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Twin Scroll Turbo and Headers

OK, I have read about a twin scroll turbo and why it is an advantage many times and never quite got it but I think I have had a brake through moment.

First, a 4 stroke motor must turn 720 degrees to complete a full cycle of every cylinder.

Second, a stock 930 cam is open for 200 deg of exhaust duration, an SC 220 deg, and a C2 230deg.

Thus, in 720 deg of motor rotation there is an exhaust valve open at least 1200 degs of the time! That is , 40% of the time, or more, tw

That means that 100% of the time there is a 40% or more over lap where two exhaust valves are open at the same time with a 6 cylinder motor!!!

On a connected exhaust manifold, any time there is overlapping of exhaust events, pressure will be elevated in the exhaust manifold and this will inhibit the exhaust cycle of the closing valve.

Now the fun stuff:

If we split the motor to into what is principally two 3 cylinder even fire motors things can improve dramatically.

With a split system having three cylinders on each side we then have an exhaust valve open 600 deg out of 720 degrees of motor rotation.

This gets us no over lap of to exhaust events!

Now we can get more exhaust out of each cylinder for less contamination of the incoming charge, it will run cooler, and we can even run more timing for a further increase in motor efficiency (VE) at the same boost levels.

Potentially important stuff with significant opportunities to improve our performance.

Cool!
Old 06-01-2010, 03:14 PM
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I suspect that if we identify a proper sized split scroll turbo and divorce both sides from each other we will see an improvement in performance and I now understand better why.

Of course a fully split system would be the best. That being a twin turbo system.

A split turbine turbo on an equal length would also be a best of the best system and easer for most of us to obtain.

Following this further and working toward divorcing the two sides of a 911 motor could generate several new products opportunities.

By just modifying one's old B&B style headers one would be able to fully eliminate the 'overlapping of exhaust events' for a potentially solid bump in performance.

Even the stock J pipe 930 system. If we replaced just the J pipe with separate tubes and waste gates to the split turbine turbo we would also be eliminating the overlapping exhaust events issue.

I suspect even on a stock 3.2 NA Carrera converting from a joined to a split system, including muffler, could make for a bump in performance.
Old 06-01-2010, 03:28 PM
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A twin scroll is a twin turbo with a common compressor. The power pulses from each bank are both transferring exhaust gas momentum to turbines linked on a common shaft but at different times so the spool-up is more linear and slightly faster. This benefits the compressor on the same shaft.
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Old 06-01-2010, 04:55 PM
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Flieger,

Agreed.

The benefits told are it improves spool and also that it extends the turbine's efficiency up top.

I had read that many times but did not really feel like I got it other than it was a mechanical advantage at the turbine.

Not that it also might make for in increase in VE.

Splitting the two banks on a flat 6 seems like it make a difference in clearing the cylinders over a joined system.
Old 06-01-2010, 06:22 PM
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Crotchety Old Bastard
 
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Keith this is what we did with my headers. If you take this line of reason one step further and add equal length primaries and secondaries you not only alleviate overlap but line up the pulses resulting in a tuned system. This style of header makes great power as we have seen, and a beautiful sound. You can utilize the benefits using a conventional turbo as well. The key is to have the plenum divide completely flush with the turbo flange. Many headers stop short of this.

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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 06-01-2010, 08:08 PM
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Brian,

I keep learning but here is what I believe at this juncture:

A common manifold equal length system by its self can not stop cylinders from seeing exhaust pulse overlap on a 6 cylinder. --- It is mathematically impossible.


Equal length seems a solid improvement.

A fully split system seems a solid improvement.

Equal length with twin scroll turbine would seem even better.

Subject to there being an ideal split scroll turbo for one's goal and application.


Quote:
Originally Posted by RarlyL8 View Post

...The key is to have the plenum divide completely flush with the turbo flange. ..
Only if there there are divided wast gates and the turbo is a true split scroll design.

Last edited by 911st; 06-01-2010 at 08:50 PM..
Old 06-01-2010, 08:47 PM
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Crotchety Old Bastard
 
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No you do not have to divide everything to see some benefit, there are benefits to using any part of this design with a conventional turbo.

OPTIMAL is a different story and we have done custom headers that use it all. That includes equal length primary and secondary, dedicated waste gate to each bank, split plenum flush with the turbo flange, and the twin scroll turbo. Essentially you have an equal length twin turbo system that comes together at the turbine outlet.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 06-02-2010, 06:55 AM
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Your system is in the best position to make a split scroll equal length happen.

It would be interesting to see a controlled test comparing a non equal length split and an equal length split system.

Though I do not see anyone going to the effort.

However, Turbo new car makers seem to be going to split systems on most there 4 & 6 cylinder cars.
Old 06-02-2010, 08:19 AM
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A complete split system and true equal length tubes per cylinder are separate designs and each has unique merits and influence on power delivery.
You can compare equal length vs unequal and split vs merged and then extrapolate equal split vs non-equal split. I'm dead certain that equal split will yield the best results. How much better depends on many things.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 06-02-2010, 12:59 PM
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Brian,

As you just said, equal and split are two distinct and different systems and I agree.

We both have now stated that combining both technology's has a high expectation of being synergistic. I to firmly accept this.

However to me, though it may be a good idea, adding the division between the secondaries just at the turbo as you pictured above has nothing to do with any of the benefits associated with a 'split scroll' system.

I can see that it might positively benefit reducing turbulence at the collector.

The attributes associated with a split scroll system are 1) separate intersection of the exhaust pressure's at the turbine wheel and 2) no common connection to that point. One might try associating just one of these attributes alone in the hope of benifiting. However, it probably would not be prudent to try to guess or 'exrapolate' if one might have any positive effect without the other.


We understand you make great headers, we all know you make a great 'trully' equal length system, and thanks to your efforts I think most here should know you have the flexability to make them in to a split system to. We are lucky to have you to supply such and I hope you are very successful so they will keep being available.

However, that is not the question but thank you for reminders.

The question is -- Twin Scroll, better or not, by how much, and 'why' is it better?
Old 06-02-2010, 04:06 PM
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Twin scroll is best...period, where possible.

A fully split AND equal length exhaust system is the only (feasible) method for harvesting maximum exhaust pulse energy to improve efficiency and spool performance (there are theoretical methods for achieving this otherwise, which I won't bore you with). Just like wrapping your exhaust to insulate it and keep more heat energy (and minimize exhaust gas contraction and thus keep gas volume at its highest) in the exhaust tubes for best performance, maintaining the sine wave integrity of the exhaust pulse with a minimum of wave clipping (which destructively misapplies this energy in directions you don't want it), gets more energy to the turbine, as opposed to wasting in places of no or (negative) benefit to our purposes.

I have read a lot of technical discussions and documentation on these points, which is where I draw the above high points from. The Subaru and Mitsubishi enthusiast sites are full of deep and technical to light and anecdotal information on twin scroll; I'll post links here as I dig them. For now, here's a brain dump of "the state of the union of twin scroll" today:

There are numerous permutations of turbos that can be described as "twin scroll", all of which have different levels of 'discreteness' in terms of how clean they keep the exhaust pulses in pursuit of extracting the "free" additional energy in the exhaust stream. As a result, by designing the exhaust system to harvest unmuddled exhaust pulse energy to drive the turbine more efficicently, you get quicker spool.

This requires an equal length exhaust system that mates the cylinders that fire at even crankshaft degree spacing from one another. The Porsche 911 and Japanese 4 cylinder pocket rockets are perfect for twin scroll turbos in this regard. On the flip side, you will never get a twin scroll system to work on a 5 cylinder Audi (without some truly insane engineering wizardry), nor will you be able to use two twin scroll turbos with any positive effect on a 930TT. A V8TT on the other hand, would be able to.

The next step beyond simply keeping the exhaust discrete until it hits the flange are turbo designs that divide the exhaust stream past the flange, then comingle exhaust gases just before the turbine - maybe a few centimeters before the turbine. The longer the gas signatures stay discrete, the less energy is wasted.

Taken even further, the latest twin scroll designs not only have a divided housing, but actually have completely segragated cavities which ensure that the exhaust streams stay competely separate until the gases are actually in contact with the turbine wheels. The latest Mitsubishi and IHI turbos literally keep the exhaust streams separate until just a few thousandths of an inch before the vanes of the turbine wheel, so in reality aren't ever in contact until after they have crashed into the turbine, exactly where you want all available energy to be expressed (see the Mitsubishi Evo VII/IX/X or Subaru WRX STi).

The final (theoretical) permutation would be any flavor of the above twin scroll housing with variable geometry, whether single vane, dual, linked vanes (one for each scroll path) or even (although I have never seen anything like this) dual vanes with independent pitch control.

Net-net, more energy expressed in the direstion you want to apply force is always a good thing. This is what a twin scroll turbo designer's ultimate goal is.

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Old 06-03-2010, 02:58 PM
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Outstanding.

Excellent info Kenik!

Also I am thinking, as long as there are three or less cylinders per split turbo feed, the cylinders on said feed are equally spaced, and the exhaust cycle duration is less than (720/3) 240 degs -- we also get an increase in Volumetric Efficiency in the cylinders because of the lack of overlaping common exhaust events. That is all the other exhaust valves will be closed anytime one is open.

Double whammy!!

The best split system is probably a twin turbo system.

As you notes imply, all this is just an exercise if we do not have good Split Scroll turbos to choose from???

Any referance info is appricated.

Thx!!!
Old 06-03-2010, 05:25 PM
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My pleasure to chip in. I spent a looooong time researching turbo tech for my upcoming 2.3L turbo build. In the end, I went with a Mitsubishi Evo IX TD05-16G with 10.5cm hotside for all of the reasons above. Of course I had Chad Block and Blouch modify it into custom 650 cfm, 400+ HP capable beast that hits 1 BAR @ 2500 RPM on a 2.3L motor and blows 1.5 BAR+ without boost taper to 8000 RPM.

I agree with you that Brian's headers are by far the best 911 Turbo header on the market. Once my short block is done, these will be purchased.

Yes, TTs are a great alternative if you can't twin scroll. I mapped TTs for my motor and the boost curves and transient response curves I extrapolated from existing data looked identical. Draw your own conclusions... smilewavy
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Old 06-03-2010, 05:57 PM
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Keith, I wasn't saying these things because I like to type. The reason I took the time to share my experiences is to show that there are benefits to using even partial bank separation, and the benefits increase the closer you get to complete separation. This helps in understanding fully the benefits of a true split scroll system. These are results that I have witnessed in the evolution of my header system. Below is a picture of my Generation 1 flange. You will note it does not have a split volute. With that change alone I saw a difference in both how the engine ran and sounded indicating increasing efficiency.
The things Kenik is talking about take this evolution to the extreme, and you see increases in performance with each step.

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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 06-03-2010, 06:49 PM
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I am not saying it dose not work.

If your sound test shows a difference we know it has an effect.

If the dyno proves it is a positive effect that is excellent.

I suspect two things going on.

First the header cross section restricts down from the secondary tubes to the turbo buy almost 2 to 1 .

Second, the secondaries are in effect at almost a right angle to each other as they approach the area into the the turbo.

The dam should help reduce turbulance and help compensate for both of these factors.

On top of this, it would seem the 'bridge' wast gate conection will already have functioned as a 'cross over' and had a significant equalization effect to balanced each sides pressure waves.

Thus, I do think the 'cross over' and 'dam' are probably both positive features.

However, I do not see how we can attribute any of the benefits of a 'split scroll' system to them.

Just my unexpert opinion.

The best.

Last edited by 911st; 06-05-2010 at 12:30 PM..
Old 06-05-2010, 07:58 AM
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Anything that crosses over the gasses is detrimental. For the wastegate, the crossover design is less than ideal as anything that acts as a pressure equalizer is detracting from pulse integrity. The best solution is to either use two separate, independent wastegates or use a turbo that has an internal wastegate. Gases should ideally never comingle anywhere before the compressor wheel.
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Old 06-05-2010, 01:01 PM
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Brian's system is a true equal length and not a twin scroll system.

Thus, I am guessing the cross over might be a positive on this type of a system.

A cross over connection probably helps to further reduce the peaks of the amplitude of pressure changes inside the headers. This is mostly the same thing an equal length system dose.

The net effect being lower back pressure and even more orderly exhaust.

Anything that sooths the highs and lows should let it better flow, with lower restriction, less noise, and less back pressure.

Said lower back pressure's further aids cylinder filling and increases VE.

I could be mistaken but this is what I am believing at this point in my education.
Old 06-05-2010, 02:13 PM
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The only reason it isn't full twin scroll is the crossover pipe and a non-divided flange. Ben and I have chatted and when I am ready to buy, we'll be working together to omit the crossover and fit the pipes to my divided flange. Then the header system will be truly twin scroll.
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Old 06-05-2010, 02:33 PM
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Excellent!

Assuming a good twin scroll turbo being available, I would do the same thing!

Another thought.

We spend a fair amount of effort on the primary tube collector. Seem to not be to concerned with the collector into the turbo, and almost ignore the collector that splits exhaust to feed the WG's! Seems that on some turbo installs that run smaller turbine to compressor ratios, the WG circuit in some cases might be approaching the volume flow of the turbo.
Old 06-05-2010, 02:54 PM
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Back to better understanding Twin Scroll.

Twin Scroll is said to be most effective on a 4 cylinder motor, there is some benefit on a 6 cylinder, and on an 8 cylinder it is not worth the effort (even on an even fire).

As an example, if we run a cam that has say 230 deg duration like a C2 cam:

Two cylinders of a 4 cylinder motor will have an exhaust valves open for 460 of 720 deg of rotation which leaves a fair amount of time for the pressure to drop between exhaust cycles and exit the header.

This would give us a lower average header pressure than if all cylinders were merged.

On a 6 we would have 3 cylinders per side with an exhaust valve open 690 of 720 deg. The time for each exhaust event to exit the header before the next is greatly reduced and the benefit lessened.

By the time we get to a V8 and have to put 4 cylinders together. We end up with exhaust valves open 920 deg of 720 deg of crank rotation. Now we an overlap of exhaust events with all exhaust cycles and much of its advantage has been lost.


Per above, all exhaust valves closed as follows:

4 cylinder, 36% of the time.

6 cylinder, 4% of the time.

8 cylinder, 0%. In fact we now have two exhaust valves open 28% of the time per side.



Cool stuff!!!

Now I better understand why a 4 cylinder sees more benefit with a split system.

Last edited by 911st; 06-05-2010 at 04:34 PM..
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