enough to make any usable boost. As the smaller turbo hits it's max boost pressure, the valve in the exhaust side starts to bypass some of the exhaust air around it just like a wastegate does. The difference here of course is that it is still going through the second turbo. Now the larger turbo starts to spool up and faster than it would have on it's own as there is more air passing through the engine from boost. The more exhaust that comes out of the engine, the wider this valve opens until full open is realized.

Meanwhile on the intake side all of our air has gone into the larger compressor first. Then it goes out and into the smaller turbo. As the larger turbo starts spinning and making boost, a valve opens which starts to bypass the air out of the first turbo around the second one into the engine, thereby keeping total boost pressure down. As this turbo winds up, the valve opens all the way diverting all airflow around the compressor section of the second turbo. Now pressure is the same in the front and back of the compressor and exhaust sections of the smaller turbo and it is effectively not doing anything anymore. The larger turbo has taken completely over.

At low rpms, the small turbo did it's job. There is a smooth handoff assuming the valves work properly. For a time both turbos will be contributing. At high rpms the large turbo is doing all the work.

Opel shows the use of 2 intercoolers but one can probably work just fine. I'd love to see someone do this on a gasoline engine. Imagine doing this in a Supra. Now you can have a dyno queen that makes tons of high end power but also have a good lower and mid powerband that would finally make those setups drivable and usable on the street.

I think the idea is great. It may have drawbacks sucah as space requirements but I definitely see performance potential in it over any other turbo setup. This is a way to in a sense simulate a variable area turbo without actually trying to find one that is sized right and can actually stand up to the heat. The difference being this is keeping the intake and exhaust sides of the turbos in their efficiency ranges as opposed to a VATN only altering the exhaust side. They'd have to also vary the intake as well to have the same potential.

I like it. What do you guys think?
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Old 08-02-2006, 01:56 AM
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That looks very interesting and viable. One thing I don't understand is why at low/mid rpms does the smaller compressor need to feed off the larger comp? Why not have that intake valve seperating the two compressors, instead of after the larger comp? Does that make any sense?
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Old 08-02-2006, 09:16 AM
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Very cool. I saw a turbo diesel once that had one huge turbo that was fed my a smaller turbo. Only one that I have ever seen. This seems more viable.

Turbo sizing would have to be just right in order for the flow to be right though.
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Old 08-04-2006, 12:27 PM
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Compound twin turbo setups have been common in diesels that needed very high boost for a while. High boost with a single turbo results in a pressure ratio that is out of the best efficiency range of most turbos. The solution was to run one turbo feeding into the other. If you have one turbo feeding 15 psi(2:1 pressure ratio) into the inlet of a second turbo running at a pressure ratio of 2:1 as well, you end up with 30 psi at the engine, while the turbos are operating more efficiently than a single turbo at the 3:1 pressure ratio necessary to get 30 psi out of it.

All that Opel did was add a check valve to the intake side of the second turbo which effectively makes the turbos operate sequentially.
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Old 08-04-2006, 01:59 PM
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