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?

http://www.worldcarfans.com/news.cfm...ine-technology

 

nice find .... simple....

 

Could you do a similar thing with a small clutched Supercharger and a big Turbo? Less plumbing on the exhaust side and about the same on the intake side. It would also relieve the space problem.

Then again, I know nothing about engine technology so I should just shut up.

 

very interesting stuff

 

compound turbo setup?

 

More or less but with a twist.

 

The Volkswagen Golf GT uses this. Along with Direct Injection. Below 2400rpm all the boost comes from the Supercharger. 2400rpm to 3500rpm both the Supercharger and the Turbo are providing boost. Above 3500rpm only the Turbo.

There was a 1 page write up in the July 2006 issue of Car and Driver. It was the same issue as the PTP Turbo kit article.

 

no joke... all through high school, this was exactly how i thought a twin turbo setup worked.

 

sequentialish compound

cool beans, i'll get started next month :D

 

hehe cool

 

so in summary when the first smaller turbo hits it limit on the hotside of the turbo a wastegate bleeds the excess and the hotside air from the first turbo into the hotside of the second larger turbo. Sounds easy enough though youd probably have to do some R&D on selecting the proper size turbo and determining the most efficient wastegate pressures. This will proabably be the next big thing in twin turbo's. Who knows supra's may even make power below 5,000 RPMs...

 

Turbo sizing is the hard part but I suspect it won't be too bad. There's no reason to not use the same compressor side on the large turbo that a large single would be. The exhaust side would be larger though. The small turbo compressor side probably wouldn't be too ahrd to figure out either leaving only the exhaust side in question. As a start to get in the ballpark, I'd size this turbo for midrange power on a 1 rotor engine. That's just a guess though. Once you've got them compressor trim figured out you can probably even guess the exhaust trim that will work and just vary the exhaust a/r housings as an adjustment.

 

Okay, so at high RPM some exhaust is still flowing through the small turbo, but the intake is being routed around it's turbine? Why not just close off the exhaust valve to the small turbo at the same rate that the valve for the intake closes? Seems wasteful to spin it for no reason.

edit: I just re-read it and I think you're implying this, though you don't state is specifically like you do for the intake.

 

Going back and reading this closer...

How does this differ from the 3rd Gen RX-7's? I thought that was the hype about the Twin Turbo 7's was that the Turbo's were sequential. One small Turbo for reduced lag and a larger one for higher rpms, just like you are describing.

I'm no expert but I thought this was the case.

 

Sequential Twin-Turbo

Sequential Twin-Turbo refers to a set up in which the motor can utilize only one turbocharger for lower engine speeds, and both turbochargers at higher engine speeds. During low to mid engine speeds, when available spent exhaust energy is minimal, only one turbocharger (the primary turbocharger) is active. During this period, all of the engine's exhaust energy is directed to the primary turbocharger only, lowering the boost threshold, and increasing power output at low engine speeds. Towards the end of this cycle, the secondary turbocharger is partially activated (both compressor and turbine flow) in order to pre-spool the secondary turbocharger prior to its full utilization. Once a preset engine speed or boost pressure is attained, valves controlling compressor and turbine flow through the secondary turbocharger are opened completely. At this point the engine is functioning in a full twin-turbocharger form, providing maximum power output. Sequential twin-turbocharger systems provide a way to decrease turbo lag without compromising ultimate boost output and engine power. Examples of cars with a sequential twin-turbo setup include the 1993-2002 Toyota Supra Turbo (JZA8x), the 1992-2002 Mazda RX-7 Turbo (FD3S), and the 1986-1988 Porsche 959. With recent advancements in turbocharger design, sequential twin turbo systems have fallen out of favor because they are seen as unnecessarily costly and complex.


http://en.wikipedia.org/wiki/Twin-turbo

 

I think the difference here is that the new system uses 2 different turbos and work somewhat independently at the extremes of the RPM bands giving you maximum efficency.

I know damn sure id rather have a big single spooling 5-8k and a smaller turbo spooling 2-5k then 2 mid size sequentials turbos running in a true series. This is all assuming of course that I know what im talking about...

 

Sequential twins such as the 3rd gen RX-7 run them in parallel to each other. This runs them in series. In other words in series you ultimately run one through the other but in parallel you run them next to each other. In parallel, air from one turbo never flows through the other one. The 3rd gen system needed several actuators, electric vacuum relays, and over 70 vacuum lines plus computer control to work. They also needed to spin at insanely high rpms which made them inherently unreliable. This needs 2 valves. Each turbo is in it's sweet spot. It's elegance is it's simplicity.

At high rpms, this completely bypasses the intake and exhaust side of the smaller turbo which effectively takes it out of the picture. There is now equal pressure before and after each turbo wheel. It may still spin but it makes no power. At low rpm's the small turbo works and even though air passes through the large turbo, there isn't enoug flow through it to make any boost. This means that it effectively isn't working at low rpms. At the transition point they each contribute in a small way.

 

very interesting stuff. thanks for this

 

I see. I've never actually had a good look under the hood of a 3rd Gen 7. I just assumed that since it was called "Sequential Twin Turbo's" that it was in series. To me "Sequential" implies an in series setup. I didn't realize that they were in a parallel setup and exhaust gas directed towards the appropriate turbo.

Learn something new everyday.