Ian - I look at a dual filter head as a fluid manifold...the pressure provided by the oil pump is provided to both filter inlets. You are correct in that most of the flow will follow the path through the full flow filter, but there is still sufficient pressure available upstream of the filters for the Trasko to function. The valve in the Trasko regulates a constant pressure differential in the range of an input/output of 1.00:0.89 to direct oil through the filter element. The filter element in the Trasko is not as restrictive as it looks...flow travels longitudinally (down/up) between the layers of the TP vs outside/in like on a full flow. The internal parts:
Another option to the Trasko is the MotorGuard bypass filter...it is not a spin-on type though and requires modifications to work with oil. It's quite a bit larger too. When you eliminate the stock filter head, you can replace with an adapter with a 1/8" port or you can T the line to the remote filter to feed the MotorGuard. It requires an orifice in this feed line to limit flow. About the only place to mount it is where the charcoal canister is on USDM Supras. The return line to the pan can use the old cooler return port. I use a MotorGuard on my Toyota truck...it uses a real roll of TP for a filter element, so the replacements are cheap
I made the aluminum parts in the 2nd pic on a lathe...the short pieces seal the top/bottom of the TP roll and the tube supports it. I pressurize my MotorGuard from the bottom, through the roll, flowing out from the top to bottom through the tube.
The 3rd pic is a basic diagram of how a bypass filter like the MotorGuard is installed in the circuit.
Rennat - this whole bypass filter/oil analysis drill is not intended to be cheaper overall. What it does is allow you to run expensive (true) synthetic oil longer and to know what is going on inside the motor. For example, if you have coolant in your oil, you will know about it way before other symptoms of a BHG become apparent. One other thing, the benefit of running an ester based oil like Red Line vs a PAO based oil like GC is minimal. What is really important is the additive pack...both are excellent.
