A new thought for twins - Compound Turbos? Maybe?

[RotaryProphet]

Originally Posted by TitaniumTT

Unfortuneately for me, I only hav the option of running one stepper motor with the Motec that I'll be using. I was going to use it for the OMP but I like the throttle body butterfly for the intake idea more. I suppose I could use an internal style wastegate actuator connected to a butterfly valve as well.

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.

Originally Posted by TitaniumTT

My main question would be, how would the turbo's handle working against each other? The larger T4 turbo in your example would be moving more CFM at the same boost pressure as the T3 style. Pressure is pressure and if held constant between the turbos, would the T4 over power the T3 in anyway?

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.

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.

Originally Posted by TitaniumTT

The setup I had in mind is similar. Here's the setup that I'm contemplating

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.

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 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.

[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.

[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.