Turbo Compounding.

[KiwiBacon]

The reason for compounding is to have two turbos both working at the most efficent point ie both say 15-20 psi each, this then gives you roughly 30-50psi at the manifold. Try doing that with a single or even a variable turbo. At these pressures you are flowing a lot of air, the secret in making high horsepower.

Andy

Exactly.

To get 30psi with two turbos, the little one can provide 15psi boost, the larger one feeding it only needs to provide 7psi.

[blurredvision]

Just wanting too clarify what you meen by stacking turbos?
Is this staggering the sizes so that you have the two of them being efficent in different parts of the rev range (ie the last of the RX7's and 1jz motors), or are you talking about one turbo dumping into another turbo and being compressed twice like they do on some of the tractor pull Motors?
Also i thought that when adding pressures it was not simple addition, but a value between the two pressures with the reference to the rario of the volumes of gas?

[Dzltec]

Look at the pics of the turbos on a manifold, not the bmw pics. Yes the big one feeds the little one which then goes into the engine.

Pressure in the inlet manifold is detemined by pressure ratios, not adding the two pressures together, hence the word of compounding


Andy

[rocknferoza]

Hi Andy

How much power was the cruiser putting out with the twin turbo setup and what is the patrol also putting out

Have u got any videos of them on the dyno etc.

Thanx
James

[tweak'e]

o i get ya. sorry, my pics of the BMW setup was sequential not compounding as you call it.

you would have to be really carefull on sizing the turbo's, it will still be limited by flow of the smaller turbo. for a street vechile i would look at trying to intercool between the turbo's, otherwise you will loose a lot due to trying to compress hot air.

mind you for a diesel wouldn't supercharger + turbo be better? get better low down off the line power rather than having to wait for boost to build. exspecialy in the low gears.

out of interest i hear the nextt generation of variable turbo's will be interesting, variable on compresser and turbine

[KiwiBacon]

o i get ya. sorry, my pics of the BMW setup was sequential not compounding as you call it.

you would have to be really carefull on sizing the turbo's, it will still be limited by flow of the smaller turbo. for a street vechile i would look at trying to intercool between the turbo's, otherwise you will loose a lot due to trying to compress hot air.

mind you for a diesel wouldn't supercharger + turbo be better? get better low down off the line power rather than having to wait for boost to build. exspecialy in the low gears.

out of interest i hear the nextt generation of variable turbo's will be interesting, variable on compresser and turbine

That BMW picture is compounding in several of it's stages. Study the pictures closer and it's quite clear that the output from one turbo can feed the input of the other.

Intercooling between stages is a good way to lose your engine under a maze of hoses. IMO the extra complexity isn't worth the small gain.

Superchargers suck.

With compounding you can flow a lot more air through the smaller turbo as it's compressed first. It's still restricted by volume flow, but no longer by mass flow.

[tweak'e]

That BMW picture is compounding in several of it's stages. Study the pictures closer and it's quite clear that the output from one turbo can feed the input of the other.

it only partly does it, it has an exhaust valve to seperate the exaust flow between turbo's and a valve on the compressor side to bypass the smaller turbo. the compounding is proberly an inbetween turbo's stage.

Intercooling between stages is a good way to lose your engine under a maze of hoses. IMO the extra complexity isn't worth the small gain.

depends on how its done. i was thinkin more along the lines of a small short water/air inbetween the turbo's.

Superchargers suck.

yeah right....

[KiwiBacon]

it only partly does it, it has an exhaust valve to seperate the exaust flow between turbo's and a valve on the compressor side to bypass the smaller turbo. the compounding is proberly an inbetween turbo's stage.

Compounding boost does not necessarily need compounded exhaust stages.

Water/air intercoolers are hard to come by, let alone compact ones suitable for placing between the stages.
This is one of the few I could find, it's rather large. Those inlets are 3" each.

[zagan]

Just wanting too clarify what you meen by stacking turbos?
Is this staggering the sizes so that you have the two of them being efficent in different parts of the rev range (ie the last of the RX7's and 1jz motors), or are you talking about one turbo dumping into another turbo and being compressed twice like they do on some of the tractor pull Motors?
Also i thought that when adding pressures it was not simple addition, but a value between the two pressures with the reference to the rario of the volumes of gas?

Fom what I've seen on TV shows where the makers talk about these systems.

Is that you use a little turbo to start compressing the air this gives you very little lag because the weight of the turbines is a lot less than a bigger turbo.

the air already partly compressed helps to spin up the next turbo quicker.

the bigger turbo compresses the air again and this allows you to fit in even more air into the motor itself.

For a diesel this basicly means more power due to having heaps more oxygen to burn up.

Think of it like this:

1st turbo compresses to 2-3bar
2nd turbo compresses to 6-8 bar

as per the BD manual you don't need an intercooler but if your wanting a bit extra HP, then after the second turbo fit an intercooler then into the motor.

a larger exhuast outlet off the turbo helps to reduce EGT temps, so you'll want a big exhuast to go with the kit as well, this is the major differance with the 3rd party diesel turbos and the factory turbos.

[tweak'e]

Just wanting too clarify what you meen by stacking turbos?
Is this staggering the sizes so that you have the two of them being efficent in different parts of the rev range (ie the last of the RX7's and 1jz motors), or are you talking about one turbo dumping into another turbo and being compressed twice like they do on some of the tractor pull Motors?
Also i thought that when adding pressures it was not simple addition, but a value between the two pressures with the reference to the rario of the volumes of gas?

Fom what I've seen on TV shows where the makers talk about these systems.

Is that you use a little turbo to start compressing the air this gives you very little lag because the weight of the turbines is a lot less than a bigger turbo.

the air already partly compressed helps to spin up the next turbo quicker.

the bigger turbo compresses the air again and this allows you to fit in even more air into the motor itself.

For a diesel this basicly means more power due to having heaps more oxygen to burn up.

Think of it like this:

1st turbo compresses to 2-3bar
2nd turbo compresses to 6-8 bar

as per the BD manual you don't need an intercooler but if your wanting a bit extra HP, then after the second turbo fit an intercooler then into the motor.

a larger exhuast outlet off the turbo helps to reduce EGT temps, so you'll want a big exhuast to go with the kit as well, this is the major differance with the 3rd party diesel turbos and the factory turbos.

thats sound more like sequential ie the BMW pic. the small turbo spins up quick and provides boost (2nd turbo bypassed), then the vavles redirct for the 2nd turbo to work (small turbo feeds big turbo), but then as the small turbo can't flow enough its bypassed and the big turbo runs on its own.

if i get the idea of the compounding, its simply a 'normall' turbo with one added before it to compress the air a bit. turbo's work more efficently with denser intake air, so the first turbo is simply there to make the air denser so the main turbo will run more efficently. downside is you don't get the low down boost like a twin/sequnetial setup. also the lag of the 2nd turbo would be a problem for a street vechile.

[Dzltec]

There seems to be a little confusion on how this works. Ignoring the bmw system as it is both compounding and sequential.

In a true compound system with no flaps in the exhaust or inlet system, the smallest of the 2 turbos is mounted to the exhaust manifold. This is driven first and directly supplies the engine with air, like a normal turbo would be fitted. Once the exhaust gasses exit this, they go to drive the bigger turbo. This turbo is drawing air from the air filter, pressurising it then feeding the inlet of the first turbo.

The first turbo does no know or care that it is being fed pressurised air. All its doing is adding its own pressure to it, hence compounding it.

If this system of turbos are matched and set correctly, there is no lag, just a broad span of boost pressure with no falling off of boost pressure when one turbo gets out of its flow capabilities.

Ther is no lag of the larger turbo coming in, just immense pulling power.
This is designed like this to supply a hell of a lot of air, more air = more fuel can be added, = more power/fun

[-Scott-]

Andy, I'm still trying to get my head around this - I think I'm almost there.

Is the following (sort of) correct?

At low engine rpm/early in the sequence, the small compressor is providing most of the boost, and the large compressor is doing little.

As engine revs and/or exhaust flow builds, the larger compressor compresses the air more, and the small compressor compresses the pressurised air - the "compounding" bit.

When the small turbo is reaching the limits of it's flow abilities, it doesn't really matter, because the larger compressor is doing more of the work, and the smaller compressor is just "helping" as much as it can - but still adding pressure.

I suppose I'm trying to imagine the relationship between "boost pressure" in the intake manifold, and "boost pressure" between the compressors. It's very non-linear?

Also, would this require VERY high turbine speeds?

[Shadow]

Andy, I'm still trying to get my head around this - I think I'm almost there.

Is the following (sort of) correct?

At low engine rpm/early in the sequence, the small compressor is providing most of the boost, and the large compressor is doing little.

correct

As engine revs and/or exhaust flow builds, the larger compressor compresses the air more, and the small compressor compresses the pressurised air - the "compounding" bit.

correct

When the small turbo is reaching the limits of it's flow abilities, it doesn't really matter, because the larger compressor is doing more of the work, and the smaller compressor is just "helping" as much as it can - but still adding pressure.

incorrect,

The smaller compressor is always doing the bulk of the work. The larger compressor simply comes on boost and increases the overall pressure differential between atmosphere and inlet manifold.

I suppose I'm trying to imagine the relationship between "boost pressure" in the intake manifold, and "boost pressure" between the compressors. It's very non-linear?

Also, would this require VERY high turbine speeds?

You are correct, if you look at the system from 3 pressure perspectives, at low rpm the pressure either side of the large compressor is effectively equal, as the large compressor comes onto boost, its own pressure differential ramps up, and this also forces the overall pressure differential to ramp up.

Turbine speeds do not to be any higher as the smaller turbo is now compressing denser air, (for every turn of the compressor wheel, its moving more oxygen).

[tweak'e]

If this system of turbos are matched and set correctly, there is no lag, just a broad span of boost pressure with no falling off of boost pressure when one turbo gets out of its flow capabilities.

its always going to lag because the smaller tuurbo is as big as a standard turbo, it will lag as badly as a stock motor.

At low engine rpm/early in the sequence, the small compressor is providing most of the boost, and the large compressor is doing little.

As engine revs and/or exhaust flow builds, the larger compressor compresses the air more, and the small compressor compresses the pressurised air - the "compounding" bit.

afaik thats sounds about right.

Also, would this require VERY high turbine speeds?

i don't think it would be that bad until you get to extremly high boost. the turbo is a lot more efficent at compresing dense air so won't need really high speed like a single turbo. however really high boost always trys to leak back through the turbo as the turbo has no seals seperating the inlet and outlet sides (unlike some superchargers).

if i understand it right one problem is the wastegate size of the smaller turbo, if its stock sized (eg internal) then it won't flow enough to cope with the extra exhaust gas (exspecially if you use a vairable turbo) and can cause it to over rev.

[KiwiBacon]

if i understand it right one problem is the wastegate size of the smaller turbo, if its stock sized (eg internal) then it won't flow enough to cope with the extra exhaust gas (exspecially if you use a vairable turbo) and can cause it to over rev.

The wastegate size is no different to that of a normal single turbo.

Remember the small turbo has to flow exactly the same volume as it did before. Both into the engine (compressor) and out of the engine (turbine and/or wastegate).
The density has changed, but the volume hasn't.

[zagan]

Andy, I'm still trying to get my head around this - I think I'm almost there.

Is the following (sort of) correct?

At low engine rpm/early in the sequence, the small compressor is providing most of the boost, and the large compressor is doing little.

As engine revs and/or exhaust flow builds, the larger compressor compresses the air more, and the small compressor compresses the pressurised air - the "compounding" bit.

When the small turbo is reaching the limits of it's flow abilities, it doesn't really matter, because the larger compressor is doing more of the work, and the smaller compressor is just "helping" as much as it can - but still adding pressure.

I suppose I'm trying to imagine the relationship between "boost pressure" in the intake manifold, and "boost pressure" between the compressors. It's very non-linear?

Also, would this require VERY high turbine speeds?

basicly that would be correct.

Your small compressor would have to be matched to your size of engine this way you'd get the max out of it, the small compressor is what spins up the next compressor the big one.

In the BD manual they call the second/big turbo the main turbo as this one is compressing the air the most.

The first/smaller turbo is called the second turbo.

If that helps you out, it's basicly the other way around more or less.

They get around the boost problem by simply using larger piping, and upgraded header that can handle the boost, you would also need a much bigger exhaust BD power's smallest exhuasts for F250's etc is like 5" they go up to 7 or 8" exhuasts.

A bigger exhuast helps to reduce EGTs as well, also helps the diesel motor to pump more air in and get more used air out so doing that mod will help to add extra power if you havn't done it I notice a lot of people don't say anything about exhuasts much on here.


You will also note that shop page link isn't talking about pounds per inch but HP at the rear wheels, 550HP at the rear wheels from their dodge compunding turbo kit and if you add a shot of nos and and a intercooler you can get 800 hp at the rear wheels.

On 2 guys garage they have a show at BD power they had a F250 spinning the rear wheels at 8-10kph for 5-10 seconds then the driver must put the foot down then it just spins the wheels up, these are 33" AT tires.

I had no idea you could control the power to the rear wheels like that with a diesel before, they used it as a going to AD break wipe or whatever was really good to see.

I should have that show recorded I'll try to find it and maybe put it up.

Once you see stuff like that it gets you thinking of what you could do to your diesel as well.

[tweak'e]

if i understand it right one problem is the wastegate size of the smaller turbo, if its stock sized (eg internal) then it won't flow enough to cope with the extra exhaust gas (exspecially if you use a vairable turbo) and can cause it to over rev.

The wastegate size is no different to that of a normal single turbo.

Remember the small turbo has to flow exactly the same volume as it did before. Both into the engine (compressor) and out of the engine (turbine and/or wastegate).
The density has changed, but the volume hasn't.

compressor yes but exhaust, no way. for eg.....your doubling the air in which gives you at least double the exhaust gas (and thats without adding fuel) all of which has to squeeze through the small turbo/wastegate. if waste gate is standard size wouldn't the backpressure be huge and would cause the turbo to severely over rev not to mention waste of power and increase EGT's. unless of courese you actually need that high backpressure to be able to turn the turbo ie load on compresser is increased due to the high density air on the intake so you need extra push from the turbine.

if the standard turbo can handle the extra exhaust it generally will be able to make the required boost which means there is no need for the 2nd turbo

[KiwiBacon]

compressor yes but exhaust, no way. for eg.....your doubling the air in which gives you at least double the exhaust gas (and thats without adding fuel) all of which has to squeeze through the small turbo/wastegate. if waste gate is standard size wouldn't the backpressure be huge and would cause the turbo to severely over rev not to mention waste of power and increase EGT's. unless of courese you actually need that high backpressure to be able to turn the turbo ie load on compresser is increased due to the high density air on the intake so you need extra push from the turbine.

if the standard turbo can handle the extra exhaust it generally will be able to make the required boost which means there is no need for the 2nd turbo

You are forgetting something.

Backpressure increases pretty much linearly with boost. The pressure the small turbo is venting into isn't atmospheric, it's the drive pressure for the big turbo.

Once again, higher density = higher mass flow but same volume flow.

The only piece of the system which sees atmospheric pressure is the start of the intake pipe and the end of the exhaust pipe.

The exhaust pipe gets a pressure ramp up from the exhaust tip to the outlet of the big turbo. (also a small pressure step in the muffler).
There's a pressure step across the big turbo's turbine and another one across the small turbos turbine.

The standard turbo can only take the extra flow because the big turbo increases the density (and mass flow) of the air through it.
Both side of it, hot and cold.

[tweak'e]

i hope i'm looking at this the right way around....

say the small turbo, which is the first to get the exhaust gas has a 3" outlet overall (turbo outlet + wastegate) in standard trim. now you add more boost/air into the motor, double/triple the amount of exaust gas. it all has to go through the 3" outlet which mean pessure in the manifold must go up. if it was a single turbo with that amount of boost/air the outlet would be a lot bigger.

The standard turbo can only take the extra flow because the big turbo increases the density (and mass flow) of the air through it.
Both side of it, hot and cold.

so i take the 2nd turbo restricts the exhaust enough so the there is limited pressure drop across the small turbo hence it won't over rev ?

[Dzltec]

Tweak'e,

have you ever seen inside a turbo turbine housing?

The hole that feeds the turbine wheel is very small to start with. A 3" exhaust will feed up to 350rwkw, its not that restrictive. We actually measured the exhaust manifold pressure or drive pressure. On any turbo system it will be 1-2 times boost pressure. Standard being roughly 1 1/2 time boost. So if your making 10 psi boost pressure the exhaust manifold pressure before turbo will be anywhere from 10-20psi.

When we measured our drive pressure with the twins on the landcruiser, the drive pressure was roughly 50 psi, with a boost pressure of 46psi. This was with a standard ct26 turbo.

I think your theory is restricting your practical efforts. As to over revving of the turbo, the ct26 is still going strong, the l/c now has 375000km,
the last 18 months in a twin setup, and it wasnt driven lightly. I think it it was going to over rev it would have done it by now, and i would have a lunched turbo.


Andy

[KiwiBacon]

so i take the 2nd turbo restricts the exhaust enough so the there is limited pressure drop across the small turbo hence it won't over rev ?

Kind of.
Basically without the big turbo spooled, there isn't a problem getting flow through the little turbo.
Backpressure from the large turbo builds slightly faster than boost from the large turbo, so extra flow doesn't arrive without the extra pressure to keep it to a managable size.

For a single turbo to flow the same as a compound setup, it has to be the size of the large turbo. As you can imagine this gives some serious lag issues and a very small usable boost range.

[tweak'e]

Tweak'e,

have you ever seen inside a turbo turbine housing?

The hole that feeds the turbine wheel is very small to start with. A 3" exhaust will feed up to 350rwkw, its not that restrictive. We actually measured the exhaust manifold pressure or drive pressure. On any turbo system it will be 1-2 times boost pressure. Standard being roughly 1 1/2 time boost. So if your making 10 psi boost pressure the exhaust manifold pressure before turbo will be anywhere from 10-20psi.

When we measured our drive pressure with the twins on the landcruiser, the drive pressure was roughly 50 psi, with a boost pressure of 46psi. This was with a standard ct26 turbo.

Andy

i think you missed my point entirly. i had a quick look around and http://www.cumminsracing.com/ had a small bit on it. basicly the turbo's AND wastegates where sized from measureing 'drive pressure' (as you call it).

i was only useing the 3" as the therotcal example not real life.

the old problem (exspecialy with petrol vechiles) with to much boost is it makes to much exhaust which forces the turbo to overboost. eg with the wategate jammed fully open boost still keeps climbing and 'drive pressure' increases with minimal performance increase. same thing with the coumpound, if the first wastegate is to small it becomes a big bottleneck hampering performance and it restricts the drive to the 2nd turbo. also if its to big then the first turbo will loose drive.

When we measured our drive pressure with the twins on the landcruiser, the drive pressure was roughly 50 psi, with a boost pressure of 46psi. This was with a standard ct26 turbo.

thats exactly it. most of the info i've just looked at mentioned that boost pressure was way above drive pressure. yours sounds like it was being choked. what was the drive pressure after the first turbo?
if i understand all the theory allright you could have your drive pressure a lot lower for the same boost which will give you more performance, better economy and more importantly lower EGT's.

edit: the main reason for KiwiBacon to watch this is if he uses the variable turbo. there is no wategate on these and limited adjustment on the actuator. you might have to add an external wategate to it.

[KiwiBacon]

I still don't know if I've got room to run the GT2256V, if not then a T25 will be going in place.

To get a drive pressure down to (or lower than) boost pressure requires a very hot exhaust and a large turbine housing. Essentially a turbo matched only for full power.

I also have measured exhaust backpressure and boost simultaneously, lowest I could get was a ratio of 1.5 at low revs and low boost. From there it settled into a 2:1 ratio all way up to full boost and full revs.

Where did you find mention of boost creep, what engine were they running it on (petrol/diesel) and was it an off-the-shelf turbo?
Because unless a turbo has been screwed with (hybridised or had a very small A/R exh housing fitted) it will have a turbine capable of flowing every bit of air that it's compressor can deliver.

A 2:1 pressure ratio is not choking. I once fitted a hybrid T2/T25 to my engine and that choked. Backpressure went to 4x boost at full rpm and full boost.
In that example a bigger wastegate would not help, as the actuator is closed loop control driven off boost pressure. A bigger exhaust housing and/or turbine does help.

[Vineboy]

Heres one from an Isuzu site

http://clubisuzu.proboards29.com/index. ... 1147444072

[tweak'e]

Where did you find mention of boost creep, what engine were they running it on (petrol/diesel) and was it an off-the-shelf turbo?
Because unless a turbo has been screwed with (hybridised or had a very small A/R exh housing fitted) it will have a turbine capable of flowing every bit of air that it's compressor can deliver.

can't remember, wasn't mine and was many years ago now. however i have seen quite a few comments of minor ver of boost creep happening on 2.4 surfs under high boost. mind you those turbo's are not that great.

think of the backpressure/drive this way. once the turbo is up to full boost extra backpressure doesn't achieve anything, you don't get any power from it. exspecailly as your trying not to get any more boost from it. if anything you loose power from it. with the compounding turbo's the 2nd turbo can make use of any extra exhaust gas that comes through via the 1st turbo wastegate. that way the extra pressure that would normally be wasted gets used running the 2nd turbo.

[physh]

How do you control the boost levels of the two turbos individually?

Wouldn't you want the 1st turbo wastegate to bypass over a certain flow to stop the little turbo overboosting and self-destructing?


Seems pretty complex, but I want twin turbos...

http://en.wikipedia.org/wiki/Twin-turbo

[Patroler]

the old problem (exspecialy with petrol vechiles) with to much boost is it makes to much exhaust which forces the turbo to overboost. eg with the wategate jammed fully open boost still keeps climbing and 'drive pressure' increases with minimal performance increase

I agree with you on this, but the exception would be in a compound turbo setup when the exhaust of the first turbo (small) feeds into the exh housing on the larger turbo, this would cause some restriction and increase the pressure in the pipe between the two turbos exhaust housing.
Therefore the pressure drop through the small turbo could be setup so as not to overboost the turbo, this would just require the large turbo to have the correct exhaust housing and turbine, for example the exaust pressure may be 50psi between the engine and small turbo, as it exits into the less restrictive pipework and larger turbo it may drop to 25psi, and finally to 0psi (hypothetically!) as it leaves the big turbo.

something from a cat acert site

The air system on the bigger Cat engines features series turbocharging, with dual turbos that together raise the inlet charge pressure to as much as 42 psi. Series turbos are used because the two-step compression keeps pressure ratios and turbine speeds down — both contributors to longer turbo life. Air exiting from the high-pressure compressor feeds directly into a water/air pre-cooler, then through the chassis mounted air/air charge cooler to the inlet on the opposite side of the head.
An interesting feature of the twin turbos is the flexible steel couplings used to accommodate the differential expansion of the components making up the air system.

and a photo of the twin turbos on a c15 cat, you can see the exhaust, pretty tight packaging tho!!

http://www.truckerphoto.com/2005_peterbilt_amisol.htm

[KiwiBacon]

Hey Dzltec

How was the economy when running compounded turbos?
I realise that if you're having fun and using all the power, you're going to pay for it. But on a highway cruise how did it go?

[Dzltec]

In my landcruiser i could get 20-21mpg at a steady run from lilydale to dromana.

If you have a heavy right foot then you will use more fuel.


Boost pressures are controlled via a wastegate on the small turbo and correct sizing of the big one, but I have seen wastegates on them to for more control.

They are fun.


Andy

[Shadow]

is the new cruiser twin turbo v8 diesel a sequential compounded or paralell twin setup?