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Discussion Starter #1
There's been some interest in this in a few other threads and in PMs, so I'll ramble on a bit about it here. Any questions, comments, advice, ridicule, etc, are always welcome. :)

This is the same concept applied to diesel motors, and different from twin turbo, sequential turbos, etc. Diesels do it to achive high pressure ratios, on a gas motor I do it to get the spool of the small turbo but the flow of the large turbo. The implementation is the same, but the tuning is a little different. I know of at least one other Talon and a Ford pickup of some kind running compound turbos on a gasoline motor.

The general idea is that the small turbo (secondary) gets exhaust first, and its exhaust outlet and WG outlet both feed the turbine housing of the big turbo. The big turbo (primary) sees intake air first, compresses it, and feed it to the small turbo's inlet to be further compressed.

The small turbo sees exhaust first and spools as it would if it were on its own. In my case, it's a t3 50 trim and it spools around 4000 rpm. Once it spools exhaust gas output goes up, eventually enough to spool the big turbine downstream. To help describe this, consider that my 2 liter with 15 lbs of boost applied puts out the exhaust volume of a 4 liter, roughly. At 30 psi, a 6 liter. This is how I can spool a 1.32 T6 s475 somewhere between 4 and 5k rpm with a 2 liter 4 cylinder.

On the compressor side, I run both turbos at a 2:1 pressure ratio, which is the equivalent of about 15 psi around sea level. Since the pressure ratios are not additive, but are multiplied, you end up with 45 psi instead of 30, and neither turbo is breaking a sweat. :)

A picture is worth a thousand words, and this is one of my favorite examples. In this case the top turbo is the small one and the bottom turbo is the large one.



And here's some shots of my setup.



http://i116.photobucket.com/albums/o23/kjewer1/Junk/rwdturbo004.jpg

You can see the small turbine outlet and it's WG outlet merge into the T6 flange.
http://i116.photobucket.com/albums/o23/kjewer1/Junk/100_1135.jpg

I ended up adding a second WG on the small turbo to keep it under control.
http://i116.photobucket.com/albums/o23/kjewer1/Junk/December008.jpg

Here are the first two full track runs on this setup. My previous best on the single turbo ( 1.01 T4 GT4294) at the same boost was almost the same ET and trap, but I needed nitrous to get it to spool. Now I can spool the much bigger turbo with no loss in performance without worrying about nitrous.

http://www.youtube.com/watch?v=vhiKLm9E6UA

Now that I've proven it works and I'm happy with the setup, I have to copy it all over to stainless and get some new pictures, I've changed a few things in the design since these were taken, and have a few more to make that should really improve efficiency and spool up, and really take advantage of the benefits of this type of system. This has been a big learning experience, and a lot of fun. :)
 

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Re: Compound tubo charging

Very cool setup man I thank you for all the help you have given me and dealing with my stupid questions on the subject! Let me know when you get that beast to the track again!
 

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Re: Compound tubo charging

Thanks for the write up and pictures, thats really cool. You dont hear of people doing that on gas, nice someone is pushing the limits!
 

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Re: Compound tubo charging

looks good. i've never really knew too much about this sort of setup, but in my head i always thought that the smaller turbo would create a bottleneck in the system. but obviously it works. thanks for the write up.
 

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Re: Compound tubo charging

sweet setup, I helped to the math and other stuff on paul volks setup. I've been wanting to build a compound setup for my 2.3l but haven't been able to afford it quite yet. have you done any egt, or drive pressure logs? I'd love to put one together and dyno it at work, just for the information we could learn.
 

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Re: Compound tubo charging

so do both gates dump into the bigger turbos housing? so its like you bypass the small turbo once its at its boost level? how do you control the big turbos pressure?
 

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Re: Compound tubo charging

you have one gate before the smaller turbo which dumps into the inlet of the larger turbo, then there is a gate at the inlet of the large turbo which dumps to atmosphere.
 

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Re: Compound tubo charging

have you ever logged temps on the compressor side between the turbos and in the intake?i am curious if it would be lower than using a single turbo that you would have to push harder to get the same amount of airflow.
 

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Re: Compound tubo charging

I've done this math over and over again, temps can be lower in a compound setup, but it GREATLY depends on where in the efficiency maps both turbos are... I had planned on running each turbo at different PRs to try to keep them as efficient as i could... if I have some time to I'll dig up all the math i did awhile back.
 

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Re: Compound tubo charging

At what point or when would compound turbos be better then twins? Assuming both set-ups are properly sized for the combo? Or is compounding mostly for the real small CID guys like 4 cylinders?
 

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Discussion Starter #14
Re: Compound tubo charging

It does seem like the small turbo will be a restriction, but in the end it's not. It doesn't know that you're force feeding it since it only cares about moving volume and can't tell what the density is. We all know what happens to a turbo between sea level and 5000 feet, this is like going 5000 feet below sea level.

In my case I ended up with two Tial V44s bypassing the small turbo feeding the big turbine. Boost control on the big turbo is just one V44 (off of the back small turbo V44) dumping to atmosphere. Boost from the big turbo is fed to the top port of the small turbo WGs so that as the big turbo starts to make boost, the small turbo comes up with it 1:1. It's all automatic, the only boost I adjust is on the big turbo. The small turbo is based on its spring pressure, and adds 27 psi to whatever the big turbo makes. Boost pressures are additive in this configuration.

I only measured temp at the manifold so far, and the post IC temp at 40 psi was identical to post IC temp on the single turbo at 40 psi. Post IC temp is too watered down to really be sure though. I measured back pressure on the last day of the year and it's just over 1:1, but, the sensor I was using was just starting to clip at peak back pressure. I'm going to swap in a bigger sensor and test it again this year just to be sure the data is good. The back pressure even being close to 1:1 is amazing, this is the big gain here. I think on the air temp side, there won't be appreciable gains until you get into interstage intercooling, which is where I'll be going with this setup this year. Along with lower temps, there should also be some further reductions in drive power, and with any luck, I'll catch it all on the sensors.

Paul's car is sweet, I can't wait to see more results from it. For everyone else not familiar with it, it's a second gen talon/eclipse with a 62 lb turbo feeding a 42 lb turbo. Same compound turbo setup, but geared more toward a street car. 600+ hp with spool up around 3500 rpm. It was actually all of the naysayers in Paul's thread, and some comments from kiggly, that motivated me to try this and see for myself what would happen. :)
 

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Discussion Starter #15
Re: Compound tubo charging

At what point or when would compound turbos be better then twins? Assuming both set-ups are properly sized for the combo? Or is compounding mostly for the real small CID guys like 4 cylinders?
I think 4 cylinders lend themselves to compounding, since twins really aren't an option with one bank of 4 cylinders. It had been tried over the years, and I've never seen anything good come of it.

Compound turbos on gas motors are done for spool, which usually isn't much of an issue for V8s. And there are some downsides to it as well, mostly in wieght, complexity, and cost. There were times when I was ready to scrap all this junk and just go back to a big single and nitrous. :)
 

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Re: Compound tubo charging

Compounds are really slick. Their forte is high pressure ratio - mathematically you end up multiplying the pressure ratios. The OP mentions running both at 2:1 to gain final PR of 4:1. It's not a stretch at all to run PR of 3 on each unit....getting you to over 100 psi real quick!

A huge portion of the diesel & alky pro stock tractor pullers run a 2-stage compound setup with 3 chargers - 2 smaller chargers act as one gigantic primary (low pressure or atmosphere turbo), with only a huge charger for the secondary (high pressure turbo). 200 psi manifold pressure.....yeehaw!

There are a few guys running a triple setup on a Cummins in a Dodge pickup and it's pretty neat.

Very interesting to see a gasser running a compound setup.
 

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Re: Compound tubo charging

the one thing i never could fathom is how the little turbo doesnt explode with the higher volume of air going through it at higher boost. i understand that you are bleeding off the exhaust volume before the turbine on the smaller turbo, but is the incomming compressed air from the larger turbo going to try and over speed the smaller impeller of the small turbo and put that small turbo on some extreme pressure differentials between the cold and hot sides ? basically how does the smaller turbo not overspeed ?

and i think i might try this setup on my dodge. it has a gt3782bb now and i have an innovative gt80r that no one wants to buy lol. i always thought that would be a wicked setup.
 

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Discussion Starter #19
Re: Compound tubo charging

the one thing i never could fathom is how the little turbo doesnt explode with the higher volume of air going through it at higher boost. i understand that you are bleeding off the exhaust volume before the turbine on the smaller turbo, but is the incomming compressed air from the larger turbo going to try and over speed the smaller impeller of the small turbo and put that small turbo on some extreme pressure differentials between the cold and hot sides ? basically how does the smaller turbo not overspeed ?
This is probably the most asked question when it comes to this setup, and was one my key questions when I considered building it myself. The point to remember is that the small turbo is not moving any more volume of air that it otherwise would. I touched on it above with the 5000 ft below sea level example. To use some conveniently round numbers, let's say my small turbo is rated at 500 cfm, which ends up being 50 lbs/min when fed air at atmospheric pressure. If you then use the big turbo to raise the pressure to double atmopheric pressure, or 15 psi boost, the density of that air is doubled. That same 500 cfm, which the small turbo still moves, now weighs 100 lbs/min. The small turbo still moved the same exact volume of air, but with it being twice as dense, it's twice the amount of air, and provides twice the horsepower. But it still only takes roughly the same drive power to move that air, as evidenced in the low back pressure. This is where all the magic happens. :rock:
 

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Re: Compound tubo charging

It's all fine for the small turbo, until you blow a charge tube or band clamp or a boot....

I bet the little bugger goes TO DA MOON! :shock:
 
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