Twin Turbo Question

So I saw that thread that was made a little bit ago about the GT with the twin turbos and it made me think a little. Why exactly do two turbos create higher pressure for the forced induction? I understand pretty much how turbos work so my understanding of physics and pressure must be a little flawed. The two turbos use the exhaust to spin a rotor that powers a compressor. In the twin turbo setup I'm guessing that each turbo has its own bank to use the exhaust from. That means that each turbo has "X" volume of exhaust running through some conduit of diameter "Y". In the single turbo the rotor is spun by the exhaust from both banks , meaning the rotor is spun by "2X" volume of gas through a conduit of diameter "Y". Doesn't that mean the single turbo will have its rotor spinning twice as fast as each rotor in the twin setup because there is more gas running through a conduit of the same size so the velocity is greater? and so the compressors will ultimately deliver the same amount of compressed air to the engine in each setup?

There is a good chance that my thinking is pretty flawed but can someone clarify the idea for me? thanks. and sorry for the long post but i felt you all should be a part of my daily car-related ponderings...

 

[Jakee]

To put it in simple form, two turbos are picked so that smaller turbo's can be used. What this does, when set-up correctly, is you'll be under boost thru-out the entire RPM range, as opposed to some single turbo set-ups, where you don't have boost until a certain RPM. Also, you can have less lag on some twin turbo set-ups VS a single turbo. It's NOT about pushing more boost into the engine (In general) because most single turbo's can give way more boost than an engine can handle.

Also, not all vehicles will perform better with a twin turbo set-up. A good example is the 4.0 SOHC. A twin set-up on these motors does nothing. You can get a good correct sized single turbo and get the same results.

There are some pretty smart tubo guys in this section, so If I've said something wrong, I'm sure I'll be corrected, but this is what I've learned with the research I've done this far.

 

[MountaineerGreen]

At SEMA 05, I saw a twin turbo Focus. The builder explained it to me patiently although he had obviously been asked before many times. There was a larger turbo and a smaller turbo, the smaller one spools up faster, the larger one comes up slower, but after the smaller turbo passes its peak. So, in short, no turbo lag = fast from the beginning.

 

[Spdrcer34]

I saw an ad for the new F-Series truck...

It read:

Even our Turbos, have a Turbo!

It does exactly what MountaineerGreen said. Small turbo builds boost fast, as it peaks, the larger turbo picks up the slack and builds more. The picture on the ad is pretty cool ,with the 2 Turbos mated together the way they are...

Pretty cool!

Ryan

 

[steventadams]

Alright that actually makes a lot of sense. The little turbo goes first and then the big one picks up where the little one left off so you get power through the entire rpm range. I guess I just assumed they would always be the same size. Thanks guys.

 

[Exploderpilot]

What you guys are talking about is sequential twin turbos. the smaller turbo "feeds" the bigger turbo, to avoid lag. there are however, straight through twin turbos, where each turbo is on it's own bank, but with both simultaneously feeding the intake.

 

[steventadams]

is one setup more efficient/advantageous than the other?

 

[justin146]

is one setup more efficient/advantageous than the other?

-Twin turbo- two symmetrical turbos
-compuound turbo- small turbo feeds large turbo
-Sequential turbos- one small turbo for low rpm, then large turbo takes over

All of these can be called "bi turbo"

if PROPERLY DESIGNED these can be more efficient than a single large turbo. The compound design is used for really high power diesel applications because one turbo alone cant create the pressure needed. The pressure of the first is multiplied by the pressure ratio of the second(not added).
The new 6.4 Powerstroke uses sequential turbos to help combat turbo lag. The RX7 used this design too. The small one of the two spools fast then a valve diverts exhaust to the large turbo.
With twins, it is usually just for quicker spool. It is usually not to make more power, unless you are talking really high horsepower cars that are using twim 100mm or larger turbos. These guys use twins because there isnt a single large enough to make their power goals(3000hp+)

 

[Jakee]

so what's next? A turbo for each gear?

 

[IZwack]

A turbo for each wheel.

The wheel is bolted to a portal box with a ridiculous gear ratio and the final top gear is splined directly to a turbo. And of course there are four turbos -- one per wheel. This eliminates both the transmission AND the transfer case, which means there's less energy wasted due to friction :


Portal box:

 

[MountaineerGreen]

I am guessing military stuff?

 

[Jakee]

cool - so the fitting up top is not only a grease fitting, but a blow off valve too? heheh

 

[IZwack]

cool - so the fitting up top is not only a grease fitting, but a blow off valve too? heheh

Woot! We're on a roll here..

That blow-off valve, can have a bypass that would go directly to the wheels (like the H1 wheels with the integral filling system) - controlled with a solenoid. And so you can wheel with 0-PSI and at the end of the day, actuate the solenoids and refill the tires in no time at all!

I am guessing military stuff?

Most Portal boxes are -- like the Mercedes/Unimog 404 axle, the Volvo C3x series, and of course the Hummer-1 IFS/IRS Portals

[/thread hijack]