sorry to rain on your subie parade... but this is nothing like a twinscroll turbo you know in your import world. (e.g. Evo)

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VTG already provides the functionality of twin-scroll and a bit more. So you can be sure this isn't twin-scroll.

Twin-scroll is one method (like dual entry and VTG) to better utilize the low-velocity airflows at low revs. All 3 work like putting your finger over the end of a hose to increase the force of the stream coming out.

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OK, then explain: "Two-stage-snail"

That sounds a whole lot to me like a bifurcated exhaust turbine housing, that takes input from two ports instead of a common port, for balanced exhaust pulse input, and improved cylinder scavenging fluid dynamics.

VTG is not the same as twin-scroll. They may have similar effects at the dyno, but go about it two different ways.

One improves the fluid dynamics of the exhaust system for inherently more efficient spool-up for the turbine.

The other varies the shape and angle of the turbine to the tapering cross-section turbine housing (usually one chamber in standard turbos, a divided housing in a twin-scroll) with aerodynamic advantages for the turbine itself, to make the turbine efficient at low and high turbine RPMs.

So why not both? If twin-scroll sorts the exhaust pulses and makes the head-to-turbo exhaust system inherently more efficient, and the Variable turbine can further improve turbine efficiency, as well, why not reap the benefits of both?

And like I said, if two-stage-snail doesn't refer to a twin-scroll design, then please enlighten me as to what it actually does, and where the "alternate" design came from, and how it works. Prove me wrong, if you can.

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My understanding is that dual-entry is when the exhaust gas is put through two different entry holes, one large, one small depending on revs. The small would be too restrictive at high flows, but at low flows works to increase the velocity of the air hitting the vanes.

A twin-scroll has two entries, to two different sets of vanes (or slightly different areas on one vane), with the vanes in the areas optimized for high or low flow, again, it redirects based upon flow.

http://www.mazda.co.za/servlet/ContentServer?cid=1163396227462&pagename=Page&site=MSA&c=DFYPage

VTG works like dual-entry, except the intakes are continuously varied in size to be large or small, a continuously-variable dual-entry if you will.

I've heard of this thing with preserving exhaust pulses, but I don't know what it is called. Explicitly mentioned above at Mazda's site is two unequally-sized vanes, making the idea of equal parallel paths for both rotors almost nonsensical.

I don't think a single turbo needs both a VTG and either of the other two low-flow technologies I mention above, it would be redundant.

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That is a stock borg-warner cutaway that was used to show the Porsche 997 Turbo's Variable Turbine Geometry mechanism, which was the first application of BW's design.

That is not necessarily indicative of the Ford diesel's turbo, which is another, new iteration of that design. The turbine is probably very similar, but a twin-scroll turbos would do no good on a 997 Porsche, which has two independent turbos, three cylinders feeding each, which makes twin-scroll turbine housings superfluous.

Twin-scroll is for when you have at least four cylinders or more feeding a turbo, and where the exhaust runners may not be exactly the same length, causing backpressure effects in the header tubes. Separating some header tubes from others tends to straighten the exhaust flow out, not having some pulses crowding others in the runners.

two-stage-snail is not twin turbo, either. That would be twin or dual snails (plural).

With a diesel V8, it probably has a very generous-sized single twin-scroll and VTG turbocharger, with each side header feeding each of the two turbine inlets.

A diesel turbo is fed by a lot of exhaust pressure and heat, but not as much gas velocity as higher-revving gasoline engine. the heat and pressure drive a single bigger turbo just fine, even at low engine RPMs, and that single turbo can put some compressor pressure back into the engine, and the diesel can handle it.

Gasoline turbo engines have heat and pressure, but also more velocity at higher revs than diesels can handle, so smaller turbos tend to do better, at lower compressor pressures, and twin scroll works there even better with exhaust organization, as well as less back-pressure at low exhaust velocities. Moderate single, or even two small turbos tend to handle spool characteristics, and less compressor pressure, but smoother delivery, better.

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So what is wrong with the turbo setup? Seeing as we have four new 08 F-550s with the 6.4L and I have hauled numerous things with them, I can honestly tell you it's a damn good setup. Unlike our 6.0s and 7.3s, the 6.4 has a lot of power off the line and definitely has a lot of power throughout the whole RPM range. Hauling a bed of whatever isn't scary like with the 6.0s and 7.3s. And towing a trailer is simple.

I was honestly surprised by this engine....

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For your uses, maybe nothing.
But the perfomance potential is limited because the air always goes through both turbochargers. There are no bypasses.

It is like balancing a knife on edge. The small turbocharger can not be a restriction to the larger turbo, so the vanes are designed appropriately. So the only thing to give bottom end power is the adjustable guides.

Can I ask at what engine speed does it really start to pull? Ford quotes peak torque at 2000rpm for the 6.4 & the 6.0, while the '03 7.3 was at 1600rpm.
The 6.0 was willing, the turbo was just waking up at 1500, and only pulls from 1750+ (and is a high revving engine, 3750rpm-decent)
http://www.pickuptruck.com/html/2003/ford/superduty/first_drive/page2.html
So with the serial turbo setup, hopefully the torque output under 2000 rpm is much greater than the 6.0.

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Turbos on Diesels aren't exactly the same as turbos on gas engines. A turbo on a Diesel can improve mpg at highway speeds, often significantly. So even if you don't need more power, a turbo on a Diesel can still be your friend.

Although I'm not completely sure I understand why a system only using a turbo to increase steady state efficiency needs to have sequential turbos.

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Funny- its actually the smallest diesel you can buy in a truck here in the States. A 4l V8 diesel wouldn't do any better in the same application. Keep in mind that these are 4-5t trucks and might tow another 8t on top of that.

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For real! An old Ford 6.9 would do me just fine. We don't haul any more today than we did twenty years ago.

Folks new to trucks have been spoiled by power. For the cost of ten minutes more on my trip, I'll skip the "sucker" payment! ;)

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