A new thought for twins - Compound Turbos? Maybe?

[antman0408]

Boost Logic has been working on a kit for the supras, you should search supraforums and see what they have done or give them a call. They probably have the most knowledge of a compound kit on a gasoline engine.

[TitaniumTT]

Originally Posted by RotaryProphet

The turbo doesn't care what pressure the inlet is at, it just compresses it further. So our charge gets compressed to ~320CFM at 3.75 bar, or around 55PSI absolute; think about 40 psi of boost.

This right here is what I was unsure of, thank you for the clarification. It was my believe that if the smaller turbo is seeing say 2bar Absol, and running at a 2:1 ratio, that it would be producing 3 bar of BOOST. While it makes perfect sense in theory, I was unsure if the turbo would physically be able to do it. I'm sure it's not simple absolute math as ineffeciancies do exist.

Originally Posted by RotaryProphet

It's not really a device to increase response like a twin setup, it's a device to get around the boost/airflow limits of turbochargers. The larger turbo flows enough air for the entire system at a reasonably low boost level, while the smaller turbo, instead of having to worry about massive airflow numbers (The numbers I used are probably par for 1/3 to 1/2 throttle in a big diesel application at that boost level) can compress a smaller charge further.

In your opinion, is this something that COULD be configured to provide the response that I'm looking for? My main concern here is the response. It's pretty common to see a 450-500rwhp rotary. There are a number of basically bolt-on kits, turbo and mani, that allow for this. None of them however, address the response that I so dearly love in my FC. I understand this is an outside the box kind of thinking as the general rule is you have to give up response for power and vice versa, but Mazda figured it out almost 2 decades ago. Granted they have engineers and foundarys that I don't, but still, anything's possible.

Originally Posted by RotaryProphet

Heat buildup, however, is pretty intense, since unless you absolutely match the turbos with regard to efficiency ranges, they'll both be running outside of their efficiency ranges most of the time. Even if they are well matched, you're getting heat from two turbochargers in all of the intake air. Hence why they generally run massive intercoolers in diesels.

Heat I can deal with. It's a matter of fabrication as far as I'm concerned. Not to sound arrogant but my FC on the dyno on a 65* day can run 4-5 4th gear dyno pulls in 20 minutes and the coolant will stay at or BELOW 185*, oild ~170* and AIT's will start ~60* and end BELOW 80*F. If need be a small inline Water-to-Air could be utilized to lower the charge temp of the smaller turbo's intake back to about ambient before the smaller turbo compresses it, reheating it, then it's just a normal intercooler after that.

Originally Posted by RotaryProphet

In short, if your goal is to run > 30psi or so of boost, this might be something to look into, and if you want to run more than about 45, it's almost a necessity, but otherwise, it doesn't do what you're looking for.

My goal is 360-375 ish torque with it being as early and as flat as possible. A PSI goal hasn't entered my mind yet. I would obviously like it to be as low as possible as I would like to avoid AIT related problems and Meth or water is something that I don't want to run either. Although I think it might come down to two retardedly small for a rotary turbos (GT25ish size) and some meth to keep it under control, although that defeats the really plush easy daily driveably charateristics that I want to keep with this car. I love driving my FC around, but it has become a little raw.

[TitaniumTT]

Originally Posted by classicauto

Ive seen a number of trucks in here with compound setups, and always had wet dreams about one on a rotary.

I know there's a kit for supra's using a compound plumbing setup, and its laying down RIDICULOUS spool and insane top end on the 2JZ. Search around the net, can't recall the name of the kit ATM.

Thanks Joe, I'll check it out.

Originally Posted by classicauto

Personally, I think taking the general idea and adjusting it to suit the rotary's strengths is the best approach. I would personally like to see the small turbo fed first, but reversing the direction of the intake air. Have the large turbo suck through the smaller one. Thoughts behind that being: the smaller turbo would have -essentially- vaccum on the compressor which is going to help it gain shaft speed very early. These babies like the air volume as we all know And once that snowball effect starts to roll on as the exhaust pumps out more and more pressure and volume - well we're off the races

I'd also love to see this scaled for a rotary as well. The dyno charts that I've seen for the diesels look exactely like what I'm after. Instant and constant torque.

What I'm wondering is, could we have the smaller turbo feeding the the larger turbo, both intake and exhaust. What would the larger turbo do when it sees compressed air and spinning at low shaft speeds? Would it pose a restriction? Or just let it pass? If the smaller turbo is running a 2:1 and the larger is just starting to spin, would it allow the 2:1 to pass through, wold it start to spool up with a positive charge thus evacuating more exhaust? Or would it just act like a plug?

Originally Posted by classicauto

I do feel though that cooling will become a larger issue with this setup. We know its a problem that you have manage well on these things already, and 2X the exhaust plumbing, 2X the compressors etc is going to create a larger easy bake oven then a single turbo. It may be worth considering a small cooler between the first and second turbo (which ever you mount first) like one of those tubular style babies. Just something to add some extra density and take some load off the main intercooler.

Heat can be managed, I think size constraints in the engine bay are going to be the limitations

Originally Posted by classicauto

I love the idea, and think that with a well tohught out setup, we can see a true example of what Mazda may have designed if the intended the FD to be 550whp from the show room

How sick would that be?

Originally Posted by classicauto

If there was ever a man I would have hoped to tackle this task - its you Brian. If you head down this road, I know its going to be all it can be.

Thanks Joe! Something will be done, and it will be done in an unconventional way. I just don't know what it is yet.

I've done some research on VVT's as well. Garrett has a GT37 designed for diesels with this technology. I need to call them soon and get some information. There's also a company callled Aerocharger that's offering some promise as well. I was VERY skeptical about this until I saw all the military applications for it. Still seems "gimicky" to me though.

[TitaniumTT]

Originally Posted by antman0408

Boost Logic has been working on a kit for the supras, you should search supraforums and see what they have done or give them a call. They probably have the most knowledge of a compound kit on a gasoline engine.

Thanks Anthony, I'll do a little searching around.

Originally Posted by RotaryProphet

The big turbo sucking through a small turbo is the same as a big turbo sucking through an inch and a half inlet; the big turbo is going to pull vacuum on it as soon as it starts to spool, and then the smaller turbo is just a restriction.

What you'd want is a sequential system with a reasonably large turbo for the top end, and a small turbo to create boost down low; that's what I'm working on with my sequential controller. It uses electronic controlled valves to control the secondary turbo and the wastegate, and allows a small turbo to boost to say, 14 PSI almost immediately, and allows the big turbo to make as much boost as you care to let it when it spools. And since the wastegate is electronic, it doesn't even start to crack until the second turbo is at full boost.

Would you mind sharing your idea for this? This is something else that I'm considering. Without doing any math, the sizes that I'm looking at are GT25/28 for the small, and a GT3071-R for the large. My thoughts for this type of system are

Exhaust - an equal length mani, each with a wastegate and an internally gated turbo just to give the turbo the chance to vent out as much exhaust energy as possible. The two individual gates will plumb back into the exhaust AFTER the second larger turbo. The larger turbo is also internally gated.

Intake - First, I'm undecided if the intercooler will be a dual inlet or not. For simplicity and to give the first turbo the least amount of space to pressurize, I believe the turbo's will be merged as close to the first turbo as possible and an electonic cut-out will be utilized. So, the first turbo is plumbed strait to a merge collector. The second turbo is plumbed to the merge collector as well but inline is an electric cut-out to keep the first turbo from back-spinning the second. There also needs to be a vent to allow the turbo to spool without creating a TREMENDOUS amount of exhaust backpressure.

Control.... well I'll be using a Motec M800 so the control's are virtually limitless. I'm thinking there needs to be a 3d map setup such that when MAP = the charge pipe for the seconday turbo, the valve opens and the vent closes. Easily accomplishable. Two different WG controls but the closed loop being on the larger turbos internal gate and the WG on the mani's being run off duty cylce at a preset limit to control "absolute boost"

Thoughts?

[TitaniumTT]

It's all so clear now
http://forums.hybridz.org/showthread...=145379&page=2

[need RX7]

Well after seeing the above design, the exhaust side makes alot of sense, but I'm still having trouble understanding why the air from the larger turbo has to pass through the smaller one, wouldn't the smaller compressor become a bottleneck at a certain level? What would happen if you ran a manifold like the one above, but had the compressor sides set up like normal, eventually merging before the intercooler?

[TitaniumTT]

For the rotary I think that makes the most sense HOWEVER - if you're going to do that than you need compressors that flow the same. Essentially making a sequential twin turbo. If one turbo can out pressurize another, it will most likely backfeed and overun the smaller.

The point of having the larger one feed into the smaller is to further compress the intake charge. The smaller one will compress whatever it is being fed. Whether it be atmospheric pressure, or 15lbs of boost, it will further compress it.

I'm coming up with a few ideas for a sequential twin turbo idea. I'm hoping Rotary Prophet shares some of his as well.

[dudemaaan]

Compound charging is one of those things that looks like it wouldn't work right, but apparently with all the success in diesel engines, and now Supras and other cars it apparently works quite well. The easiest method is to use at least an internal wastegate on the first turbo (smaller one). And for a rotary you'll want to make sure the wastegate and housing can flow enough so as not to become a bottleneck.

[TitaniumTT]

The EMAP is what really concerns me. So many things can and do go wrong when this is left unchecked.

[NoDOHC]

A few thoughts from yesteryear:

Believe it or not, 20 years ago, a lot of people were going the staged route. This was before turbos could make more than about 2:1 pressure ratio. The trick to success on a staged setup was (and still is) dual intercoolers.

The boost from the first turbo is cooled before it passes through the second turbo, and then it is recooled after the second turbo. This allows for reasonable charge air temperatures, while minimizing lag.

On a diesel (as has been noted) the intake air temperature is not an issue for engine longevity (except for possibly the valves). It is an issue for charge density and thermodynamic efficiency. This (and new NOX emissions standards) is why almost all modern diesel engines have intercoolers.

I have seen many diesel engines with staged turbos and a gaseous-fueled engines with staged turbos (good detonation resistance).

Benefits of staged turbos:
Quicker spool of smaller turbo
Insane power potential from larger turbo
Cooler charge air with very little if any more boost lag due to intercooler volume.
Very high boost potential (120 psi + boost).

Drawbacks of staged turbos:
Underhood piping disaster
Increased EGT from engine
Increased EMAP from engine
Higher cost
More potential failure points.
Insane underhood heat problems

Here is the reason that the turbos should be connected in the standard staging configuration:
Turbo Flow Maps are measured in inlet volume (or mass) per time. (Typically CFM, lbs/min, etc.) This means that at 2:1 pressure ratio on the primary turbo, with a well cooled charge, the secondary turbo will flow twice as much as it is rated on the curve, as the air is twice as dense going into it. This means that the secondary turbo needs to be 1/2 the flow rate of the large turbo (for this example).

Actually, the density ratio is what actually determines the turbocharger flow rate ratio. a 1.5:1 density ratio across the primary turbo means that the secondary turbo should be 2/3 the size of the primary turbo.

On the exhaust side, the air is very hot coming out of the engine. It is also under very high pressure. The exhaust turbine on the turbo charger is rated in pressure ratio. This pressure ratio across the turbine varies with air flow rate.

High pressure air from the engine does not occupy as much volume as lower pressure air coming out of the secondary turbo, thus the flow rate through the large turbine is a factor of the exhaust gas density ratio across the secondary turbine (the temperature will decrease some). Thus the output flow rate of the secondary turbine is significantly larger than the input flow rate (in volumetric units, obviously the mass flow rate is constant, due to the conservation of Mass limitations in our non-nuclear internal combustion engine). The larger turbine requires a higher flow rate, and must therefore come after the secondary turbine on the exhaust stream.

If the large turbo was first, it would spool very slowly, while the small turbine would quickly overspeed.

[RotaryProphet]

Originally Posted by TitaniumTT

Would you mind sharing your idea for this? This is something else that I'm considering. Without doing any math, the sizes that I'm looking at are GT25/28 for the small, and a GT3071-R for the large.

Certainly. My initial design is something like this (and keep in mind, this is a budget buildup to even see if it's plausible, so the parts combination is all whatever I could find in a junkyard; I'm currently using a T3 turbo off of some 4 cylinder, and a Masterpower 70mm T4.

The exhaust manifold is based on a standard, short, fat runner design that merges into a T4 divided flange, in about the same position as a megan racing 13b manifold. About halfway between the engine and the T4 flange, we run a second, somewhat smaller pipe off of each runner, and route it to a T3 flange, in a position to make mounting as easy as is possible in a setup like this. At roughly the same position that the T3 splits off from the main runners, we split on the other side, route the pipes out toward the exhaust, and mount the electronic wastegate. I'll get back to that.

So basically, each runner of the exhaust goes three places; a wastegate, a small T3 turbo, and to a T4 turbo, which you'll note has exhaust priority. In my setup, the wastegate pipes come off of the top of the runners, between the exhaust and the intake, and run down towards the downpipes. The T3 runners go from underneath, and are routed up towards the front of the car, to mount the turbo somewhat in front of the T4, and higher up. It's also worth noting that neither turbocharger is internally wastegated.


The only thing of note in the intake system is a pair of valves on the turbocharger outlets. Basically, you use a reed valve on each one to prevent back flowing through either turbo. This could happen through the T4 when the T3 is doing it's initial spool up, or from the T3 if you choose to run more boost from the T4 than the T3 is capable of pushing.

As for the control, I'm an embedded systems programmer by trade, so I chose to build my own microchip driven control box. In it's current form, it's dead simple, with just two stepper motor outputs to control the two exhaust valves (we'll get back to those), and two MAP sensors, one connected to the outlet of each turbocharger. The capacity is there to input RPM, throttle position, gear, or whatever else, and to map the boost just like a full fledged boost controller.

In the down pipe for the T4 turbocharger, there's an electronic valve. It's basically a throttle body that's driven by a stepper motor, and designed to deal with the kind of heat that we're going to see in a rotary exhaust. The part I'm working with now was designed to route jet engine exhaust, actually. This valve, when closed, prevents any exhaust from flowing through the T4's exhaust turbine by blocking the outlet. This is preferable to blocking the inlet, as the temperatures are not nearly so extreme as in the manifold.

There's another valve located where the wastegate would otherwise be, and functioning as it.

To help you visualize how all this comes together, here's a play by play.

You floor the engine at low RPM. The control box reads vacuum on both turbo outlets, and closes both the turbocharger select valve, and the wastegate valve fully. This routes all exhaust through the smaller turbo. Once the primary turbo shows that it's reached it's pre-set full boost limit, the system begins opening the turbocharger select valve. The system will gradually open this, working much like a wastegate, releasing any exhaust pressure not required to run the T3 at capacity through the T4 turbocharger, which gradually brings it up to speed.

Once the MAP sensor for the T4 charger reads that -it- is at full boost (some time before the turbo select valve is fully open, generally), the system begins opening the wastegate valve in the same way, metering it exactly to keep the boost solidly where you want it. It also fully opens the turbocharger select valve, if it wasn't fully open yet.

Anyone familiar with how a wastegate works knows that as a control device, they do a terrible job, as they waste energy by opening before the turbocharger reaches full boost; by using an electronic valve for a wastegate (which has been done before, but not very often on road cars, usually due to cost), we keep the wastegate closed until it's absolutely necessary to open it to prevent over boosting. By doing this, we pick up significant torque during the engine spooling phase.

With two properly sized turbochargers, there's no reason you couldn't have the rock solid straight-line torque numbers you're looking for with a system like this. A small T3 can spool before 2000 RPM on a 13b, but I would investigate a slightly larger one that spools by, say 2500 RPM if I were planning to run high boost numbers with a relatively large secondary turbo. This would help keep the transition between the small and large turbos smooth and unnoticeable from the driver's perspective.

In a way, the smaller turbo selection is more important, because the goal of the entire system is to run the small turbo into it's peak efficiency range, and then keep it there by gradually moving work to the larger turbo. This will help the low end torque numbers by keeping heat out of the incoming air charge. Then you just need to select a secondary turbo that, when coupled with the primary, will leave both of them in their peak efficiency at about 75-85% maximum revs.

I know I've already got a book here, but it's worth looking at pros and cons versus a single turbo setup.

As far as pros go, you've got the very quick spool up of a small single turbo, the high end efficiency and power of a large single turbo, and a very smooth power band and seemless transition. Operationally, the sequential setup takes it every single time.

The list of cons is entirely logistical. It requires two turbochargers, a custom manifold (although, I'm hoping I can start producing these once I get it up on the engine dyno and start tuning, and can put out some numbers of my kit vs a single turbo vs stock twins.), some not inexpensive valve hardware, an electronic control box, and let's face it: a f*cking plumbing nightmare under the hood.

[TitaniumTT]

Sounds like a VERY intreresting project! I like the thought that went into it. 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.

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?

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

Two equally sized turbo's as far as compressor goes. Possibly changing the A/R on the secondary turbo to reduce the inherent backpressure that would be created between the two turbo's. Basically what I'm contemplating is an equal length mani with a T3 flange and the largest WG possible right under the flange. The primary turbo would get the full brunt of the rotary exhaust. This primary turbo MAY be gated on the snail, and if so, that WG will run to the DP, I don't think the will be necessary though. Routing from the turbines exit of the first turbo is plumbed directly to the turbine inlet of the secondary turbo. The first WG is also plumbed back into that same exhaust stream allowing the exhaust energy to be split. There is a second WG below the turbine of the secondary turbo as well. This WG is vented directly to the actual DP. That's the exhaust side of things.

The intake side is rather simple as well. THe primary turbo is plumbed directly to the intake track. THe secondary turbo will have a vent valve placed between the compressor discharge and a TB style butterfly valve to keep the two turbo's seperate.

Play by play -

Theory being that when you floor the go pedal, the smaller turbo gets the full power and flow of the exhaust and such will spool VERY quickly. The WG associated with that turbo keeps the primary turbo;s boost in check. The secondary WG will COULD be open at this point to reduce the backpressure between the two turbo's to allow for a bit faster spool in the lower RPM's. (If I need to build seperate vac and boost storage tanks to aid in this, so be it. Easy enough to do, They're on my setup now actually) As RPM's and EMAP increase prior to the first turbo, the primary WG will open to keep the primary's boost in check, when the prim WG is fully open, the secondary will be in charge of controlling boost in the entire system. Once the secondary turbo begins creating boost, the seconday WG will most likely begin to close to aid in the secondary creating boost. Once boost is realized in the secondary turbo, the vent will close, the butterfly valve will open and both will be running in parrallel. The point of the second WG on the snail of the first turbo would be to vent the exhaust energy from the first turbo if the difference in flow of the exhaust system was such that the primary was receiving more energy than the first.

In my Datalogs of the stock sequential system, the rear turbo will always run hotter by ~75-100*. I've switched signals, repeated runs and gotten the same result so I know that it is not a function of different sensors, wiring, or signal amps. My theory is that the exhaust flapper in the rear of the housing creates enough resistance to increase the backpressure there, keeping the gas hotter. There is a direct correlation with the seperation of EGT's and EMAP.

The system that I am proposing is very similar to the stock sequential system. In my eyes, the drawback to the stock system is two fold. The inability of the hitachi turbo's to tolerate more than 15-16 lb's of boost reliably. More important though is the exhaus mani itself is a convoluted mess. A tubular mani in my eyes would yeild significant gains.

The plumbing would be a bit of a chore. However, I believe that after scouring the goodridge catalog, they offer enough fittings to allow for a somewhat neat packaging. I would run one -3 line to the top of the turbos and hope that I could get the CHRA's in a banjo fitting and just use an on-the-run type fitting to feed the second turbo. Drains are pretty simply if you're using an RE or an REW block, there is a drain for and aft. I would use the outlet/inlet of the WP housing to cool the first turbo. Perhaps use the nipple on the rear housing to cool the rear turbo and just weld a second -6 bung on the waterpump inlet. Although I do plan on using an REW WP housing, I plan to take the T-stat housing and outlet off and build my own to suit my needs. In that case the AST would be contained there and I would just have a ton of fittings welded on to it. I think the exhaust plumbing would be much more of a headache than oil/coolant plumbing it thats what you guys are reffering to with the plumbing nightmare comments. Hardlines would be awesome to have here as well and are very easily made, even when using banjo fittings.

[TitaniumTT]

I do love this snail though

[dudemaaan]

could also do a twincharge setup with a roots blower and a large turbo. The roots blower would add power down low, any extra power downlow = more exhaust pulse and faster spool of the larger turbo. The large turbo would allow the engine more power potential.

[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.