Ok, dug out my book...
The Calc for airflow rate is as follows, CIDxRPMx0.5xEV/1728
The .5 is due to the engine being a 4 stroke, and filling the cyls on one half of the revolutions, and EV is volumetric efficiency.
So, for a 6.9, we have 420 CIDx3750rpm(mine turns this)x0.5x.85(probably a bit low, typical of a gasoline engine though) divided by 1728(converts CI/min to CFM.)
So, at 3750 RPM, a N/A 6.9 moves 387.37CFM, this is our basic cfm.
Now, Lets figure out our Pressure ratio for the turbo, so we can figure out our required flow rate under boost.
Pressure ratio is total absolute pressure created by the turbo divided by atmospheric pressure. For the purposes of these calculations, the design goal will be 20 psi @ 3750.
So, 14.7+20(to get absolute pressure, we add atmospheric pressure to boost pressure) 34.7/14.7 = 2.36 pressure ratio, which means we are moving 136% more air through the engine than it could move on it's own.
So, our airflow rate under boost = pressure ratio x basic engine cfm, so
2.36 x 387.37, which = 914 CFM. Some compressor maps are in lbs of air/min. To get this, (lbs/min) we multiply Atmospheric pressure x cfm x 29/(10.73 x Temperature(in Rankine, which is F+460.)
So, 14.7x914x29= 389638.20
10.73 x (75+460) = 5740.55
389638.20/5740.55 = 67.87 lbs/min.
This may just like like a bunch of useless math, but, it is all very important when sizing the compressor, it makes it possible for us to read compressor maps, and have a clue as to what we're looking for.
What we want to see for a non intercooled setup, is a compressor that will flow 914 cfm/68 #m, at a 2.36 pressure ratio, and give us at least a 60% compressor efficency.
Looking at the following charts, we can see by where the lines intersect that a T04 62-1 compressor will just barely do the job, and the efficiency will be below 65%
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But, a T66 will do the job nicely, and give us around 70% compressor efficency.
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All that math, and we just have a compressor picked out. And, I'm half asleep, so, my math could be out....
For a twin turbo engine, we would cut our required CFM rating in half, but, keep the same boost pressure requirement, so 914 CFM @ 20 psi becomes 457 CFM/34#m. Our pressure ratio hasn't changed, but, our airflow has, so, lets look at some compressor maps.
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The V trim compressor looks like a VERY good match
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A H3 would also do it, but, the efficency isn't as good, so, the V trim would be a better choice, as it will create less heat.
For the exhaust side, I don't have nearly the information on hand to calculate it, nor the formulas, and of course, all the sizing charts I can find are based on gasoline engines which turn considerably more RPM than what we are dealing with.
Most charts actually have my too large, late spooling non-gated Banks setup down as having a small exhaust housing for the displacement of the engine.
So, just to get a rough idea as to what would be close without calling up a turbo supplier(I doubt any are open at this time) and quizzing up an expert, a turbine sized for an engine half the displacement, that turns twice the rpm, should be roughly about the size we need, so, for a single turbo, a T4, O trim turbine, with a .58 housing should be about the right size, which is about what I had figured before doing any math, and shows just how oversized the 1.0 a/r on my existing Banks unit is, it was clearly designed to have a high boost threshold(lowest rpm at which useable boost is created) and spool well into the engine's rpm range so as to limit peak boost, without use of a wastegate.
For twins, a T3 turbine with a .48 housing.
Again, this bit is only a rough guess.
It looks like those .48 T3's off of a 2.3 may be useable based on the exhaust side, but, I'll need some info on their compressor to figure out what kind of flow and boost they are capable of.
I know on a 2.3, from the factory, they put out 15 psi(85.5/86 SVO) and a 2.3 turns about 6200 rpm. So, back to out inital formulas. 140x6200x.5x.85/1728 = 213cfm
15+14.7/14.7 = 2.02 pressure ratio
so, 2.02x213= 430 CFM/32#/min. Knowing this is a T3, and assuming it's sized so it's reasonably efficent, it's probably a super 60 compressor.
Now, lets see how it would work on our twinned 6.9 at 15 psi, 2.02x387 = 781.74 CFM/2(remember, twins) 390.87 CFM/29#/min per turbo
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Looks like for a 15 psi design goal, a T3, super 60 trim compressor would be a very good choice. Like I said, I need info on the compressor side of that TC T3, and, I need to find better information on sizing the exhaust side, which, is really the hard part.
Hopefully I did more to help you guys understand turbo sizing than confuse you....
Boost pressure is a direct product of how much CFM your turbo(s) flow, the more air they move for a given drive pressure, the more boost they will make, provided they flow more than the engine would by itself.