Paul,

  Good news!  After spending many, many months (years) trying to get my oil
  temps down, I finally discovered that the problem was a defective oil
  cooler.  It appears that the only oil cooling was from the oil/water
  exchanger.  While temps were too high for comfort, especially during
  summer,
  it did cool the oil well enough to keep me flying throughout the Texas
  summer.  After just recently discovering the defective cooler, I replaced
  it
  with a Modine of similar size & style and flew it for the first time
 today.
  WOW!  What a difference, possibly enough to remove the oil/water exchanger
  altogether (more testing is requried).  Addimitedly, the OAT was in the
 mid
  40's, the return oil temp was down in the 150*F range, when it had been
  around 210 - 240*F.  On today's flight, in cruise configuration, the
  coolant
  was 165-170 with cowl flap closed.

  Also, it appears that Lynn Hannover is correct (big surprise) about the
  best
  horsepower being produced at around 160*F oil temp.  I saw an extra 11
  knots
  IAS today over my typical cruise speeds.  I'm sure it wasn't just my
  imagination as the fuel flow was also higher.  (Gotta take the bad with
 the
  good.)

  So, if you want to go fast(er), work on reducing those oil temps.

  Mark S.

  That has happened before. Was it a 2nd gen Mazda cooler?
  What was wrong with the old cooler? Was the oil thermostat still in there?

  Paul Lamar

 Paul,

 It was a Fluidyne oil cooler.  It is the one in the pictures you have of my
 a/c.  This is a dual-pass cooler, so the inlet/outlet are on the same end.
 The inlet/outlet tank is partitioned into two halves by a divider. For some
 unknown reason, most likely due to over-pressurization, the tanks (both
 ends) swelled.  This allowed the divider to pull away from the core (the
 divider is only welded to the tank, not to the core) and the oil could then
 pass under the divider, bypassing the core entirely.   (owner of
 Fluidyne) told me that these coolers are designed for up to 300 psi working
 pressure, and they pressure test them to 400 psi.  Since I have no way to
 pressurize to 400+ psi, I don't know how this could have happened.

 Mark S.


 OK we have seen that before as well.  Perry Mick experienced that. I had no
 idea you were having so much trouble.  Obviously Gary lied.

 A lot of oil coolers are not up to the oil pressure seen in a rotary
 engine. Here is a couple of shots of an early Griffin design. They have
 since changed the design to beef up the oil tubes. Some other brands have
 failed and caused in flight fires.

 When the oil is cool I have seen 140 psi in the oil system.

 When buying an oil cooler make sure it will endure at least 300 psi. Good
 ones are rated at 300 psi. It is also a good idea to put two oil temp
 sensors in. One ahead of the cooler and one after the cooler for purposes of
 trouble shooting.

 Paul Lamar

Paul,

I have a special fitting that I can move around to different locations when
monitoring temps and pressures.  I currently have it located before the oil
cooler.  I have the primary oil temp sensor on the filter adapter block
(after the oil cooler).  I measured a delta T of 25*F across the oil cooler
yesterday (OAT 44*F).

Mark S.

That is a cool idea. The 25F temp. difference is impressive. Do you recall what it was before? Here is a way of figuring out if your two pass heat exchanger is working as it should. Check the temperature on both side of the partition using an optical temperature sensor such as this Fluke. Also listen to it with a stethoscope.

BTW while we are on the subject a good oil cooler has fins inside the tubes such as this picture. Not all oil coolers have these. I am not sure but I think the 2nd gen RX7 had Modine oil coolers as stock. When you cut one apart it looks like this. They work well for a 2 rotor aircraft application if they are ducted properly. They have a built in oil temperature thermostat which most people disable.

Paul Lamar