Compound turbo or Bi-turbo set ups

[Smokey Eddy]

compound setups traditionally go like this:

exhaust manifold to high pressure turbo to low pressure turbo to downpipe

air filter to low pressure turbo to high pressure turbo to intake

high pressure turbo being the smaller of the two.

this way the exhaust has less volume to spool the small turbo quickly, and the intake volume after the high pressure compressor is reduced keeping volume induced lag to a minimum.

once the high pressure turbo has spooled and the motor is making enough exhaust gasses to spool the large turbo, the high pressure wastegate opens and the high pressure turbo effectively freewheels.

apparently the risk of overspeeding is low as long as a suitably large wastegate is used to bypass the exhaust past it to the low pressure turbo.

But wont the "low pressure" turbo be spinning the "high pressure" turbo from the COLD side.. traditionally backwards to what you would expect.

If you FORCE (the low pressure turbo will make more CFM than the  "high pressure" turbo can handle) air through the INTAKE of a turbo it's going to rotate it regardless.

We tend to think the turbine only spins from flow going through the HOT side but will the same effect not result from forcing air through the compressor?

Meaning the wastegate will have no effect on the "high pressure" turbo.

Or does this "just not happen"

I'm trying to think of an analogy...

ok think of a hydro dam where you have a turbine inside a pipe. Now divide the pipe into 2 like on our turbos.
You drop water through one side of it from top to bottom and it spins the other side.

Now think if you sucked the water through the other side from the bottom really fast, like faster than gravity would pull the water through the top half... the turbine will still spin on the top, in the same direction it normally would except WAY faster than it normally would...  And any kind of flow restrictor you had on the opposite side of the turbine (meaning the top, the first side) wouldn't make a difference. The top will still spin just as fast because you're forcing it to from the other side.

No?
or am i confused...
I wish you could draw pictures on here.

[truckinwagen]

the high pressure turbo will not be flowing more CFM than it is used to, the pressure at the inlet will just be higher.

turbos work on differential pressure, they don't care what the inlet pressure is, they will just compress it more.
the wastegate is to vent exhaust gasses from one side of the high pressure turbine to the other, so that it is no longer driven, but all the exhaust gasses still go to the low pressure turbo.

google compound turbo cummins and there should be lots of info on the theory and application of using more than one turbo.

there is even a drag semi with a five stage compound turbo system pushing in excess of 300PSI

-Owen

[Smokey Eddy]

300, oh my christ...

I still don't understand how running air through the compressor side of the small turbo isn't going to rotate it's turbine...

[truckinwagen]

oh it does, but apparently it wont overspeed like it will when driven through the exhaust side, the compressor is designed to push air, not be pushed by it.

[Smokey Eddy]

that's so strange but cool how it works like that.

It's like god said unto man, "Here is a turbo. It makes boost. You can also force pressure into it and it will just make more."

[truckinwagen]

most superchargers are just differential devises too, so you can stack them and make more boost without making any of them work harder(outside their efficiency range)

[Smokey Eddy]

That's so crazy! Why doesn't everyone do that and run like just insane boost pressures all the time?
is it inefficient?
I see absolutely no downside to having lots and lots of boost.
Eventually it would be like driving with your thumb down on the NOS button all the time. So long as the fuel turning to exhaust that is spinning the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades to make the boost to burn the fuel to make the exhaust to spin the blades.

Pretty straight forward if you ask me.

[Smokey Eddy]

oh my WORD
Look at these photos!!!
only 2,500 ft.lbs of torque though

LOOK how long the HP is over the 1,000 line...
http://www.dieselpowermag.com/features/ford/0808dp_custom_ford_f250_powered_by_cummins/photo_14.html

[RabbitJockey]

its not really about the pressure.

[Smokey Eddy]

"Adam decided to build a compound-turbo setup that could handle the drive-pressure spikes caused by running four stages of nitrous. Now Adam has an S362 with a 71mm wheel and a 40mm internal wastegate over an S480 with an 80mm compressor wheel. Before exhaust gases get to the turbos, however, they have to pass by a 50mm external wastegate which connects to a 2-inch diameter pipe and dumps directly into the 5-inch downpipe. With no nitrous and both wastegates shut, Adam will see about 90 psi of boost and over 700 hp from the big Ford. With the 50mm wastegate and internal 40mm wastegate wide-open, the truck only makes about 40 psi of boost, but that number jumps to 95 psi and 1,300 hp once the four stages of nitrous are turned on."

That is interesting that he would use so much NO2 and turbo's at the same time?
Would you not expect to see similar results with just "enough" NO2?
The NO2, contrary to popular belief (but i dont have to tell you guys this), does not act as fuel, the NO2 seperates from one of the Oxygen molecules and it is free to react with the fuel.

Exactly the same principal as forcing lots more atmosphere into the cylinders.
Perhaps the added NO in the mix helps spin the turbo's. But then why would he run the turbos with the waste gates wide open + the NO2?
Maybe im just dumb.
Would closing the waste gates overspool even those big turbos and bad things would happen?
His small turbo is 11mm larger [120% the size of mine](compressor wheel that is) than my holset.

[Smokey Eddy]

its not really about the pressure.

Do explain, im listening. =)

[foxracer1]

OK! So i confirmed that the exhaust turbine is indeed 60mm and the compressor wheel was about 75-80mm in diameter.
in the midst of taking the compressor wheel off to give it a good clean.

Edit: I actually managed to get it back on exactly where it was when it came off.

that was my next question. cause theres definitely some big balance marks on that compressor. and it needs to be where it was. else the turbo is gonna go *BOOM*

I have spoke to multiple turbo shops G-Pop in cluded and turbines and compressor wheels are not balanced together. They are balanced indepently. And only need balanced if material has been removed or moved from contact with the housing or bending the blades. But it is a wise move to balance them any time you rebuild a turbo just to be sure.

Also when compounding turbos you do just that you are compounding boost. So 20 from one in to the other making 20 doesn't equal 40psi.

a turbo pushing 20psi will have a pressure ratio of about 2.39
When running twins both turbos should be setup so they push close to the same pressure ratio.

So 14.7x2.39(1st stage)x2.39(2nd)= 83.96 psia or 69.26 psig thats alot of boost for a 1.6 idi even a tdi.

Both making 10psi:

14.7x1.71x1.71= 42.98psia or 28.28psig

maybe a better option but the holset would be to large and inefficent. You'd be better off with 2 stock t3s or a stocker and a merc .48 turbine as the second

Just my 2 cents. I've been looking into this myself.

[foxracer1]

its not really about the pressure.

Do explain, im listening. =)

Its all about the amount of air in an engine not the pressure. Pressure is a measure of air but if an engine flows more it could ingest more air at a lower pressure.

Check out http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

Doesn't have a way to calculate twins but interesting in the calculations. Try different engine sizes and such and see what happens to the cfm numbers.

For our diesels i use 20:1 air fuel ratio and .48 BSFC 74% VE as a baseline.

[rallydiesel]

That's why it's so important to port and use larger valves and less restrictive manifolds with compound turbos.

[Smokey Eddy]

Would my AAZ head do ok?
Im going to port it out like ... almost the maximum tolerances and get it smooth as glass.