Tracy; I hear you and the thing is that a 2-pass would make for a much better hose routing. It would shorten it and get rid of a 180 degree AN fitting. That would save a little weight and resistance and just make for a cleaner install. After cutting open my Mazda cooler I feel certain that this one will have more than enough flow capacity even as a 2-pass. I just ordered 2 Arduino barometric pressure & temperature sensors as it should be easy enough to add them to the data-logger and then I can get a better idea of pressures across my coolers.

  Ernest; I just ordered a wireless inspection camera to try to get a good look at air flows to test whether you and Tracy are in agreement ... or not :-)

Lynn; So one of my biggest concerns with welding in a plate to convert to a 2-pass is the inability to weld all 4 sides of the plate with the plate edge inside the cooler against the tube plate being inaccessible. I can make it fit tight but it would still be susceptible to an insignificant amount of oil leaking by and bypassing the tubes, if I do it right. If. Nobody, yourself included, mentioned this as a concern, so I'm going to proceed with this.

  Steve; That is some really good data. While there are certainly some differences in our installs it is still great to compare to. I see you have the RD1C so are climbing at 6800rpm. I have the RD1B and so only run at about 5600 during climb but with about 4psi of boost, climbing at ~95kts IAS at ~950fpm with 2 of us onboard, full fuel. (well full main tanks, not aux.) 

  Most of my flying has been in the circuit as I'm working on that darn tailwheel thing, so my temps are really in a cycle up around a 110C for coolant and 110 for oil, then both dropping a little on the downwind and dropping right off on final, repeat. I have used my data-logger in it's rudimentary proto-type form to collect raw data while flying, and it works well, so now I need to apply some correction math to the data input to give me more meaning full data, so I can report hard numbers.

 Steve, I assume from the pic that you have the cooler perpendicular to the airflow, which would account for much better performance. I had to tilt mine at a 30 degree angle in order to fit it under the oil pan. Did you have to modify your cowl to fit it in there? Id like to see a pic of your cowl with inlet.