WHat is the possibility of a custom remote turbo system similar to what STS is doing? Maybe someone like RG can discuss the posibility/feasability of such a system and how it would work with an engine such ours?

 

I don't think there would be any more issues with it on this car than any other car. I've looked at alot of other different forums on the topic and there seems to be alot of misinformation and skepticism on the topic. All those who have used them seem to like it and even turbo guru Kenny Duttweiller was impressed with the setup. That sounds like a good endorsement to me. The people that are typically the most closed minded and against the setup are always those who have no experience with them. I can't say I have any experience with this setup myself but I don't see why it couldn't work decently.

I have seen people claim that it only works good on V8's and not smaller engines as there isn't enough exhaust velocity to spin the turbo. I'm not even sure how this can be considered a relevant thought. This simple airflow issue is quickly fixed with a properly sized turbo for the engine. Not too hard to understand. May as well claim that only an International 18 wheeler diesel engine is large enough to spool a rear mount turbo but not a V8. See how dumb the argument is? It's all a matter of scaling. You'd just need to find the right size for a rotary.

Here are some facts about how conventional turbo systems work. First off remember that a turbo is traditionaly placed before a cat and muffler. Anyone who has ever owned a turbo car can tell you how much less lag and how much more power there is when you remove the car and muffler to free up the exhaust. A rear mount turbo doesn't have these restrictions after it. They are before it. This isn't much of an issue though as you just size the turbo properly to account for this. Spool up shouldn't be an issue.

Alot of people like to falsely claim that a turbo runs off of heat and flow. It only runs off of airflow. Blow some cold air from an air compressor through a turbo and tell me if it spins. It will. Now place a turbo in a hot oven and tell me if it spins. It won't. Heat isn't an issue to turbo spool up. What does confuse people though is that a hotter exhaust has more energy in it and is moving faster. Hotter air also takes up more space. At the back of the car after the gasses have cooled down alot, it's velocity is much slower. The gasses have lost alot of heat energy on it's way to the back of the car. Only heat has been lost. The airflow is still the same. This means that a turbo near the engine must have a larger exhaust housing/wheel than one at the rear of the car. Yet each setup may still spool up just as quickly and be the same effective exhaust restriction. A restriction of the same size has a greater effect the closer you get to the engine as more energy is passing through it. Figure that many cars that come with turbos have turbos that don't spin as freely as they could and have far more restrictive exhausts after the turbo than they should and it shouldn't be too hard to see that the extra distance that the exhaust gasses need to travel to the rear of the car to a turbo wouldn't have much of a lag issue if done properly. At the very least it wouldn't be any worse than what many are used to. Aot of people who worry about lag and who already own turbo systems probably have overly large, slow spooling turbos already. This is true with many RX-7 owners.

A rear mounted turbo wouldn't heat up as much and this would definitely lead to longer turbo life notto mention the fact that it wouldn't be near as hard on your oil. Since turbos can get glowing red hot, it is not reasonable to think that this heat isn't transferred in some way back to the intake side. There would definitely be a benefit in this area to a turbo that runs cooler.

There is one claim that STS makes that is absolutely false. First off, turbos can not increase your gas mileage on a gasoline engine. They claim it will. In order for a turbo to work, it has to use energy. Exhaust energy is not free as people think it is. You still have to restrict it down a little bit to get te turbo spinning. If it took no power to run and actualy increased your efficiency, I would call it the perpetual motion charger. There is always some loss even if slight and this can not lead to better mileage under any circumstance.

There will also always be some form of lag. There is no turbo system out there that has no lag despite what people claim. If you have no lag, you have no turbo or the sudden surge of power that happens afterwards. Even the Greddy kit has lag. Don't worry about the long intake pipe from the back of the car causing excessive lag. For low boost levels that don't require an intercooler, this especially isn't an issue. It's all about pressurizing a certain volume of air in the system. A large intercooler will add alot of volume to the intak piping. The pipes running to the front of a car and back can sometimes be excessively long as well. Even adding a small intercooler to the SDS system shouldn't hurt lag times much. The total volume in a couple of extra feet of pipe only 2" or so in diameter isn't very much more airspace. I do feel that ultimately an engine mounted turbo with a free flow exhaust and short intercooler piping will have slightly shorter lag times than the rear mounted turbo but this doesn't mean it will be bad or undriveable. I doubt that would be an issue at all. We are really only talking about milliseconds.

I'd like to see this setup tried on the RX-8. With the muffler off this car has the perfect setup to be able to do it. As always engine management has to be dealt with. SDS did get patent approval on the setup so we aren't going to see any other companies doing this for that reason alone. It wouldn't be hard to build though. I'm definitely open to the idea. All of the concepts make sense and the list of endorsements goes on and on. Nothing speaks more than actual results. I hope someone tries it. It may not be the ultimate solution in forced induction but for street use it should do fine.

 

unfortunately plumbing the compressor all the way back to the intercooler/intake is a real b-tch ...

 

Here's a newbie idea:
What if you had the exhaust-side fan in the back, but the intake-side fan in the front connected by a wire - kinda like that dremel extender tool thing.The exhaust would spin the rear fan, which would turn the wire, which would turn the intake fan. It would be a lot easier to route a small wire than a 2" air duct, and none of the exhaust heat would be transferred to the intake.

 

I would be a bit concerned about that as the turbos are spinning at similar speeds to a dremel, but with much higher loads. The wire acts as a spring and will introduce a lot of strangeness, but I imagine with proper design it might work.

 

Thanks for the insight RG!!

Maybe I should buy an Interceptor-X and take 2 months vacation in Houston


If we were to remove the muffler and put the turbo there, would it be dangerous to have a spinning/hot component so close to the gas tank?

 

no it would not

 

i asked sts how that would be done way back when and i never got an answer

 

I don't think this is totally true. In any introductory thermodynamics course, there are plenty of turbine examples taught. Temperature change across the turbine is one of the most important parameters in all of them. If the gas is cooler at the exit of the turbine than at the inlet (thats where your oven example breaks-down,there's no change in the internal energy of the gas between the inlet and exit of the turbine), then the turbine has converted some of that heat energy to work -assuming the turbine is insulated such that its not just wasting that heat energy. Thats why good turbines are well insulated (adiabatic). We don't want to waste the available heat energy that we could be taking advantage of. In fact, a lot of turbine calculations are done assuming that the change in kinetic energy of the gas passing through the turbine is negligible. That means that the change in gas velocity (blowing cool air across the blades) is really not that important afterall. Powerplants use steam to drive a turbine because steam contains a helluva lot of interal heat energy. Thats not an accident. They are using that internal heat energy and converting it to work. Now, its not lost on me that a powerplant turbine has a lot more engineering behind it than an automotive turbine, but I don't think we can discount the similarities, either. Turbine calculations are all done with enthalpy at the inlet and exit which has units of BTU/Lb. The mass (Lb.) flow is the same at both locations, meaning the only thing changing in these simplified models is the thermal energy (BTU). Those units alone should be enough to say that the turbine does use heat to do work.

rebuttal?

 

Of course you would want to calculate for change in temperature at both the inlet and outlets. The turbo will absorb much of the heat and the properties of the gas will change on their way through the turbo. This is why you would optimally want it insulated very well. You want the turbo to absorb as little heat as possible to keep the properties of the air moving through it more constant and predictable. Some heat energy is converted to work. This is why the turbo is sized larger near the engine. There is more energy moving through the turbo in the form of flow and heat and you have to account for the amount of additional space they need at the front vs the back. The thing is though that the heat isn't doing work on the turbo to spin it. It is doing work on the air going through the turbo causing the air molecules to vibrate more and take up more space. This is what you size the exhaust side of the turbo differently to account for. Take some of the heat energy out of the air and the air now takes up less space. Since it is air and not heat that spin the turbo, we must now size the turbine accordingly to adjust for the amount of space the air now takes up. A steam turbine while very hot is still moving air and building pressure. Release this pressure and it is now converted into airflow which spins the turbine. If airflow through the turbine isn't important but heat is, it would spin in the oven and not when you blow through it. We know this isn't true. We just need to understand how heat changes the properties of the air. This is why heat is important. In a classroom they want to teach you every last little detail so that sounds like an excercise that would probably be done in school.

 

I enquired about a year ago, and they said they planned one for 2005.

I could see their kit would work, there is lots of room back there once you take out the huge muffler - the real problem would be fuel/mixture management.

I was actually contacted by the Canadian rep, back in April, asking if I was interested in being the prototype......I said of course I was!

Since April there has just been an awkward silence.

S

 

can't you just say the ideal gas law says that if you heat up a gas, the pressure will rise, and the gas will have more energy to do work on the turbine - and its really pressure that applies a force to turbine blades and makes them move?

BTW - heat is very critical in a steam turbine as the steam entering the turbine needs to be superheated to the point that after going through all the stages and losing all the associated energy it doesn't condense before leaving the turbine... or else your going to have turbine blades that look like they have been hit with a sledgehammer

 

Seems less efficent than right off the exhaust manifold since the exhaust gases are cooler and moving slower by the time you get to the back. I can see where a system like this would be benificial if you had no space in the engine bay, but the rx8 has plenty.

Also when you go get emissons tested and they throw the sniffer in your exhaust it's probably going to run up into your turbo and make a mess (I know someone this has happened too).

 

That would be an easier way to say it but I never say anything in simple terms.

 

Well, which is it? You are contradicting yourself:
All I am saying is that heat is VERY important to the equation. You can't claim (as you did in the first post) that heat has nothing to do with how much work the turbine can do. That's simply incorrect. If the statement from your second post is correct, which it is, then you have more potential to do work with hot exhaust gasses (near engine) than cold exhaust gasses (farther from engine).

A turbine is a thermodynamic device - keyword 'THERMO'. In order to understand it, you have to analyze the the thermal cycle thats taking place. You can't claim that cold air and hot air moving with the same mass flow rate, have equal potential for doing work. The hot air has more energy available to use. Its simple thermodynamics. Since the mass flow rate is the same at the engine and at the exhaust tips, you are giving-up a lot of available energy if you choose to use the cold gasses to do work. Some of the available energy that we would like to use was lost as heat escaped from the gasses to the exhaust pipes and convected to the atmosphere. THAT is the reason you insulate the manifold and turbo, not ,to quote you, "to keep the properties of the air ... more constant and predictable." In fact, if the properties of the air (enthalpy) were constant across the turbo, you wouldn't be doing any work, now would you... You actaully want the enthalpy to change by the largest possible amount across the turbine! To calculate the work/mass that a turbine can do all you need to know is the change in temperature of the gas and its specific heat. (work/mass)=Cp*(Tin-Tout). Therefore, 1) change in gas properties is good 2) Heat lost to atmosphere is available energy wasted.

Yeah, that would be engineering school. That's where they teach us that a turbine is a device for converting heat energy to work. Don't believe me? Here's at least one online definition you can see for yourself.: http://www.engineersedge.com/thermodynamics.htm
"Turbine - A device that converts heat energy into work. "

If heat weren't important, we'd call it a WINDMILL, not a TURBINE

 

You can't use a blanket statement like "a turbine is a thermodynamic device." Some turbines are. Wind turbines, water turbines, etc are not. The basic principles are the same for all turbines.

Rotarygod didn't contradict himself. Without airflow, the turbine isn't going to spin, no matter how hot it is. The heat energy provides work to move the air molecules around. This provides a little more pressure, causing a higher d/p across the turbine, which adds a little airflow. He never said to ignore heat completely, just that it's not the most important aspect of turbochargers, like most people seem to assume. A hotter exhaust output generally means more potential for flow, but that also means a hotter exhaust at the muffler making the rear mount turbo sound pretty good for the rotary.

When building a header mounted turbo you design to take advantage of the heat, you also design to try to minimize the negative effects of that same heat. When building a rear mounted turbo, there's still alot of heat just not as much, so you design for the less heat and have less negative effects. It's all compromise, no matter which one you build. Both can work extremely well. I don't understand why people still doubt the rear mount turbos, when there are hundreds out there and quite a few making alot of power.

 

I agree with you on most of your points, therm8. But just because you don't percieve that water through a water turbine and wind through a wind turbine are hot, like steam and exhaust, doesn't mean that a thermodynamic cycle isn't taking place. The first law of thermodyamics state that you can't do work without a change in enthalpy.


really?

I guess it sounded that way to me....

 

Oh I agree with you there. But enthalpy depends on more than just temp. Temp is the internal energy component, but it also depends on pressure and volume. It gets much more complicated to compute when dealing with a system full of pressure drops. Even more so in pressure-velocity compounded steam turbines, which are really the only turbines I actually know alot about.


I guess I'm hearing it differently. I read it as heat doesn't move the turbine.


In the end, I really like the rear mount turbos. But I don't want to turbo my 8. I don't want to deal with overboosting, spikes etc. Gimme a supercharger

 

i've noticed.....

 

Well the long and the short of it, it works and it works on small engines, see the magazine reviews on their web site.

its not like the rx8 exhaust gas, even as far back as the muffler, isn't feaking hot. This is the only turbo system I would personally consider. having all that crap right by the engine, especially as hot as our engine gets, just doesn't sound like a good idea.

 

The difference in heat from one side of a turbine to another is important because a greater difference between the hot and cold side means more power is there to be extracted. A stirling engine makes more horsepower when the hot side is really hot and the cold side is really cold. Heat motivates the air molecules, which in turn motivate the turbine. Applying a heat lamp (well okay, a reeealy big one!) to a turbine would cause it to move...but only if you seal the system off and insure that the turbine is the only path the hot air can escape through, as opposed to just floating straight up.

That's the thing though, we're talking about a closed piece of pipe. The conservation of mass principle tells us that M1 = M2, unless you've got an exhaust leak. If we have 1000 CFM at the header outlet, we will have 1000 CFM inside the exhaust tips. The (relatively) cool exhaust molecules pushing against the STS turbine are themselves being pushed by the hotter exhaust molecules by the header; they have nowhere else to go but through the turbine. Instead of a high velocity/high temp/low density exhaust stream hitting the wheel at the header, you've got a somewhat lower velocity/low temp/higher density exhaust stream at the back of the car. But it's the same number of molecules per second passing a given point, assuming no exhaust leak of course. Insulating the header should increase the turbine power in a conventional turbo, but it should also increase the turbine power in an STS setup. Power turbines cannot allow the steam to cool down because it wrecks the turbine (unless you're talking about a bladeless turbine), but that isn't a concern for a car's turbo.

Or at least that's how I remember it, although my grades in Intro to Thermo weren't stellar.

BTW, does STS have specially designed turbine wheels made just for them? Would it be possible for someone to just order a special turbo, and construct a similar system, if STS is dragging their feet? Not that I exactly blame them, the RX-8 market is fairly small compared to the other vehicles they have kits for. I'd expect them to have a Mustang kit before a RX-8 kit. Fabricating the long intake pipe to the engine won't be fun, but probably not any worse than fabricating a header, which isn't necessary in this type of setup. Also, will a catalytic converter break down over time, and send bits of trash into the turbine?

 

I'm not certain, but I believe they use off the shelf wheels. But there are so many, it shouldn't be hard to find one that fits any particular application. I'd be worried about the cat as well, particularly the 8's stock one. It'd probably be better to swap to a high temp/high flow one anyway.

They now have an '05 Mustang GT twin turbo kit, but it's $9000. They do have CARB certification for their GM 5.7, 5.3, 4.4 and 6.0 liter kits.

I would think the market for the 8 would be just as big as the Toyota truck market. How many people are going to turbo their truck when a TRD supercharger is warranty covered? Perhaps they were intimidated by the 8's fuel/ignition tuning.

 

Yeah, you're probably right about the Toyota trucks and the fuel/ignition control. Maybe they could release a tuner kit like for the Acura though.

edit: oh and about their claims for higher MPG. It makes more sense if you realize they were talking specifically about the Toyota trucks. Apparently the trucks aren't powerful enough to cruise on the freeway, on hills, towing situations, and against headwinds and such, without kicking down to 3rd gear. So if the turbo makes enough grunt that you can stay in overdrive, I guess it's concievable that you could actually see MPG gains.

 

So, RG... How would one go about sizing the right turbo for an application on the rx8?

 

Its all about what you want. The greddy unit is sized on the smaller size so that it adds power at lower revs, but ultimately runs out of flow at higher boost levels. All turbos have an area of boost and flow that they operate best in. Learn how to read a compressor map and look at what's available in terms of turbos.

This is a good site: http://www.rbracing-rsr.com/turbotech.html