Sizing a turbo for rear mounting

What are you looking for? Sizing for front mount is all over the web but what changes when considering sizing for rear mount.

I can't really help with your specific question, but I'll chip in my 2 cents and say that I recall the main challenge with rear-mount turbos being an oil supply. Cars with rear-mounts tend to blow up oil pumps rather quickly, IIRC.

I am just exploring all options on my 924s turbo project. The muffler in these cars are huge so putting a turbo back there seems to be a no brainer. I am looking at this versus the mounting I was considering up ground just because space on these things is so limited.

As for oiling and cooling etc I was thinking of using a remote resiwar, a trans cooler (from a explorer because I have it on the shelf) a possible a 12 volt cooling fan (or two) from radio shack a remote oil filter mount and filter and a Bosch fuel pump to deal with oiling duties. I was going to use a thinner oil. I was actually thinking of synthetic ATF as I really don't have to deal with as much heat and it is very low foaming while having a higher detergent additive package than oil used in motors. I was going to put all this into one of the rear fender wells and some how duct some air into the cooler or just mount it under the car if I can figure a way to protect it from rocks etc. This is just logistics there is plenty of room back there.

The technical side of it seems easy. Heck I was even considering inter cooling in the rear some how. Thus not needing to have anything in front of the motor. Again the fender wells behind the wheels are huge. I could also use the spare tire area. This would early house the 951 inter-cooler I have. I will then just use a piece of 2" hot dipped thin walled galvanized conduit for the pressure side. and run it back to the front. Available in the Home Depot electrical department. The bigger issue I have is dealing with the BOV and how to make the afm/maf work. I would rather do a draw through with the bov dumping back in front of the turbo. This means another bit of hose / tube running back. I may just use a piece of 3/4" galv conduit. Take stainless steel hose clamps and attach it to the 2" piece.

Like I said the actual fabrication of this seems to be much easier than trying to front mount this. What I am lacking in is the actual sizing of the turbo. I have been told / read that with the cooler exhaust gasses you loose the added push from the expansion of the exhaust gases as they exit the motor. I would surmise from this that you need a small hot side but the same size cold side.

Compressor sizing doesn't change. Turbine however gets smaller because the flow is cooler and more dense, meaning volume and velocity are much less. Most of your energy is in the heat (not the pressure) so insulate the exhaust to get as much as possible to the turbine.

The trick is finding a turbo with a turbine a good bit smaller than the compressor. I'd love to see a Grassroots how-to on taking the smallest T25 turbo and swapping in a larger compressor wheel and housing. Which part numbers to grab, what rebuild parts, etc......

I was talking to Corky once about that issue and he said he'd downsize the turbine "one size" and look at what that did.

He was very suspicious of the whole process and said it wouldn't work, but time has proven him wrong and yes it is pressure differential not heat that runs the turbine. Heat simply expands the gases to give a greater pressure differential.

Since you don't have that heat forget about it and worry totally about flow.

From Squires Turbo Systems FAQ and a quick rehash of basic thermo (this analysis neglects alot of variables including wall friction, flow around corners, heat exchange rates based on material properties, etc):

Doesn't heat create the velocity in the exhaust gasses to spool the turbo? No, heat doesn't create velocity. Heat creates volume. If you look at any of the physics laws for gasses, you will find that pressure and volume and heat are related. PV=NRT is a popular one, The V isn't for velocity, it is for Volume.

The turbine housing is what creates the velocity. The scrolling design that reduces the volume of the exhaust chamber as it scrolls around causes the gasses to have to increase in velocity and pressure to maintain the same flow rate.

Hotter gasses have more volume, thus requiring a higher A/R which in effect means that it starts at say 3" and scrolls down to approximately 1". Lower temperature gasses are denser and have less volume, so they require a lower A/R housing which would start at the same 3" volume, as the turbine housings use standard flanges, and scroll down to say 3/4".

Now if you were to reverse the housings in application, the conventional turbo would spool up extremely quick, at say around 1500 rpm but would cause too much backpressure at higher rpms because the higher volume of gas couldn't squeeze through the 3/4" hole without requiring a lot of pressure to force it through. On the reverse side, the remote mounted turbo with its cooler denser gasses, wouldn't spool up till say around 4000 rpms but once spooled up would make efficient power because it doesn't require hardly any backpressure to push the lower volume of gas through the larger 1" hole.

So a quick rehash: size the turbo conventionally, then see about resizing the turbine housing down at a minimum (based on sizes available), many turbos have multiple housing applications for various sized engines, a good example would be T3/T4s which come in many various sizes and can be hybridized as well. An alternative would be Holsets as there are tons of turbine housing sizes on the same turbo

Here's a post by a fella who works for STS on an LS1 application: http://ls1tech.com/forums/forced-induction/1014057-custom-rear-mount-turbo-3.html

carguy123 wrote: and yes it is pressure differential not heat that runs the turbine. Heat simply expands the gases to give a greater pressure differential. Since you don't have that heat forget about it and worry totally about flow.

Nope. If you lose the heat you lose the lion's share of the enthalpy. The power extracted by the turbocharger turbine is the change in enthalpy over the turbine multiplied with the massflow. The higher the temperature the more enthalpy will be availible at the turbine inlet. Which is why the flow coming out of a turbine is drastically cooler than what went in, along with a lower pressure. For a pressure-only driven turbine you need WAY more backpressure to power the compressor (excessive backpressure=bad).

Think of it this way: you need energy to compress the flow going into your engine. Your exhaust has energy in the form of pressure and temperature (volume is not energy!), most of which is temperature. The turbine extracts the energy in the flow - if you lose most of your temperature energy, you may not have enough left to compress the flow going into the engine.

Oh and there is a good bit of "marketing" on the STS site.

Ya I agree that it would be great if GRM could take a look at this and dispel some of the myth and legend about this. And oh ya take a look at 4 cylinder motors. Just about everything I see out there deals with V8's (LS1's and the like).

I think I may see about moving forward on this. I now know that the td04's from the Volvo's 13 and the 15 the difference is the cold side. The hot side is identical.

This is what I am planning on my Milano. Remote mount with a TD04-13C from a Volvo 740.

I'll have a separate oiling system in the rear of the car, with oil cooler, reservoir, pushed by a Shur-Flo pump. The pump will suck through the cooler, so it should be OK temperature wise.

I'll be using megasquirt.

If you are keeping the AFM, why not extend the AFM wires to the rear of the car?

Heat doesn't turn the turbine wheel, the flow of gases does. You can put a blow torch on a turbo all day and never get it to turn but blow thru the intake with your mouth and it'll turn. You size the turbine for the amount of gas flowing thru the system and the amount of air you want out. It's a simple as that.

Now here's the issue with a rear mounted turbo, the amount of heat in the system (which means the volume of the gas as Raze pointed out) varies greater than one mounted close to the engine. So you need to find some way to equalize the temp so that you equalize the flow.

On the drag strip or a track is one thing, but driving around on the street will cause a fairly wide variance in temps and therefore volumes.

From, my limited understanding, cooling the intake charge to the plenum is accomplished without the need for a heat exchanger mainly because of the length of the plumbing. I would think a smaller turbo for a small 4 cylinder would be okay compared to a larger one on a big a$$ V8. Though you'd have to consider the velocity of the exhaust gasses at the far end of the system when sizing, and what not. my post may actually have no real information in it, this is all from the top of my head lol.

Also from my limited knowledge the length of pipe from the turbo to the front is supposed to be equivalent to an intercooler in volume which means there's supposed to be no more lag. That doesn't seem possible, but that's what I've read.

carguy123 wrote: Also from my limited knowledge the length of pipe from the turbo to the front is supposed to be equivalent to an intercooler in volume which means there's supposed to be no more lag. That doesn't seem possible, but that's what I've read.

Our front mount system has something like 7' of ducting from the compressor outlet to our intake, plus a modest sized intercooler, we don't have any appreciabe lag but to believe me do some calculations. I wanted to see before I built it what kind of volume increase it had to the system and therefore how much lag I could expect based on the flow rate of the turbo, it was really small, as in the added delay due to the volume increase of the longer intake path was less than 0.1 seconds at an averaged flow rate (i.e. not full boost, remember here that the delay is variable based on the rest of the components of the system, hence why sizing your turbine for your given flow rate is important, not just looking at compressor maps). Honestly, where people get into trouble with rear mounts and plumbing is correctly sizing not only their turbine, but also the cold side plumbing size, if it's too small you'll create alot of backpressure on the compressor (not good), too big and you'll add lag due to the disproportionate volume differential vs flow rate. All of this is theory however, in application an individuals sensitivity to lag is variable, however backpressure and choked flow is not...

dean1484 wrote: Ya I agree that it would be great if GRM could take a look at this and dispel some of the myth and legend about this. And oh ya take a look at 4 cylinder motors. Just about everything I see out there deals with V8's (LS1's and the like).

I think you hit the nail on the head - V8s seem to tolerate higher exhaust backpressures so they can drive the compressor for moderate boost better than a 4 cyl.

carguy123 wrote: Heat doesn't turn the turbine wheel, the flow of gases does. You can put a blow torch on a turbo all day and never get it to turn but blow thru the intake with your mouth and it'll turn. You size the turbine for the amount of gas flowing thru the system and the amount of air you want out. It's a simple as that.

That's the tail wagging the dog - temperature (~1100F) drives the flow of gasses just like afterburners (reheat) in jet aircraft. The volume stays constant across the turbine - upstream pipe is a fixed diameter, and downstream is roughly the same. So put your blow torch upstream of the turbine in a fixed volume and the expanding gasses will drive the turbine. That's how you make a home brew turbojet. It's called the Brayton cycle. Look here:

http://en.wikipedia.org/wiki/Brayton_cycle

Heat (q) is added under constant volume. The turbine extracts work from the gas stream, and gas cools (-q). In the turbocharger, most of that work comes from temperature. Not pressure. For some reason we understand pressure diff turning a turbine (because they think it is a fan device?) but don't believe thermodynamics class when another form of energy contributes to the work done on a turbine. Kinetic energy in the form of temperature and kinetic energy in the form of pressure BOTH do work on the turbine wheel. When you have 1100F coming out of the exhuast mani, you've got a bunch more thermal energy than 6 psi of back pressure energy.

There is some energy from the dP - but to ditch the thermal energy is neglecting 60%-70% of the energy that can turn the compressor wheel.

SVTF wrote: There is some energy from the dP - but to ditch the thermal energy is neglecting 60%-70% of the energy that can turn the compressor wheel.

But is still almost free energy, and you can tap that. It of course is not the same amount as if the turbo was upstream (because energy is lost) but to ignore the free energy the comes out the tail pipe as heat, noise and pressure would be folly.

P38, P47 and other military aircraft use this set up. Of course they had much more exhaust going to the turbine and produced 2000hp, but my point is this is not a new concept

Well, not really free, but yes you can use the dP to turn the turbine. I think the problem is that without the thermal contribution you'd have to run a really high exhaust back pressure to get significant boost on the compressor side. 8 cyl engines seem to tolerate that better than 4 bangers.

Let's assume I want a pressure ratio of two on the compressor (say 10 PSI at the engine inlet); that means I'd need an extra 20 PSI (accounting for losses and such) of backpressure IF the turbine was that same size. But since the turbine is much smaller due to the more dense flow, you need more dP to spin the larger compressor. So maybe we end up with 30+ PSI total backpressure on the engine? That might be okay for a V8 that only makes 1/4 mile runs, but I don't think a DD 4 cyl will tolerate that for long.........?

You are still saying that the heat causes an increase in volume which gives higher pressure or a better way to say it is that you get more work out of the same amount of gas. That's all true, but if you size the turbine to use the amount of gas flowing thru it then you get out of the system what you want.

Another biggie is that all the math is already done for a turbo mounted near the engine so it's much easier to determine the proper size.

If all you want is heat, then mount the turbo just aft of the cat.

As far as back pressure, sure there's an increase in both types of systems. So just don't run a muffler. I'm not buying the argument that it takes twice as much pressure to spin it just cause it's a 4 vs a V8. They make dual rear turbos for the V8s and that's only 4 cylinders per turbo.

I've seen all kinds of specious arguments as to why the rear turbo doesn't work, but the bottom line is that it does. They've made the same types of arguments about the bumble bee not flying either and it still does. As they say, the proof is in the pudding.

SVTF wrote: Well, not really free, ... But since the turbine is much smaller due to the more dense flow, you need more dP to spin the larger compressor. So maybe we end up with 30+ PSI total backpressure on the engine? That might be okay for a V8 that only makes 1/4 mile runs, but I don't think a DD 4 cyl will tolerate that for long.........?

I agree, but here is were I support the idea. Simplicity, if space is an issue, if you don't want a totally dedicated system, if you have the parts already and if you run low boost.

I thought about this for myself, but once I did the math it didn't pay for the amount of power I wanted. I figured I might as well install a bigger engine and come out cheaper. Example STS wants $4000 for a system. I could put an LS based truck motor (L33 or something) in my Suburban with a homemade turbo system for that amount and have some money left over. But, then I would have to deal with premium fuel in a thirsty truck. I figure my truck will stay a truck

carguy123 wrote: You are still saying that the heat causes an increase in volume which gives higher pressure or a better way to say it is that you get more work out of the same amount of gas. That's all true, but if you size the turbine to use the amount of gas flowing thru it then you get out of the system what you want. If all you want is heat, then mount the turbo just aft of the cat. As far as back pressure, sure there's an increase in both types of systems. So just don't run a muffler. I'm not buying the argument that it takes twice as much pressure to spin it just cause it's a 4 vs a V8. They make dual rear turbos for the V8s and that's only 4 cylinders per turbo. I've seen all kinds of specious arguments as to why the rear turbo doesn't work, but the bottom line is that it does. They've made the same types of arguments about the bumble bee not flying either and it still does. As they say, the proof is in the pudding.

I guess I'll have to agree to disagree. Heat does nothing to the volume, the volume is fixed and doesn't change: exhaust piping in, exhaust piping out. A muffler adds maybe 1-2 PSI, and that is downstream of the turbine. You simply can't get 14 PSI boost on the compressor side using only a couple PSI backpressure on the exhaust to drive the turbine. Using only pressure you HAVE to have more pressure ratio across the turbine than you do across the compressor, and that for the same size wheels. Do you also consider eBay electric superchargers another great way to add power?

Yes a rear mount turbo can work - in particular applications for particular driving cycles with many compromises. I'm not against rear mount turbos, as I am in the process on designing my own. But I want mine to actually work.......and that means conserve all the heat I can.

The volume isn't fixed. You can prove it to yourself with this simple little test. Blow up a balloon, now put it in the freezer. See what happens to your volume.

It's kinda like what happens when the male member hits cold water. It's all still there, it's just been compacted and can't do as much work.

http://www.balancemotorsport.co.uk/main.asp?category=Project+Car hmm I really like this one

carguy123 wrote: The volume isn't fixed. You can prove it to yourself with this simple little test. Blow up a balloon, now put it in the freezer. See what happens to your volume.

This analogy is not relevant, since it is a constant pressure process. The pressure in the balloon and outside the balloon stay the same - P1xV1/T1 = P2xV2/T2 (Boyles Law). With pressure held constant the volume changes are driven by the temperature change. Your exhaust pipe on the other hand is a fixed volume - cooler flow means less pressure in the fixed volume. Hotter flow means high pressure in a fixed volume (think pressure cooker).

SVTF wrote:

carguy123 wrote: The volume isn't fixed. You can prove it to yourself with this simple little test. Blow up a balloon, now put it in the freezer. See what happens to your volume.

This analogy is not relevant, since it is a constant pressure process. The pressure in the balloon and outside the balloon stay the same - P1xV1/T1 = P2xV2/T2 (Boyles Law). With pressure held constant the volume changes are driven by the temperature change. Your exhaust pipe on the other hand is a fixed volume - cooler flow means less pressure in the fixed volume. Hotter flow means high pressure in a fixed volume (think pressure cooker).

What about the rate of the gas? Is the exhaust pipe really a fixed volume? It is open at the end and the gas gets to go out at differing rates depending on the volume of gas? Does it not? How can a "fixed volume" have "cooler flow". I am asking, as I am not acquainted with fluid physics in any meaningful way.

You guys fail to realize that the exhaust flows flow out of the engine in a series of pulses, not a continuous stream. As the exhaust gases cool, they expand. This expansion means that the pulses are less pronounced. The pulses help drive the turbine more efficiently than a steady stream.

Lets get away from saying "fixed volume". There is no fixed volume, it varies with pressure, temp, rpm, volumetric efficiency, etc. Say "mass flow rate." With all the variables held constant, the mass flow rate does not change throughout the exhaust system, even though the pressure, temp, and volume change.