I would think the restriction would be "downstream" of the media so that the dP across the media would be low. A high dP would likely damage media, even synthetic, if allowed to get too high. There is an age-old adage that goes thus: "when in doubt, meter out". It means if you have to restrict something, do so on the outbound. That keeps pumps from cavitating, media from blowing out, etc.
The need for the restriction is to create an artifically slow volumetric flow across the media; gives the media a good chance to grab the particulate. Much easier to hold a particle in the media bed with a slow flow than a fast flow. It also provides the dP to create flow direction.
I should have done a better job defining what I mean by high dP. Very little dP would not push past a high-density BP media very well. So several PSI or more is beneficial here. But you do not want 40-80 PSI differential; that would damage media. As long as you meter with a restriction downstream of the BP element, using as-supplied pump flow, that would be fine. You have to think of this in terms of both partial and total flow. In a parallel path, the pressure/flow response will be just like those in electrical paths. It's a mathematical relationship. If you used a BP filter fed off the main circuit along with a FF filter off that same circuit, and they are in parallel flow, then the restriction behind (downstream) the FF filter is the engine itself, and represents the largest portion of the back-pressure. If you didn't have a restriction on the BP side, the engine would attempt to pump as much as it could through the BP element and damage it.
- the FF filter loop uses the engine circuit itself as a "restriction"
- the BP filter loop needs a restriction GREATER than that of the engine, or the fluid would prefer the BP loop.
- the flow will not go 100% one or the other; it will find a balance between the two to match the overall pump desired flow for any given rpm.
- the artificial restriction you place in the BP loop will create both a flow restriction and a significant pressure drop
First you have to size the artificial restriction in the BP loop so that the fluid will bias to the FF filter loop. Then, you have to choose where to put the restriction in the BP loop. If you put the restriction ahead of the BP element, then the dP could become great enough across the BP media to do some damage. But if you put the restriction after the BP element, there is little risk of a blowout because the dP across the media is reasonably controlled.
Generally, I think a .050" orifice, downstream of the BP element, will suffice at least as a starting point. However - CAUTION HERE !!! You need to make sure you do the right thing. Because your trans-dapt filter mount uses only one path for the return oil, you cannot put the restriction in that outbound stream or the entire oil feed would starve out quickly. You have to put the restriction after the BP element ONLY, but let the rest of the system flow in it's natural state. So given the way that trans-dapt is made, you need to figure out how to put a restriction somewhere in that hunk of metal that ONLY restricts the outbound BP flow, and not all flow. Now, just maybe, if you put the restriction in the inlet side of the BP element mount, and the return oil from the BP element went back into the main return stream, the balance may be reasonable enough to provide a safe operating environment.
When you see a BP element completely separate from the FF, it needs the restriction downstream of the element. If you see a BP element in parallel on the same mount with the same singular return path, it's concieveable that the restriction could be "upstream".
It has always been my preference to have a completely separate BP mount fed off the FF mount, and then return that BP flow directly to the sump pan (atmospheric), using a restriction AFTER the BP element. You are assured of the right flow, going the right way, with no risk of back-feeding, and not blowing out an element. Here, as long as the restriction is sized even remotely well, you'll never have an issue.
The challenge with your proposed system is that you have to get the orifice restriction size correct, because you're intending to co-mingle the return oil from both filters in the same mount. You MUST get this size right so that the dP of the BP is greater than what the engine represents for any given rpm output of the oil pump. And that is a much narrower window to hit without causing a media blowout, but also not creating a bias to the BP element. You could protect yourself from the risk of backflow by installing a check-valve; such as a captive ball-spring unit? But the real issue here is you're now trying to install both a check-valve AND a restriction in a very small amount of real-estate in the BP element side of your trans-dapt unit.
Essentially, you're trying to make this formula work in a hydraulic sense:
1/RT = 1/r1 + 1/r2
where RT is total resistance, and r1 is the FF resistance (engine) and r2 is the BP element resistance drop
but you want a heavy bias to the engine so that the majority of lube feeds the FF loop.
What you are trying to make at home is not unlike some of the dual-element filters that use a single can to house the elements. They will stack a FF element on top of a bypass element, and use the return inside for both. See this link:
http://www.napafilters.com/filters/oil-filters/heavy-duty-oil-filters/xd-extended-drain-filter/
open the little gallery picture link and you'll see what I mean.
So, you're going to have to accept some risk and play with it a bit. Don't get discouraged. Just realize there is some risk in engineering something yourself.
I have never owned a BP filter set-up. Several years ago I was about the put one one and was going to buy from Gary, but we talked and I could not justify the cost structure for the miles I drive in my various vehicles. Therefore, because I've not owned one, I cannot give a direct comment on how the Amsoil is constructed; never held one in my hands before.
Google this as a starting point:
"hydraulic flow in parallel paths" and start reading!
http://www.carldyke.com/parallel-flow-paths-part-1
this contrasts series and parallel in electricity, but hydraulic works the same way believe it or not
http://www.allaboutcircuits.com/vol_1/chpt_5/1.html
Maybe that helps it make sense?
Do you see why this is not really done often, and why most folks just pony up to get a pre-made unit from a major OEM?
It's not that you cannot do it, but it's a challenge to get the balance of pressure and flow correct all the in same mount when you return the oil to the same path, using that little mount you selected.
I'm not trying to talk you out of it. I'm trying to get you to understand why most folks abandon the efforts, and what challenges you have to make it right without harming your filters or engine.