Question for the Tuning Shops??????

Riddle me this Batman... Explain to me why when I pump a fire apparatus with a capacity of 1500gpm, which uses a switchable centrifugal compound pump (your turbo designed for water), in pressure mode, it uses both pumps in series, I get full rated pressure (about 250-300 psi), but only 750 GPM out of it. I go to volume, which is both pumps in parallel, I get 1500 GPM, but even at full engine speed, I might make 150 psi.

Both pumps are turning the same speed, on a common shaft, But pressure multiplication is the same. Both are the same size. The advantage to this system is I can get a pretty good operating range to move water at without needing several hundred HP to do it (doing it on a 250HP DT466). Our newer rigs have a single stage pump rated for 2000 GPM, but it's also fed by a 430 HP 60 series engine, and it's spinning at close to redline for rated output.

In our turbo configuration, you have two different sized compressors. (you must have missed when I wrote speaking of same size compressors the first time) The first turbo pretty much sets your volume, and "base pressure" for lack of a better term. The second compressor be it larger or the same size will determine your final output pressure and volume. The pressure can be higher, but the volume can not be any greater than the initial stage. I'm sure there's some voodoo for figuring out the loses from the first compressor, because in this instance you have a fixed power source, the motor's exhaust.

I would suspect (and more experienced minds can prove me right or wrong) the the second compressor's drive wheel would have to be configured to achieve power with reduced exhaust gas energy (I'm guessing larger) even if the compressor was the same size. Since work has been done in the previous stage, the available energy remaining is less.

I think a safe statement is to say that under any given circumstances, you would want more volume than pressure if you had a choice. I think we're saying the same thing, but referencing it from opposite ends of the spectrum.
 
Kleetus said:
Riddle me this Batman... Explain to me why when I pump a fire apparatus with a capacity of 1500gpm, which uses a switchable centrifugal compound pump (your turbo designed for water), in pressure mode, it uses both pumps in series, I get full rated pressure (about 250-300 psi), but only 750 GPM out of it. I go to volume, which is both pumps in parallel, I get 1500 GPM, but even at full engine speed, I might make 150 psi.

Both pumps are turning the same speed, on a common shaft, But pressure multiplication is the same. Both are the same size. The advantage to this system is I can get a pretty good operating range to move water at without needing several hundred HP to do it (doing it on a 250HP DT466). Our newer rigs have a single stage pump rated for 2000 GPM, but it's also fed by a 430 HP 60 series engine, and it's spinning at close to redline for rated output.

In our turbo configuration, you have two different sized compressors. (you must have missed when I wrote speaking of same size compressors the first time) The first turbo pretty much sets your volume, and "base pressure" for lack of a better term. The second compressor be it larger or the same size will determine your final output pressure and volume. The pressure can be higher, but the volume can not be any greater than the initial stage. I'm sure there's some voodoo for figuring out the loses from the first compressor, because in this instance you have a fixed power source, the motor's exhaust.

I would suspect (and more experienced minds can prove me right or wrong) the the second compressor's drive wheel would have to be configured to achieve power with reduced exhaust gas energy (I'm guessing larger) even if the compressor was the same size. Since work has been done in the previous stage, the available energy remaining is less.

I think a safe statement is to say that under any given circumstances, you would want more volume than pressure if you had a choice. I think we're saying the same thing, but referencing it from opposite ends of the spectrum.
Click to expand...

Wow.. I cant begin to compare what differences there might be between liquids to gasses.. But food for thought.
I have used 2 almost similar sized compressors in series but have not seen a pressure increase... Reason being is that both compressors flow about the same amount of air. For 2 stage setups to work there must be a smaller HP wheel and a larger LP wheel.
lets look at it with maths
HP wheel 14.7psi + 14.7 / 14.7 = Pressure ratio of 2
Lets say that the Naturaly aspirated flow was 10cfm so 10X 2 = 20cfm
To get any sought of pressure buildup in front of that smaller HP compressor we need to exceed the flow of it. So if we could put another pressure ratio of 2 in front of the HP wheel then we would have be flowing 40cfm on the LP wheel.
LP 2 X 10cfm = 20 cfm
HP 2 X 20cfm = 40 cfm
combined 4 X 10cfm = 40cfm
when combining the HP and the LP wheels pressure ratio we just add them together.
As for the turbines all of the gas flows through both turbines. The bypass on the HP turbine insures good flow onto the second LP turbine.


Clint
 
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If you did twins (ala Callaway), wouldn't the exhaust flow be split, therefore, the driving force on the exhaust-side turbines would be less, which means the compressor-side production is less. What is not clear to me, is where, on the compressor map, the twin set up would run. If it is in a more efficient region at peak exhaust flow, you might be better off, and, in fact, produce more, cooler flow.

The real question to ask is, what makes more sense, compounding or twins? Either way, you have a crap load of tuning and other stuff to do, so I would only recommend this twin approach if you can address some other issues. Twins will still have the lag-top end issue. If you size them using the original turbos (just two instead of one), I do not think you can tune out of the lag. They are just too big and we already know that at full exhaust flow (with only one turbo), there is a bit of lag. It can only get worse. On the top end, it depends on where you are on the map. Someone who knows the 6L stock map should chime in.

My gut feeling is that compounds will out preform twins rather easily. While I have my concerns over some of Ford's decisions on the 6.0, the 6.4 compound motor is strong evidence that compounds are the way to go, not twins.
 
Hey Ralphinnj,

You bring up some very good points in this post.

ralphinnj said:
If you did twins (ala Callaway), wouldn't the exhaust flow be split, therefore, the driving force on the exhaust-side turbines would be less, which means the compressor-side production is less. What is not clear to me, is where, on the compressor map, the twin set up would run. If it is in a more efficient region at peak exhaust flow, you might be better off, and, in fact, produce more, cooler flow.
Click to expand...

To do twins properly, you would want to use smaller A/R’s with everything would need to be down sized (turbine wheel, compressor wheel). The original post on here was using the factory charger. I think you are right, too big for an efficient, quick responding setup.


ralphinnj said:
The real question to ask is, what makes more sense, compounding or twins? Either way, you have a crap load of tuning and other stuff to do, so I would only recommend this twin approach if you can address some other issues. Twins will still have the lag-top end issue. If you size them using the original turbos (just two instead of one), I do not think you can tune out of the lag. They are just too big and we already know that at full exhaust flow (with only one turbo), there is a bit of lag. It can only get worse. On the top end, it depends on where you are on the map. Someone who knows the 6L stock map should chime in.
Click to expand...

With our 6.0’s, it isn’t cfm alone that will get the power to the ground. Our heads don’t flow enough to allow for big power without the boost pressure to fill the cylinders. In addition to this, our cam profile is not the most favorable, and we really cannot utilize the limited flow we can get out of our heads with the stock cam. So, for the 6.0, compounds will win probably 99 out of a 100 times.

This may be an interesting concept for a 24 valve Cummins or a Duramax (Banks is not a good example of this. He uses twins for a different reason).
 
Air is a fluid, just as water is, the physics are the same, just the viscosity changes. But that's not really the issue.

I can appreciate what you're saying, and maybe the difference here is that the compressors I'm working with are on a common shaft, that might have something to do with it. Honestly, I don't know.

As far as a pair of stockers go, I took it as the variable vane would be clamped down pretty tight to get it moving, and as the engine came up in output, it would back off and let it breath more normally. Since no one has tried this, at least I think, it's really tough to say what would happen in practice. You should be able to get 2x the volume out of it since they'd be separated, but again, you're relying on the VV to make up the difference on the low end. May not work for a dynamic environment. Stationary power might do just fine. I can't say that I'd believe that 2 separate exhaust paths could be asthmatic, you cut the effective engine volume in half.

I wish I had garage space and deep pockets to experiment with stuff like this because in some instances I think a lot is left on the table. But I guess that's the curse of being an engineer... you can't leave well enough alone!
 
I don't think you'd generate the exhaust flow and thermal inertia to take 2 stockers and get them to deliver 2x what one stock one does with all the exhaust driving it. If the stocker was way undersized, maybe, but it is not. The exhaust is not stout enough to drive two turbos to the point that they both are outputting at the same max as the stock one does by itself.
 
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