A new thought for twins - Compound Turbos? Maybe?

[TitaniumTT]

So here's the deal, Most of you that have read my sig know what I've accomplished with a set of stock REW turbos.... 405 RWHP with all the squential hardware intact. I'll be on the dyno in a few days time to dial in the sequentials and get a base dyno for what I want my next project to be.

I recently acquired an FD that I have yet to pick up actually (BIG Thanks to Rico and Tray for hooking me up BIG) and the wheels have already started turning. I plan on using basically the same engine that I have now. A ported, polished, portmatched 13B-RE with various RX8 internals, stat gears, e-shaft mainly and the cosmo 9:1 rotors.

What I want to do is simple, beat the twins that I have now. This is easy in top end performance, but really lacks in the low end. This is where I need the most help. This is a street/auto-x/track day car. I drive my FC every nice day that I can and it honestly is a blast to drive. Non-sequentially it lacks but when I run them sequentially, she's an entirely different car. God willing I'll have the dyno sheets Tuesday and I'll post them up for sure so we (NoDOCH) can start doing some math

This setup will be in an FD and I would like to see 450-500whp and a fairly flat torque curve. Reasons for the goal, FD has better suspension, can fit wider tires, she's heavier, and frankly, I want to outdo myself.

I've been doing alot of research on the evil forum, when IB decides to let it work & I've seen a few things that jump out at me. Howard Colemans twin setup, Jason @ RX7Stores setup, A guy named Marcel Burnett (that used way oversized turbo's) and that's really it. Howards setup is everything that it claims to be. It makes 507 RWHP on pump/AI, there isvirtually nill as far as EMAP (something the stocktwins cant boast+) the turbo's aren't working all that hard, and the entire system sees pretty reliable for what it is. What I don't like about Howards setup is the fact that if you look at the dyn chart, it looks like every other single turbo chart out there. This is not what I want to see.

Jasons setup, well, it was never really finished. I thought it had potential based on the ONLY dyno sheet that was posted. It seemed like a good idea, good turbo choice (same one I was looking at for the most part, there are a ton of variants in the GT28 family) but the packaging was just aweful in my eyes. It looked rushed to me. There was little thought put into the intake plumbing to the turbo's, or from the turbo's to the intercooler. The exhaust mani was not equal length, or even close to it for that matter. I give him credit for going against the grain and trying it out, I understand he is trying to run a business, but at the same time I believe that system left alot on the table. I'm not going to let that "failure" discourage me.

Then I saw something interesting in a diesel application - compound turbo's. Aurura Turbo has a few kits' out for the diesels that are rather interesting. Basically there are two turbos of different size. The smaller turbo pulls it's air from the compressor housing of the larger turbo. Think about that as one of my questions is directly related to it. The exhaust is first routed through the smaller turbo and then to the larger turbo. All the exhaust passes through the smaller turbo before it enters the larger turbo.

This seems like a great idea. The way I see it, the smaller turbo is compressing a small amount of vaccuumed air becuase it has to pull through the larger turbo until it spools ever so slightly. However, once that larger turbo is spooling and creating positive pressure.... is that smaller turbo going to create resistance? Or is it going to further compress the charge air?

Looking at the dyno results of this system is impressive and appealing. It is EXACTELY what I'm looking for. Torque builds fast, and stays constant. Now, is this a function of the turbo's? Or the unique characteristics of the diesel engine?

I would like input on this. I am dead set on upping the ante with my FD build in terms of power. I would like to see a sequential looking torque curve that builds quickly to 360ish and holds there throughout the RPM band.

There will be no factory style exhaust mani's. I believe that is the limitation of the FDs turbo's. It's really a mess in there and it doesn't matter if I'm making 10 lbs or 14+ lbs, the EMAP is pretty much the same at around 280-290 kpa at redline. There is a direct almost perfect 1:1 ratio as boost builds. Once boost target is met though, the EMAP continues to climb. There is just too much exhaust flow and it can't effeiciently be routed out. Anyone that has seen the stock mani will understand why. In Daves words, Yeah man, it's a fucking mess in there.

So ideas and thoughts, lets here them. This will take a year if not more to come to fruition, and I don't expect any hard data until 2011 @ the earliest but mark my words, something will happen.

[Herblenny]

Interesting thought... I guess I'm trying to picture the diesel turbo set up you are talking about.. I'm not quite picturing how it works. Maybe a diagram of some sort??

[TitaniumTT]

A couple pics, good call Phil. I should of put these up immediately

Originally Posted by Aurora's website

Compound Turbo Charging (One turbo compressor wheel feeding the other) Compound turbocharging is not the same as twin turbo charging. A compound turbo utilizes different sized turbos feeding off one another to do the job of one turbo.

Air enters the low-pressure turbo (the larger of the two) and is fed into the high-pressure turbo (the smaller of the two), then directed into the engine or intercooler. This is why this type of turbo charger arrangement is referred to as Compound Turbo System. Compound Turbo Chargers work in series.

Multiple turbo chargers are also commonly referred to as twin turbo chargers; twin turbos are just that "twins". The two identical sized turbo chargers are the same in nature or identical is size and work in parallel. Typically each turbo is used to do one half of the work and is not in a compound configuration. The air that is compressed into the engine is not fed from one turbo compressor to the other; the air that goes to the engine is simply split between the two of them.

The compound turbo charger set up offers all of the attributes of a small and large turbo charger with out the negative effect of ether one of them. There is a small turbo charger and a large turbo charger plumbed in series to one another. The small turbo charger is referred to as the high-pressure turbo charger and the large turbo charger is referred to as the low-pressure turbo charger or the atmosphere turbo charger. In a "Compound" turbo charger set up the small turbo is responsible for generating quick turbo response and rapid air flow right off idle. As the engine is accelerated from idle the small turbo begins to produce boost immediately wile the large turbo slowly starts to produce a positive pressure to the small turbo. The large turbo when matched properly should be supplying boost pressure to feed the small turbo shortly after the small turbo is making boost in to the engine or intercooler. If the large turbo charger is not matched properly it will cause the small turbo charger to stall and the low-end performance will suffer.

Turbo charger balance is a fine art; the exact balance must be achieved to produce the desirable results or the turbo charger design will not perform efficiently. In order to design a compound turbo charger package, the exact pressure ratios must be matched between all of the stages of the turbo chargers. This balance is only achieved by sizing the turbine housings, turbine wheels, compressor wheels, diameters and an array of other calculations.

The ATS Aurora(tm) Compound Turbo Charger System provides the utopia in the turbo charger world. Quick spool up off idle, extreme midrange torque and incredible top end performance is what you can expect from the ATS Aurora(tm) Compound Turbo Charger System. The Aurora 2000 and 5000 turbo chargers are placed in specifically designed turbine and compressor housings to provide a positive boost ratio through out the entire engine operation range. A positive pressure ratio means there is more boost pressure feeding the intake side of the engine than there is exhaust pressure exiting the engine. Maintaining a positive boost pressure is a major step in maximum performance and economy. This means the engine is always operating near maximum efficiency, burning all of the diesel fuel during the combustion process and reducing exhaust temperature. 100 percent of the exhaust energy is passed through the turbine wheel maximizing the available wasted exhaust energy.

The secret to power and economy is airflow; increased airflow is the lack of resistance along with a positive intake charge. The ATS Aurora(tm) Compound Turbo Charger System is designed for power levels up to 600-rear wheel HP; exhaust temperatures remain low and drivability is responsive and enjoyable.

During the design and layout of the turbo system we were sensitive to the needs of the every day requirements such as heavy haulers and RV-ers. Unlike many other turbo systems currently available the ATS engineers elected a more reliable method of manufacturing the complex components that create this system. All of the high stress and high temperature exhaust sections of the system are specially designed and made from high temperature iron with an internal exhaust diverter valve mechanism to maximize reliability and performance. There are no welded joints or "pipes" used in the construction of the turbos or the mounting of the system. All of the high stress/heat areas are designed to be self-sealing requiring no problematic gaskets that are common to fatigue and blow out after severe heat cycles. The low-pressure turbo has been placed in the exact position of the factory turbo charger requiring no exhaust modifications to be performed during the retrofit and allowing adequate room for the over sized intake system.

The ATS Compound Turbo Kit comes completely assembled with all of the necessary mounting hardware, a new ATS Exhaust Manifold, Turbos, and Intake for an easy bolt on installation.

Very vague for a reason I suppose. I like the theory though.

[Herblenny]

Hmm.. Interesting..

I wish there were more real data on these things.. What I would like to think about is, would smaller turbo getting air from the larger one eventually hit limit of one or the other. Example, the smaller one will spool first, than exhaust goes the the second, larger turbo, but the air is being pulled from the second.. Would the air eventually hit a limit as rate of pull would be different?? Also, how about exhaust rate after going thru first than the second?

Also, how about air temp?? Air seems to be warmed up already and then warmed up again by second turbo? To me, it might be better if two merges vs. one shoots warm air to the next.. Also, I've heard air temp isn't as crucial to diesel vs. rotary.. Again, thats what I've heard and I have no real experience in diesel.

again, i'm not an expert in this.. just putting down what I'm thinking in my head right now.

[Whizbang]

i have similar suspicions

[Herblenny]

I just read the description a bit more..

So, it looks like you would have to 'balance' the two turbos (sizes). Which sounds like you need to know what kind of power/response you want and set one turbo to match the other. I don't know, it sounds interesting but I feel like you could achieve this by going with a right single turbo.

[TitaniumTT]

Originally Posted by Herblenny

I don't know, it sounds interesting but I feel like you could achieve this by going with a right single turbo.

I don't see it happening in single form unfortunately. I'll be calling Garrett soon enough but I want the response of a GT25/28 with the top end of a GT35/37. These are two disparate goals and I don't see one turbo accomplishing both. I'll have to get this sequential dyno chart up and compare it to the myriad of GT35R dynos that are floating around.

My main concern is the area under the torque curve under 4.5-5k rpms. This is where I want the difference to be. This is where the sequentials really shine for a street/auto-x car. I SO wish Dave and I had more time last saturday right before the last auto-x on sunday

[TitaniumTT]

Spew all the thoughts that you want in here, that's the purpose of the thread.

Diesel AIT's vs Rotary AIT's - not all that critical in diesel applications as diesels compress the air to superheated status THEN inject the fuel causing combustion. Typical diesel comp ratio's are 20+:1 and boost is in the 40+ range....... yeah. We all know what a poor intercooler can do for a rotary.

As for the function of them..... This is what I'm wondering as well. My friend was thinking about going twin charged - and infact there are some Yanmar diesels (I'm a marine guy too) that use twin charging. The way the Yanmar system is setup to work is with a supercharger and a BIG turbocharger. The supercharger gives the instant response and is run off a clutch system. When the ECU detects mani pressure above what the supercharger can produce, the supercharger is shut down and the turbo does the rest. Interesting.

What I'm gathering from these compound turbo's is the smaller gives the instant reponse while the larger is spooling up. This of course hurts backpressure which is a major hindrance for a rotary. But how much? Dunno until it's tested. For YEARS internet myths surrounded the twins saying they were a backpressure nightmare. In reality they are no worse than MOST other turbo's. I stay with in the generally accepted 2:1 ratio but I believe I am right at the limit of the effeincy of the stock hitachis. I would love to see what the BNR's could do BUT it's not all that much of a challenge.

After my ramblings....... the compound system, it seems like the smaller provides the instant repsonse that I'm looking for while the larger is gaining momentum. But when the larger somes online is the positive pressure from the larger being further compressed by the smaller turbo? I would say..... yes, yes it is. So then air temps as Phil pointed out can come into play. Such to the point that a LARGE Air-to-Water intercooler are needed. OK, not a problem.

Maybe the thing to do is not plumb the outlets of the turbo's in series, but instead use some sort of boost activated exhaust cut-out instead? Size the turbo's such that as one is out of breath, the other is ramping up. But then will that throw the smaller into surge? Is that why they are plumbed the way they are in the first photo?

[Herblenny]

How about Variable Vane Turbos?? Has anyone use these things on Rotaries? Those I worry about reliability aspects (especially high exhaust temp of rotary).

[TitaniumTT]

Originally Posted by Herblenny

How about Variable Vane Turbos?? Has anyone use these things on Rotaries? Those I worry about reliability aspects (especially high exhaust temp of rotary).

You're 110% right on the money Phil. The combination of high heat and sutty exhaust create havoc for those turbo's. It's a great idea for sure, but reliabilty is a factor. I do plan on talking to Garrett about those as well as I have seen them and actually driven a tank of a boat, a 25' bertram with twin 4cyl Volvo's. I was on the test drive with the mechanic, I was at the helm and he said punch it. I did. I was VERY impressed with the response and torque they generated. When we got back I talked to him about them a little, got to see it first hand, it was actually the turbo that got replaced and we chatted a little about them. Great idea, but I'm unsure if they could tolerate the rotary's heat.

[Whizbang]

the VVT have some a long way, its mostly just cost.

[TitaniumTT]

Originally Posted by Gravity Fed

the VVT have some a long way, its mostly just cost.

One properly sized VVT is probably not 2x the cost of any other GT family turbo though, although I could be wrong. Definately worth looking into though.

[RotaryProphet]

The point of a compound turbo system like that is that it provides very -very- high boost levels. Let's take an example:

The larger turbo runs at say a 1.5:1 pressure ratio, consuming something on the order of 1200 CFM of air, compressing it down to something on order of two thirds that (somewhat more than that, given heat pollution, but this is example math, so don't crucify me.). So we have 800 CFM at the outlet at 1.5 Bar pressure.

This feeds into the inlet of the smaller charger, which runs at say, 2.5:1 pressure ratio. 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.

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.

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.

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.

[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.

[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.

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 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.

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

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.