A new thought for twins - Compound Turbos? Maybe?

[TitaniumTT]

Originally Posted by RotaryProphet

The beauty of my setup is that the control box for the turbos is a separate box, and doesn't rely on the stock ECU; you just input your max pressures and allow it to go about it's business.

Very nice little system. I lack the electronic knowledge to be able to do this. I wouldn't be using the stock ECU though, I would be using a Motec which has enough inputs/outputs and software to allow me to do this.

Originally Posted by RotaryProphet

No, as long as the pressures are equal, both turbo outlets will flow freely; if one produces more pressure than the other, the reed valve on the outlet will close, preventing backflow.

I see. Pressure not flow. Even though one turbo is flowing more air, becuase the pressures are equal at the outlet, they work together and not against each other. Mazda did this with the 20B turbo's actually. One was a Hitachi HT-10 while the other was a Hitachi HT-15.

Originally Posted by RotaryProphet

If you were using a system with more secondary turbo boost than the primary, it would probably be worth using another valve to redirect air from the primary turbo outlet into the turbo inlets, basically making a loop so that the turbo keeps spinning instead of stalling out against a closed valve.

That's interesting. That's more of a compound sequential system though if I'm reading it properly. That would give very quick spool fom the smaller turbo, but then when the valve re-directs the smaller turbo's boost from the the intack to the larger turbo, boost would rize VERY quickly. Again, this is assuming we are vizualizing the same thing.

Originally Posted by RotaryProphet

Plumbing the turbos like that isn't likely to gain you anything. The thing to remember is that a turbo is a pressure differentially operated pump. In short, it relies on the difference in pressure between the inlet and the outlet on the hot side to spin the turbine.

So, you have to run your small turbocharger first, or else the exhaust, which can freely flow through the larger turbo, gets backed up against the smaller one, which means there's no pressure differential on the larger one. Because the smaller turbo must come first, the net effect is that once the exhaust runs through the small turbo, you have the difference in pressure between the primary turbo outlet and the secondary turbo outlet to extract useful energy from; far less than there exists in the manifold. Certainly there is energy there, but you'll find your larger turbo takes much longer to spool than it would in a single turbo configuration, which necessitates a larger primary turbo, which raises your boost threshold.

I think we're looking for two different goals which is why our systems are slightly different. I'm looking for a total of about 450RWHP and 360ish torque. Not all that much from two turbo's when singles are getting that fairly regularily now. I'm more interested in creating the flattest torque curve possible and extending it as far throughout the rpm's as possible. I BELIEVE you are looking for more power which is why you're using not only bigger turbo's, but slightly different sizes as well. Please correct me if I'm wrong.

Regarding my manifold setup - One of the things that concerns me, is as you put it, the smaller turbo acting as a plug in the system. Leaving the second turbo with much less energy to spool up to the same level as the first. I'm worried that the extra distance traveled, the less heat available, is going to lead to a secondary turbo that just cannot keep up with the first. The main reason for thinking about not only the very large traditional WG on the primary turbo's mani, but also an internal gate as well.

Originally Posted by RotaryProphet

Your concept would certainly work, and I believe your system of wastegates would work perfectly, too, I just don't know what the performance would be like. I would be curious to see it in action, though.

Now, running sequential twins is an option, but it requires two reasonably small turbos. I feel a small and then a somewhat larger turbo will give better top end performance, but again, I'd be very interested to run the dyno numbers on a twin sequential setup with properly sized turbos and a good manifold.

I agree that a different sized setup like the one you are building will net larger gains in the top end. With auto-x and track days and street driving being the primary role of this car, I'm more concerned with the low and midrange performance of the system. I believe that twin GT28's of some trim size will net 450 RWHP if not slightly higher. Jason's setup did 450 or there abouts, and the BNR turbo's are regularily getting close to this mark as well. It's my belief that the restriction in the BNR's lies in the factory manifold. As far as I know, I'm the only one on stock twins that can actually tell you what the EMAP is. Most people just look at it and say it's a nightmare. In reality it's not horrible. It's certainly not on par with a tubular mani and a large A/R turbine housing, but that's not an apples to apples comparision. I believe if we can get the EMAP down in the higher RPM's power and effiecency will increase. Negating the need for a larger pair of turbo's or a larger secondary turbo to get to say 450 rwhp. MOST people would argue that this is a waste of time for such a small power gain, but in terms of NET hp and the intended use, I certainly think it is.

Originally Posted by RotaryProphet

I don't know where you're at in the country, but if you're anywhere near Cincinnati, I've got my engine dyno at the shop setup to test and tune rotaries, and I'm just a bit of fabrication work away from testing my setup. I'll be testing on a stock RE motor, with stock intake and upgraded injectors, and a water to air intercooler for consistency. That way I can test several turbo setups and map them against each other in an apples to apples comparison. It'd always help to have another brain when the time comes to do it.

I live in the SW corner of CT, about 1/2 hr from the city. Honestly though, I would love to come out, see the setup, see the results, and lend my brain to the project. It's probably about a 10 hr drive out which I'd be more than willing to make. I'll check some airfares as well. Please keep me in mind when the time comes, I'd love to see it in action and be a part of it.

[classicauto]

In regards to the exhaust being choked on the smaller turbo...

What if you approach the manifold design from a EMAP standpoint, forgetting about spool at all for a second.

Example: Twinsturbo FD. Sure you've seen the crazy long, multi bend labyrinth manifold. Responsive no, but 1:1 pressure ratio - yes. Wastegate is a straight shot from the runners (although they curve all over the place) while the turbo is actually 90* to them. Aaron's manifold is similar, wastegates are a straight shot out of the port, while the turbo is getting its feed on after them..........again, low backpressure.

Perhaps the best layout for the exhaust would be to favour flow to largest then smallest "vents" as possible. Maybe wastegate(s), large turbo, then small turbo. Once everything's closed up at low RPM and you hammer the throttle, the physics of the smaller turbo vs. larger should itself spool one before the other, and hopefully plumbing in the order of largest to smallest will allow enough flow for the power needed up top without making EMAP unreal.

One would imagine, given the rotaries exhaust power, that the simple physics of the two turbos in the system would create decent spool for their size even if flow was directed entirely to the gates. As long as the gates are closed and some of the pressure building is pushing on the turbos, they'll spool while the flow favouring the gates should keep pressure as low as possible.

????

[RotaryProphet]

Originally Posted by TitaniumTT

Very nice little system. I lack the electronic knowledge to be able to do this. I wouldn't be using the stock ECU though, I would be using a Motec which has enough inputs/outputs and software to allow me to do this.

Actually, I was hoping to start producing the manifolds, packaged with the control box and the valves. Just pick your turbos, and have a shop make a downpipe.

Originally Posted by TitaniumTT

That's interesting. That's more of a compound sequential system though if I'm reading it properly. That would give very quick spool fom the smaller turbo, but then when the valve re-directs the smaller turbo's boost from the the intack to the larger turbo, boost would rize VERY quickly. Again, this is assuming we are vizualizing the same thing.

I don't think we are. In the situation where that valve would open, the larger turbo is already creating -more- boost than the smaller; the only point here is to allow the smaller turbo to keep spinning and flowing air to somewhere, so that it doesn't stall and stop spinning; it's basically useful to keep the turbo spinning during gear changes and things like that, when you drop back down to the smaller turbo for a short period. Same concept as a blow-off valve; by letting it vent, the turbo doesn't slow down.

Originally Posted by TitaniumTT

I think we're looking for two different goals which is why our systems are slightly different. I'm looking for a total of about 450RWHP and 360ish torque. Not all that much from two turbo's when singles are getting that fairly regularily now. I'm more interested in creating the flattest torque curve possible and extending it as far throughout the rpm's as possible. I BELIEVE you are looking for more power which is why you're using not only bigger turbo's, but slightly different sizes as well. Please correct me if I'm wrong.

No, I think we're looking for basically the same thing. My goal would be to imitate the FD's stock boost curve, except with higher boost levels, and eliminating that transition dip. By bringing the secondary turbo online slowly instead of all at once, I think that's possible.

Originally Posted by TitaniumTT

Regarding my manifold setup - One of the things that concerns me, is as you put it, the smaller turbo acting as a plug in the system. Leaving the second turbo with much less energy to spool up to the same level as the first. I'm worried that the extra distance traveled, the less heat available, is going to lead to a secondary turbo that just cannot keep up with the first. The main reason for thinking about not only the very large traditional WG on the primary turbo's mani, but also an internal gate as well.

The theory of putting one turbine behind another, and having every bit of exhaust flow through both is a sound one... in theory. Once the exhaust leaves the turbo, there's still energy to be extracted, but less so. But here's what I see happening:

To control boost and turbine speed on the primary, you want to put a big wastegate on the manifold, and maybe internally gate the turbo, as well. The problem is that by venting all that pressure past the turbo into the inlet of the secondary, then at higher RPMs, where there's a lot of exhaust, you've eliminated the pressure differential between the inlet and the outlet of the primary; the pressure coming in is the same as the pressure going out, thanks to the big wastegate opening a valve between the two in an effort to keep boost under control. The wastegate will stay open instead of closing, though, because it's linked to system boost, not individual turbo boost. Eventually, because there's very little pressure differential to run it, inlet pressure from the second turbo will begin flowing out of the primary's inlet.

Originally Posted by TitaniumTT

I agree that a different sized setup like the one you are building will net larger gains in the top end. With auto-x and track days and street driving being the primary role of this car, I'm more concerned with the low and midrange performance of the system.

You can get a flat boost curve with a pair of smallish twins, or with a small turbo and a moderately big turbo; either way should work about as well. The small turbo should flow enough air to make up the difference while a bigger turbo spools, it's just a matter of it working alone for longer. As long as the turbos aren't too far apart, it doesn't matter.

The question is, why leave performance on the table when you can get the same results, get the same outstanding low end response and torque when you can -also- get a higher top end? My system would work either way, but it seems silly to leave power when it's there. At very least, you could use a larger turbo, tune for high boost, and use the control box to bring it down when it's unwanted.

And here's something else to consider; in a setup like this, the larger turbo spools much -much- faster, due to the simple fact that the engine is already running under boost while spooling it. A 13b under 15psi of boost is exhaling as much exhaust as a 5.0 liter NA boinger, but with a much more favorable exhaust arrangement, in terms of exhaust pulses, and manifold setup. The primary turbo, being spooled already, is only using at small chunk of that exhaust energy, and the rest is generally wasted via a properly named wastegate.

Originally Posted by TitaniumTT

I live in the SW corner of CT, about 1/2 hr from the city. Honestly though, I would love to come out, see the setup, see the results, and lend my brain to the project. It's probably about a 10 hr drive out which I'd be more than willing to make. I'll check some airfares as well. Please keep me in mind when the time comes, I'd love to see it in action and be a part of it.

I'll let you know, and we'll try some stuff and see what happens.

[TitaniumTT]

Originally Posted by RotaryProphet

Actually, I was hoping to start producing the manifolds, packaged with the control box and the valves. Just pick your turbos, and have a shop make a downpipe.

That would make it VERY easy to sell, I like that idea. I generally don't like piggy-back systems, mainly for aux injection type situations it scares me. Your idea though controls everything related to ONE part of the system which I don't see a problem with.

Originally Posted by RotaryProphet

I don't think we are. In the situation where that valve would open, the larger turbo is already creating -more- boost than the smaller; the only point here is to allow the smaller turbo to keep spinning and flowing air to somewhere, so that it doesn't stall and stop spinning; it's basically useful to keep the turbo spinning during gear changes and things like that, when you drop back down to the smaller turbo for a short period. Same concept as a blow-off valve; by letting it vent, the turbo doesn't slow down.

I agree, especially in a situation where we're looking for the best response from a turbo, keeping as much energy in the system is critical. I also believe that the EMAP plays a huge role in the torque curve as well. So in a situation where the EMAP is on the rise, the torque will fall off. Eliminating a spike in EMAP by keeping the turbo spinning would further increase low end torque.

Originally Posted by RotaryProphet

No, I think we're looking for basically the same thing. My goal would be to imitate the FD's stock boost curve, except with higher boost levels, and eliminating that transition dip. By bringing the secondary turbo online slowly instead of all at once, I think that's possible.

I think with standard valves and what not, it'll be difficult. That's the problem that we ran into on the dyno on Tuesday. We would either see a dip, or a surge. We were logging the boost that the secondary was creating in the space between the compressor outlet and the Charge Control Valve so we knew what the turbo was producing. I adjusted the RPM resoltion down to 50 rpm increments around the transition and it was still difficult to get a smooth transition. I think your REED valve idea is going to be the only way to bring the secondary online smoothly. Looking at the datalogs we were able to get the transition dip down to .15s. Its VERY slight, but it's still there. I'm still working on the transition on the street and it's definately getting better.

Originally Posted by RotaryProphet

The theory of putting one turbine behind another, and having every bit of exhaust flow through both is a sound one... in theory. Once the exhaust leaves the turbo, there's still energy to be extracted, but less so. But here's what I see happening:

To control boost and turbine speed on the primary, you want to put a big wastegate on the manifold, and maybe internally gate the turbo, as well. The problem is that by venting all that pressure past the turbo into the inlet of the secondary, then at higher RPMs, where there's a lot of exhaust, you've eliminated the pressure differential between the inlet and the outlet of the primary; the pressure coming in is the same as the pressure going out, thanks to the big wastegate opening a valve between the two in an effort to keep boost under control. The wastegate will stay open instead of closing, though, because it's linked to system boost, not individual turbo boost. Eventually, because there's very little pressure differential to run it, inlet pressure from the second turbo will begin flowing out of the primary's inlet.

You can get a flat boost curve with a pair of smallish twins, or with a small turbo and a moderately big turbo; either way should work about as well. The small turbo should flow enough air to make up the difference while a bigger turbo spools, it's just a matter of it working alone for longer. As long as the turbos aren't too far apart, it doesn't matter.

That's the reason that I'm thinking that two turbos of close to the same size would be a better choice than one small and one large unless the larger one is run first - to take advantage of the pressure differential. It's my belief that the smaller turbo would need less of a pressure differential to create some CFM to add to the larger turbo.

What I would like to try is a pair of very similarly sized turbo's collecting both runners exhaust right at the point where they would split off in a Y - one going to the primary turbo, the other to a LARGE WG. I would like to take advantage of the energy post WG-before being sucked up by a turbine. This would be routed directly to the bottom of the flange on the secondary.
Going back to your point of the lack of a pressure differential on the primary, I'm not sure if the exhaust of the primary should be routed around the secondaries turbine, or to it. I think there is response lost if it's routed around it. However, maybe the thing to do is use a slightly larger A/R on the primary to let it breath a little better.

Originally Posted by RotaryProphet

The question is, why leave performance on the table when you can get the same results, get the same outstanding low end response and torque when you can -also- get a higher top end? My system would work either way, but it seems silly to leave power when it's there. At very least, you could use a larger turbo, tune for high boost, and use the control box to bring it down when it's unwanted.

I agree it's silly to leave performance when it's there. But I am more focused on a flat torque/boost curve than higher HP. This is just based on my experience with my FC. Over 300ft/lbs, the suspension/tires really can't handle it. I may feel differently when I get a set of A6's, or get on a track where my RA1's can get up to temp. But right now, the higher end performance is wasted becuase I simply can't put it down. Now with the ability to put a 10.5" wheel and bigger tires behind an FD, and given the better suspension geometry associated with the FD, I may feel differently. That's why my goals are 450RWHP, and 360ish ft/lbs.

Originally Posted by RotaryProphet

And here's something else to consider; in a setup like this, the larger turbo spools much -much- faster, due to the simple fact that the engine is already running under boost while spooling it. A 13b under 15psi of boost is exhaling as much exhaust as a 5.0 liter NA boinger, but with a much more favorable exhaust arrangement, in terms of exhaust pulses, and manifold setup. The primary turbo, being spooled already, is only using at small chunk of that exhaust energy, and the rest is generally wasted via a properly named wastegate.

Agreed, which is why I'm torn between routing the exhaust of the primary to the bumper, or to the secondary turbo

Originally Posted by RotaryProphet

I'll let you know, and we'll try some stuff and see what happens.

Please do. I'd love to get a first hand look at what's going on, what works and what doesn't

[RotaryProphet]

Originally Posted by TitaniumTT

However, maybe the thing to do is use a slightly larger A/R on the primary to let it breath a little better.

Just remember that the more restrictive turbine needs to come -first- in the exhaust stream; if you put the less restrictive one first, exhaust will back up against the secondary turbine, and you'll lose your pressure differential, and the first turbo won't spool.

That's why in the diesel compound systems, the smaller turbo always goes first, then the exhaust runs out to the secondary. I'm not sure exactly what would happen if you ran a pair of identical twins in a compound setup.

I don't know for sure if that's what you were saying or not, but I figured I'd clarify, just in case.

[TitaniumTT]

No, you're right, and it makes perfect sense.

I'm not thinking about a pair of twins in a compound setup, but more of a sequential.

Do you happen to have any diagrams or pictures of the setup that you're working on now that you'd like to share?

[RotaryProphet]

Originally Posted by TitaniumTT

Do you happen to have any diagrams or pictures of the setup that you're working on now that you'd like to share?

Not really. The pictures of what I've built so far aren't very good, and my actual real camera refuses to turn on. Suppose I'm going to need to replace it.

But I'll see what I can come up with as far as a basic diagram, and send it your way.

[TitaniumTT]

Great, thanks so much.

As for Camera's, I picked up a Nikon Coolpix L20 specifically for shop detail. @ $130, it really can't be beat. I even use it to take in-car track video's.

[neit_jnf]

maybe I missed it but was it mentioned that in diesels with compound turbocharging a positive boost ratio, meaning more boost than backpressure, across the operating range is possible?

if the turbos are sized correctly, could this be done on a rotary?

[vex]

I have something to contribute to this thread, but I don't want to come off as an obsolete fool/youngin'. Let me go back and read this novel of a thread before I post it.

[vex]

Okay, I read some of this thread (but not all... I'm sick with the herp... I mean, cold--I got through page 2 or so). Brian, I think what you're goal is--this is an okay way to go, but there's a better way and you can easily make the uber flat torque curve, but you're not going to like what I have to say.

You should make your own sequential turbo manifold/system. If you do a compound turbo you're going to run into a few problems, especially when you want to get a specific torque curve. Since you're flowing the exhaust flow into a small turbo, then into a large turbo (st, lt, etc), you're actually removing a certain amount of energy from the exhaust flow in the st which will cause the lt to be under spooled. If you can by-pass the small turbo such that it's on pep the entire time the large turbo has matched the pressure output of the lt you will have a much flater torque curve than a compound.

The way I imagine the sequential manifold set up is by two wastegates. When the pressure of the small one hits the target value, the waste gate activates and dumps the wasted gas into the lt. lt reaches peak pressure and will be more efficient in it. For instance (and just as an example), here are some compressor maps from turboneticsinc.




As you can see from these two turbos the 60-1 will run out of steam just as the HP76 is kicking in. You then get more mass flow of air compared to the 60-1 and no bad will come of this. Since the 60-1 is just getting enough exhaust to stay on pep it will always be ready for action when you let up the throttle and the large turbo drops back out of its efficiency range, and you wouldn't have the energy losses from compounding the exhaust stream.

Put it another way: There are three different scenarios that the above performs.

1) The small turbo spools quickly providing the torque you want early on the large turbo dumps its slightly less compressed air into the intake stream of the small turbo to help the large turbo spool quicker.
2) The large turbo spools to the same pressure as the small turbo and you run with an increased mass flow of air
3) The large turbo exceeds the small turbo pressure and as a result the small turbo dumps its charge into the intake stream of the large turbo which is better at the higher rpm.

This and both turbos receive the full benefit of the temperature difference from the exhaust.

In fact, I have an idea about how to do both the intake and exhaust manifold.... I believe this very well is the only way to get the torque curve you want without the downsides of the compound turbo set up. There is another way you may want to look into:

Basically instead of pressurized air being pushed back into the intake, the turbo directly puts the energy on the e-shaft (through a series of gears to increase torque).

I found a pic to what I'm talking about:

http://www.heat2power.net/en__benchmark.php

[RotaryProphet]

The good stuff was in pages 3 and 4.

[dudemaaan]

I'm not sure I really followed what you said vex... The 60-1 is a pretty decent size turbo to be using for fast spool, it works well as a medium size turbo and is a good compromise between power and lag for a single turbo setup. Not ideal for instant response, but that really makes no difference for the theory....

If you're saying have a small turbo basically setup like normal, and then a big turbo that's run off of the wastegate exhaust of the smaller turbo... I don't think that would work well. The amount of exhaust energy coming out of the wastegate pales in comparison to that coming out of the turbine outlet. The large turbo would have very little exhaust pulses to get it moving, and would probably contribute very little extra air to the engine.

Next, the large turbo would be a restriction to the wastegate which would almost certainly cause an over-boosting situation for the small turbo.

The other system you mentioned, which seems to be separate from your first idea, is generally called a turbo-compound system. Where a series of gearing is attached to the compressor instead of a compressor wheel. This system adds so much complexity, and would only work well in a diesel application where massive amounts of exhaust gas and pressure is created.

Compound turbocharging (like we have been discussing), where one turbo flows into the other would make a very flat torque curve with properly sized turbos. With a small primary turbo you could have full boost before 2000 rpms and with a properly matched larger turbo maybe a gt42r you could make well over 700 hp if desired. I think you will lose a little engine efficiency from the added exhaust restriction, but the additional power made by the setup will vastly exceed anything lost. I would be concerned with backpressure creating too much heat and excessive EGT's. Only testing would show if this is an issue.

Another idea is to use a roots style supercharger for instant boost, and make it a compound setup using a larger turbo. This would actually remove restrictions in the exhaust since you can use a larger A/R housing, and the supercharger will give you instant response and boost. I personally think this is the simplest and most logical solution to having your cake and eating it.

[vex]

Originally Posted by dudemaaan

I'm not sure I really followed what you said vex... The 60-1 is a pretty decent size turbo to be using for fast spool, it works well as a medium size turbo and is a good compromise between power and lag for a single turbo setup. Not ideal for instant response, but that really makes no difference for the theory....

Sorry, I'm on cold meds and my thought process will be fuzzy for a few days until I'm over this. I just used the 60-1 and the other Turbo as examples of their respective comp maps.

If you're saying have a small turbo basically setup like normal, and then a big turbo that's run off of the wastegate exhaust of the smaller turbo... I don't think that would work well. The amount of exhaust energy coming out of the wastegate pales in comparison to that coming out of the turbine outlet. The large turbo would have very little exhaust pulses to get it moving, and would probably contribute very little extra air to the engine.

No, not set up like that. You have both turbos plumbed such that the first turbo (60-1 in the example) received a majority of the exhaust stream to ensure quick boost build and response. The remaining exhaust would be routed in through the large turbo so the exhaust flow would still be causing the larger turbo to spool. As soon as the smaller turbo starts to eek out past its efficiency range or the desired pressure is reached, the exhaust is then diverted via an external wastegate to the large turbo stream. The temperature difference should be negligible to allow the large turbo to spool to the desired pressure.

Next, the large turbo would be a restriction to the wastegate which would almost certainly cause an over-boosting situation for the small turbo.

That really depends on the wastegate and how the pressure in the exhaust system will behave. With nominal exhaust temperatures the large turbine should not be the restriction in the exhaust flow, but rather the small turbo will be and as such the diverted exhaust gases from the manifold, and from the wastegate will feed to the large turbine, causing the large turbo to increase energy conversion.

The other system you mentioned, which seems to be separate from your first idea, is generally called a turbo-compound system. Where a series of gearing is attached to the compressor instead of a compressor wheel. This system adds so much complexity, and would only work well in a diesel application where massive amounts of exhaust gas and pressure is created.

It's actually been used fairly effectively in aircraft applications as well. Though the gearing I don't think would be that complex. A simple unit similar to a transmission should allow for easy application to a car. From the picture I posted it is actually from a Volvo engine.

Compound turbocharging (like we have been discussing), where one turbo flows into the other would make a very flat torque curve with properly sized turbos. With a small primary turbo you could have full boost before 2000 rpms and with a properly matched larger turbo maybe a gt42r you could make well over 700 hp if desired. I think you will lose a little engine efficiency from the added exhaust restriction, but the additional power made by the setup will vastly exceed anything lost. I would be concerned with backpressure creating too much heat and excessive EGT's. Only testing would show if this is an issue.

I think it will be a vary large issue and would need to be evaluated prior to doing any testing. I think it's all in the sizing as you and others have said, though I see it becoming a much larger problem as there is no by-pass for the exhaust. Looking at my friends tuned sequential FD the torque curve was ridiculously flat, and I honestly think that's where the best performance is going to come from; something similar, but with a much more scrutinized design.

Another idea is to use a roots style supercharger for instant boost, and make it a compound setup using a larger turbo. This would actually remove restrictions in the exhaust since you can use a larger A/R housing, and the supercharger will give you instant response and boost. I personally think this is the simplest and most logical solution to having your cake and eating it.

This idea has merit, and I agree.

[dudemaaan]

Oh I didn't know you were suggesting the turbine outlet of the SmallT would be dumping into the the LargeT. That would work fine then, the velocity would still be high enough to spool the LargeT. You do realize the compound turbos have wastgates too right? They are plumbed in the same manor you're talking about. Sounds like there is really no difference in operation for the exhaust side of what you're suggesting. And in both cases you still run into the problem of two exhaust housings/wheels in the stream of the exhaust which will increase back pressure/EGT, especially if the first is a small housing to help spool.

The only difference I see now, is the cold side. In a compound turbo the LargeT cold side feeds into the inlet (where the air filter goes) of the SmallT. This is where the compounding takes place. The boost is "compounded" or multiplied. While in a sequential system, there is usually a butterfly valve to keep the turbo that's "working", flowing only into the manifold, then the butterfly opens and allows the second turbo to contribute to the total volume of air, but it in itself would not increase boost pressure. The difference is much like the following diagram of 2 pumps in series vs parallel. Series would be compounding, and parallel would be sequential.




Here you can see the wastegate in the image below of a compound turbo.