Rear mounted turbo...

And the next one:

Don't turbos have to be really hot to work properly?

Putting a torch to your turbo and getting it hot doesn't produce boost. What
produces boost is airflow across the turbine which causes the turbine to
spin. If turbochargers required very high temperatures to produce boost,
Diesel trucks and Methanol Race cars wouldn't be able to run turbos.
However, each of these "Low Exhaust Temperature" vehicles work very
well with turbochargers when, like any turbo application, the turbocharger
is sized correctly.

In a conventional, exhaust manifold mounted turbocharger system, the
extra heat causes the air molecules to separate and the gas becomes
"thinner" because of the extra space between the molecules. This extra
space increases the volume of air but doesn't increase the mass of the air.
Because the volume is higher, the velocity of the gas has to be higher to
get it out in the same amount of time.

By mounting the turbo further downstream, the gasses do lose heat
energy and velocity, however, there is just as much mass (the amount of
air) coming out of the tailpipe as there is coming out of the heads. So you
are driving the turbine with a "denser" gas charge. The same number of
molecules per second are striking the turbine and flowing across the
turbine at 1200F as there is at 1700F.

Front mounted turbos typically run an A/R ratio turbine housing about 2
sizes larger because the velocity is already in the gasses and the volume
is so big that the turbine housing must be larger to not cause a major
restriction in the exhaust system which would cause more backpressure.
With the remote mounted turbo, the gasses have condensed and the
volume is less, so a smaller A/R ratio turbine housing can be used which
increases the velocity of the gasses while not causing any extra
backpressure because the gas volume is smaller and denser.


Sizing is everything with turbos. There is more to sizing a turbo for an
application than cubic inches, Volumetric Efficiency, and RPM ranges. A
turbo must also be sized for the exhaust temperatures. A turbine housing
sized for 1700F gasses would have lag if the gasses were 1200F. This is
why turbo cars have lag when they are cold and not warmed up yet. Both
systems work well if sized correctly.
 
CobraSix,

Here's my basis for this argument:
The average molecular kinetic energy is known as pressure. The molecules fly randomly around and hit the sides of the container and create pressure. The more heat the gas contains, the faster the molecules hit the sides of the container. Pressure is naturally equal in all directions. That means that there are just as many molecules going backwards as forwards due to the average kinetic energy. This effectively cancels out the average kinetic energy of the molecules when it comes to pushing against the turbo, because to do this, the net kinetic energy needs to be in one direction. Heat does nothing to make this happen.
Now, the force that actually drives the turbo is due to the flow of the gas. This flow is actually moving in the right direction to drive the turbine. It is caused by the pressure differential between the inlet and outlet of the exhaust turbine. The pressure on the inlet side is caused by the engine constantly pumping more and more exhaust molecules into the system. This increases the "n" in PV=nRT. You'll get more volume with more heat, but you won't get more flow. The turbine housing is the thing that steps up the velocity (and kinetic energy) of this flow to usable levels. If the exhaust is hot, increase the A/R. If the exhaust is cold, decrease the A/R. You'll get the same velocity striking the turbine blades either way.

I agree that the exhaust is losing energy, but I disagree about what type of energy the turbo uses.

STS is the only one doing this sort of thing. That's not because it doesn't work, it's because they own the patent for it.

Still don't think it works well? Watch this guy run in the 8's with a rear mount turbo:

http://i.b5z.net/i/u/1473169/m/MikeRomain887.wmv

(right click and "save link as")
 
JG,

You are correct..>V doesn't equal velocity. The relationship I talked about between temperature and velocity come from a derivitive of that equation.

Let me try it this way. Heat, like you said, creates pressure and volume. The turbine is rotated as the pressure drops across the turbine blades by varying the volume to change the velocity and thus create a low pressure on one side of the turbine blade. The velocity change results in relative high and low pressures much like an airplane wing. The speed of the turbine is thus related to the pressure and volume of the exhaust gas. If you lose temperature then pressure and/or volume also decrease (depending on which you hold constant) and the turbine will operate slower since it has less pressure and volume to create velocity across the blades.

Here is a simple turbine video:
http://www.youtube.com/watch?v=yPd7QakG ... re=related

Sure, it's a axial driven turbine that I'm familiar with on gas turbine powered ships, but the principle is the same. Just replace the compressor and combustion chamber with the mechanics of a IC engine and the same principles apply.

I do agree that it is possible to to size a turbo correctly to account for less inlet pressure and volume as a result of heat loss.

And you are correct the turbos don't have to be hot to work. however it works better if the exhaust is hot. Try this experiment. See how well a turbo will spin if you just hook it up to an air compressor capable of pushing out the same amount of molecules of air at room temperature versus and engine putting out the exhaust at 800*. Even easier...see how well a turbine moves if you stick it on the end of your tail pipe. I'm willing to bet it doesn't turn nearly as fast.

Remember, the pressure the turbo sees on the inlet (which is how the turbine works) is related to the temperature of the exhaust. Since the volume of the 'container' is the length from the exhaust valve to the inlet of the turbine, let's consider it a fixed amount for now. If you are the exhaust temp at 1000*, it would normally have a higher volume then exhaust at 500*. However since we are holding volume constant, the pressure goes up as temperature goes up. The velocity created to move the turbine blades is the difference in pressures on the two sides of the turbine. Now, instead of a contain 6" long, make it 6' long and you have a much larger container to fill up with each exhaust stroke. That causes the pressure to drop (if we now hold temperature constant if you insulate it to the point of no heat loss). Sure, it's the same amount of molecules, but because it is at a lower pressure, there is less velocity across the blades. The velocity across the blades creates the relative high and low pressures on each side of each blade. Slower the velocity, the smaller the gradient between high and low pressures, and the slower the turbine spins.

Now this may be an insignificant amount of drop for such a system, but it does exist.

Again, I'm not debating that the system is not doable or won't result in measureable increases over a N/A engine because it will. Just debating that it isn't the most efficient way of arranging the system for those reasons above. I've never said that.

FR, I know you plan to put the manifold at the engine, however you are still faced with the fact of a long intake before you reach the carb which even on an EFI car will result in a slower throttle response under no boost. Once you are on boost you'll probably not notice a difference in the intake length. I know I'm older because to me, 40 series flowmasters are more annoying then open headers. I had 50 series and hated them. I won't argue exhaust note quality as that is a completely subjective matter and IMO there is no wrong answer for it.


BTW, I do love this kind of theory...makes me dust off cobwebs.
 
Even when running the rear turbo exhaust pipe keeping the temps up in the pipe keep pressure up and help reduce turbo lag.

The old Corvair turbo guys used to wrap everything in fiberglass. Not practical nowadays unless you use stainless piping. Thermotec still makes header wrap and spray on sealer to keep moisture out.

Could also get exhaust pipe thermal barrier coated to keep heat in.
 
CobraSix":21itxqkf said:
BTW...much more science and we'll have to move this to the hardcore section...lol :)
Click to expand...

Haha, maybe we should take this elsewhere. I'm having a great time discussing this.

The turbine in the video uses the increase in volume (and the resulting velocity) from the burning gasses to spin, so heat does help in that case (correct?). But in the case of the exhaust system, the expansion is taking place in the cylinders and then the engine basically pumps out the spent gasses. So by the time the exhaust gets in the manifolds, no additional expansion is taking place.

I don't think that the cooling of the gasses decreases pressure. Say you put an intercooler in the exhaust system, not taking into account the inherent losses of the intercooler (due to friction), the pressure will be the same on both the hot side and the cool side.

I'm just thinking that it takes a certain exhaust pressure to build a certain intake pressure. That exhaust pressure can be had by using hot exhaust gasses and an large turbine housing or cool exhaust gasses and a small turbine housing. The differences in size between the turbine housings does not, I believe, denote a difference in efficiencies. It simply takes more space to expel hot gasses and less space to expel cool gasses if they are both going to be making the same amount of "backpressure" in the system.

It sounds like you've got experience with this and know what you're talking about, so I'm not going to say you're wrong about anything. It seems that we're both using valid arguments from two different points of view (at least I hope mine is valid :D ). This is about the time where we just start repeating what we've already said over and over until someone gets angry and starts calling the other person names. :shock:

I'll agree to disagree (at least until I mount a turbo to the tailpipe of my ranger to see what happens ;) )

(By the way, the two questions in bold and their answers were straight from the STS website, so I can't take credit for them)
 
At what distance from the engine would a turbo simply not create boost anymore - 10ft.? 20ft.? 50ft.? When the heat of exhaust gasses is equal to the heat of the intake?
 
Good point TJ. If tempurature has a large effect on turbo performance, there must be a limit where the turbo simple doesn't work anymore. Maybe −273.16 °C (absolute zero, no heat left)? But even then, if you took a chunk of air whose molecules were not moving and threw this chunk against the turbine blades, wouldn't it still spin the blades (I suppose the air would be in solid form at that point). But still, that should illustrate that it is possible for air to spin a turbine in the absence of heat.

That would make a good project. How about we run the exhaust through an A/C core. That way we could just test temperatures against each other and eliminate the variables of longer piping and such.
 
JG,

Remember that the exhaust gases have not fully expanded by the time the cylinder gets to BDC so there is some thermal expansion after the combustion event.

Cooling of gases does decrease pressure. Easy test for that. Take a 2L bottle and cap it off tightly. Put it in a boiling pot of water (what's the melting temp of plastic?). squeeze it when cold and when hot. You should feel a difference. Or, if you are real adventorous (but I don't recommend doing this at all!!!!) you can try filling up the 2L with water and squeeze it. Now heat it to boiling and see if the bottle stays intact. (AGAIN, I DON"T RECOMMEND DOING THIS) Technically it has the same amount of molecules in it both as liquid and gas. Only thing that I've inputed is temperature and the pressure increases.

Another way to see the effect in reverse are those keyboard cleaning compressor air bottles. Every use one and feel how they get cold as you release air? Since they are under pressure at room temperature and you release pressure, the temperature of the remaining volume of air decreases. It's a simple display of the Ideal gas law since you are reducing "n" (moles) and resultant temp and pressure drop since the container volume is fixed.

The turbo blades aren't moved by the exhaust pressure directly. They are moved by a pressure difference between the face and 'bottom' of each blade that happens as the velocity increases on the top (face) of the blade. To get that velocity, the volume is gradually decreased in the 'turbine chamber' as the turbine rotates and it results in a low pressure on the face. Just like an airplane wing, the turbine blade will create 'lift' and move towards the low pressure side. That's why the blades are curved like wings. The blades are also curved so that old 'high pressure' side becomes the low pressure side for the next blade so that it rotates smoothly as it comes into rotation into the exhaust gases.

I'm getting confused now myself. I may be completely wrong...I've only been dealing with the practical side of all this for the last 10 years. Been a while since I took my marine propulsion classes.

TJ...the heat of the intake and exhaust have no bearing on each other. Remember in a turbo the exhaust only drives the turbine that is then connected by a shaft to the compressor which compresses the intake charge.

As for distance, it would depend on too many variables I think.

This is not the way to start a Monday...(headache)... :)
 
As I see it, it will work. :D However, it will be less efficient.

There is fuel still burning when the exhaust valve opens. The closser you mount the turbo the better.
While there is still a flame, there is still energy being produced.

If you mount the turbo six feet from the exhaust valve, several things happen. There will be increased volume in the pipe.
If it takes pressure to spin the turbo, it will take longer to pressurise the pipe.
I am not an engineer but I assume the heat helps to build pressure.
The more the gasses cool, the more volume you lose. You can still spin the turbo, but it will be less responsive.
The exhaust will cool considerably while cruising part throttle. Not as much while full throttle.
When you lose heat you gain density. I don't know how this affects the situation, but I don't feel it helps. I think the more density causes more resistance to flow. Again, still doable but less effecient.

Maybe it is possible to design the system with a different exhaust side of the turbo and minimise the loses, but I personaly feel it will be less hp than you could have with the normal setup.

The beauty of turbos is the fact that they don't cost power to make power.
This is great as long as you don't sacrifice drivability. The idea of mounting the carb remotely was bad and potentialy dangerous. I am glad you decided to change that.

Don't take it personal, most of us will do extra work to get that little bit of extra HP. You are talking about doing extra work to get a little bit less HP.
We are only trying to help.
If you want to explore the possibilities, get after it. Please share your results. I for one would love to see some real world comparisions by someone who isn't selling the ideas.
 
Guys, this is some really great stuff!...JGTurbo...I am curious to here the results of your Ranger...did I plant another experiment in your head?

So, what I have to do now is get a oil return pump and a boost gauge...then connect my oil pressure line with a T-fitting at the oil sender...I've been told that the oil return needs to be just above the oil level...can I get away with just tapping into the valve cover to return the oil?...will this do any damage?...my oil pan is freshly powder coated and I really do not want to remove and weld on it...the other place could be at the PVC vent at the right front side of the block...to me it makes no sense whether it's 1" or 10 inches above the oil...no different then oil being released over the rockers from the rail assembly.

CobraSix, it's really cool to have someone on board with your knowledge...the boiling bottle experiment is a very good comparison to your reason...I wont disagree, however, I believe that the velocity of the exhaust weather it be 12" or 72" from the turbo will still spin the blades because the pressure has to go somewhere...I understand what the heat difference is due to the turbo being mounted so far back...I am however counting on the turbo outlet having such a short pipe that I will hope to have a good velocity due to the quick pressure drop.

Dave 8)
 
From what I've seen, the rear-mounted turbos have to use a smaller A/R housing to get acceptable results. This in itself proves that exhaust temperature, thus volume/pressure is reduced when the distance from manifold to turbo is that far. Yes, they can work acceptably. But there is power and throttle response left on the table vs. a close-mounted turbo. IMO, they are suited only when there is not enough underhood room for a conventional mount. That's why STS got started with late-model Camaros. It makes sense in that application.
 
As I said before, I'm not going to spend the time to look at this thing quantitatively (mostly because I know the answer and I won't spend the time to do the math to prove it to myself) but I'll jump into a qualitative discussion.

For starters look at the turbine and the compressor as two separate machines which just happen to be joined together. I'll get to this a bit later.

The turbine is a device for turning energy inputs into work output. That energy basically has two forms: The kinetic energy of the exhaust gas stream and the potential energy of the exhaust gasses made up of the exhaust gas heat and the pressure of the exhaust gasses acting on the area of turbine. But it's all the same thing, energy input. You cannot say one form is more important than the others

If you look at the sum total of this energy just before and just after the turbine, the difference in these two sums is the energy transferred to the turbine output shaft or given up as losses. The more energy you have at the turbine inlet, the more work you can get out

Let's start with the easiest one: Pressure

The work done by pressure is a function of the pressure, the area upon which it is acting and the distance over which it acts. For any given turbine, the area and distance thru the turbine will remain constant so the energy available is pretty much proportional to the inlet pressure. If the turbine were to exhaust directly to atmosphere, the discharge pressure would be zero and the work done by pressure would be directly proportional to the inlet pressure. Assuming equivalent mufflers, the discharge pressure of the turbine is going to be the same or very nearly the same regardless of whether it is a front or rear mounted turbine and a very low number compared to the inlet pressure, so the conclusion is that the higher the pressure the more work done and that the gains in work are roughly linear to pressure.

Then let's look at flow:

Kinetic energy is usually expressed as 1/2 M V^2 where M is the mass that is moving and V is its velocity. (This is only true in a steady state condition; if you change the mass flow like when you go from cruise to WOT, the equation gets much more complicated because you have to account for the partial derivatives of both mass and velocity with respect to time. I haven't played with partial derivatives since college and I'm not going to try any time soon. For the purposes of this discussion 1/2 M V^2 is close enough). At steady state, the mass of the exhaust gas in and out is constant at any point in time so any kinetic energy transferred is due to a change in velocity only. If you compare two different exhaust gas charges, a denser slower moving charge increases its energy in proportion to its increase in density but a less dense faster moving charge increases its energy proportional to V^2

Finally, look at heat:

Unlike pressure of flow heat does no work directly on the turbine, but since the exhaust gasses are substantially cooler after the turbine than before, some significant amount of heat is being turned into work. The answer lies in the fact that as hot gasses expand they cool. When exhaust gasses pass through the turbine they expand from their compressed state prior to the turbine to near atmospheric after the turbine. The temperature change is energy given up to the turbine.

Heat also interacts in another way. At any given throttle opening and boost level, the engine is going to produce a known mass of exhaust gas. If that gas is hot it is going to be at a higher pressure and when released into the turbine to do work it is going to be released at a higher velocity.

Now, remember that other machine stuck on the end of the shaft, the compressor? The more work the turbine outputs, the more work is input into the compressor meaning that more fuel/air mixture is compressed and fed to the engine, making more exhaust gas, to do more work, etc. Turbine efficiency is the gift that keeps on giving.

I'm not going to flog the subject of lag other than to say that the time required for any given device to compress a mass of either exhaust gas or intake air to any given pressure is going to be proportional to the volume of the tube in which you are compressing it. Whether this number is significant or not in any given application is debatable, but the shorter the tube the better. High torque engines tend to mask the turbo lag but it's still there.

You can obviously size the machinery to work as a rear mount, it has been done. But if you were to take that same turbo and mount it under the hood where it belongs, you would see more boost and less lag. I see no point in intentionally designing something to be less efficient
 
so will the size of the exhaust pipe effect the spool of the turbo as well?
im guessing that a 3" would be bad. but would say a 1 3/4-2" be a lot better?

and would adding/moving a cat close to the turbo help but fully burning the gas(making more expanding exhaust gases)
 
StrangRanger, some great input as well...I started this thread as I wanted some input for my RMTS and got a lesson in thermodynamics...way cool!

I found this oil return pump on the web http://www.scavengepump.com/index.shtml...I had no ideal they were so expensive...$220 bones!...they also say I can run the return oil through a oil cooler before it goes back to the engine...as well as a oil filter too.

I also learned that the oil can be returned back to the engine via the valve cover would be OK...the pump and a boost gauge will be my only investment before installing the turbo...so I'll need about $400 bucks to do the install...I'm not sure how much the oil pressure line will cost but am willing to bet it too isn't cheap.

Oh ya!, I almost forgot the expense to mod the Holley...just give me a couple of weeks and I should have this thing moving.

I'll also have some pictures soon as I am building the turbo adapter with a short piece of tubing to be welded on after I cut the muffler off.

Dave 8)
 
TCIC 300ci superbeast":2g5omri8 said:
so will the size of the exhaust pipe effect the spool of the turbo as well?
im guessing that a 3" would be bad. but would say a 1 3/4-2" be a lot better?

and would adding/moving a cat close to the turbo help but fully burning the gas(making more expanding exhaust gases)
Click to expand...

Currently I have a 2.5" tubing size after the header and just before the muffler...I was planning on reducing it to a 2" as the amount of space between the two might help with the spooling effect...I'm thinking that if the pipe was a little narrower, this might help speed up the turbine quicker.
 
Falcon Ranch":2gcmnqm6 said:
TCIC 300ci superbeast":2gcmnqm6 said:
so will the size of the exhaust pipe effect the spool of the turbo as well?
im guessing that a 3" would be bad. but would say a 1 3/4-2" be a lot better?

and would adding/moving a cat close to the turbo help but fully burning the gas(making more expanding exhaust gases)
Click to expand...

Currently I have a 2.5" tubing size after the header and just before the muffler...I was planning on reducing it to a 2" as the amount of space between the two might help with the spooling effect...I'm thinking that if the pipe was a little narrower, this might help speed up the turbine quicker.
Click to expand...

thats what i was thinking as well.
 
TCIC 300ci superbeast":2mzh1b2k said:
so will the size of the exhaust pipe effect the spool of the turbo as well?
im guessing that a 3" would be bad. but would say a 1 3/4-2" be a lot better?

and would adding/moving a cat close to the turbo help but fully burning the gas(making more expanding exhaust gases)
Click to expand...

Yet another reason to go with a front mount. All you need is a J-pipe which matches the manifold outlet and/or turbine inlet and the biggest exhaust diameter you can manage after the turbine outlet.

With the rear mount, it's a balancing act. You need to keep the diameter down to reduce volume and minimize lag and also to reduce the surface area through which you can lose exhaust heat. But small diameters give substantially greater flow losses, meaning less pressure available at the turbine inlet. I'm not sure cat placement has much effect. The same amount of heat is going to be added in the cat and the same amount will be lost in the exhaust pipe. Whether it's lost before or after the cat is probably irrelevant. The net energy at the turbine inlet will be the same.

Just put the turbo under the hood where it belongs.

BTW have you considered using one of the HD 300 manifolds that pop up on eBay? They have a 2.5" outlet which is just begging for a J-pipe.
 
I was debating pulling the dynamic pressure formula out, but Strange did it for me already.

FR, I still say be proud of a turbo'd six...show it off and under the hood. But hey, it'll be an interesting project.

Slade
 
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