Cooling the IO-375 -

I’ve got a little over 350 hrs on my 801 with IO-375 now.  I really like the engine - lots of power….. and heat. Piston oil squirters shift some of that heat into the oil, and it can be a trick to manage.  I found myself needing  to make a couple of adjustments along the way to get control of oil and cylinder head temps. It has been a fairly long journey to get it where I want it. Now that I am there, I figured it was time for a write up.  This is how one guy did it…..  I’m sure there are also other ways.

I solved the oil temp problem first, so let's start there...

  I recognized when building that the oil cooler provided with the FWF would not transfer enough heat.  So I started with an AirFlowSystems 2006X, which has 13 rows and dissipates heat better than other coolers of its same size.  It looked like a good match for the IO-375.  I mounted this on the firewall, and pushed air through it with a 4 inch SCAT tube connected to the front of the intake.  In theory this should have worked great, but it did not.  I hit oil temps around 225 in early climb, and had to adjust my flying to keep the temps from climbing higher.  I measured the pressure differential across it, and found it to be about 10 in Wg, which should be enough.  So despite a good cooler, and good pressure differential I was not getting enough cooling. I verified Vernatherm function (boiling water test), Engine timing, and confirmed that oil was flowing through the cooler without obstruction.

 I got the idea that the angle of attack was causing decreased airflow through my intake - with less mass flow across the oil  cooler. 

  I moved the oil cooler to a new position directly ducted off the lower cowl where it would get more air flow with increasing angles of attack.  The air pressure differential across the oil cooler did improve, but the hot oil temps remained.

 Next I measured my inlet and outlet areas (inlet 510 cm2) and outlet areas (412 cm2 - lower than most 801s due to the float mount bar).  Despite researching the optimal inlet/outlet ratio, I was not able to determine what the ideal numbers were for our STOL craft.  So I decided to add cowl flaps to experiment with a larger outflow area.  The cowl flaps about doubled the exit area when fully open.


They are adjustable, so I can set them where they work best.  This did the trick.  Oil temps came down nicely and now only get up to about 210 with prolonged climbs in hot weather.  They stay a pleasant 180-195 in cruise.  Oil temp issues solved.

So now on to CHTs.  

Unfortunately adding additional exit area to the cowl did not correct CHTs.  They were also climb limiting, with #4 reaching 425 pretty quickly in hot weather. I made several attempts to tighten up the baffling using RTV saturated fiberglass cloth.  In the end I thought I had it pretty tight.  It passed the light test easily.  But the CHTs remained high.  After several test flights I came to the conclusion that my baffling was as tight as I could make it, and was still not adequate.  

I decided to make a plenum.   

The plenum started with a 3D scan of the engine and cowl.  I imported these into CAD, and then created a plenum shape mold plug that sat just below the cowl, and mated up to the cylinder heads. I split the plug design into three pieces so that the layup would have adequate draft to be removed from the mold.  I cut the mold plug out of polyurethane tooling board (RenShape) on a CNC router (AvidCNC).  The size of the plug meant I had to make it in several pieces, which were then bonded together. The mold plugs were bonded to MDF  to create flanges.  The plugs came out great. I sealed the mold plugs and MDF and added some wax fillets, then I laid up the molds.  

The molds came out functional, but not as great as I would have liked them.  I was rushing the process a little in order to get the plenum on before Oshkosh.  I had to do quite a bit of resurfacing  on the finished molds, but I stall have the plugs and can make new perfect molds if I ever want to.  

I decided to lay up the plenum in carbon fiber, with high temp epoxy (450F tolerant).  I would have preferred vacuum infusion for the process, but I was not able to get the high temp epoxy in an infusion viscosity.  So I laid it up using a traditional wet layup technique with vacuum bagging, and then let it cure overnight.  I also placed saturated carbon fiber into sealed bags, and then packed them up agains the lower cylinders to form the lower cylinder baffling.

I spent the next day temperature ramping the plenum parts in the oven - which is about a 12 hour process to achieve the strength and heat tolerance that this epoxy was chosen for. 

Once I had the Plenum pieces out of the oven, it was  time for some final trimming and fit up.

I trimmed them up to fit as well as I could and then mated the lower pieces to the uppers.  This mating was done mechanically with #8 screws and nuts.  Sealing up the joins was accomplished with RTV.

Once I had this all fit up, it easily passed the light test. But since I now had access to the pressure area above the cylinders (much better than with a cowl), I decided to test it with a leaf blower.  I had an assistant blow the air stream into the plenum intake, while I felt around for air leaks.  I found several, and sealed them up. I suspect that if I had been able to do the leaf blower test on the original cowl 

baffling it would have revealed large leaks.

Once the RTV was set, I made the first test flight with the plenum in place, and to my great satisfaction CHTs were outstanding.   

I no longer need to step climb, or limit HP output for Oil temp or CHTs.  I am able to climb continuously, and if It is a hot day my hottest CHT might reach 410 (which I can live with).  All of my CHTs are under 400 in cruise, and usually 370-390.

I have to say that this was a long journey.  I did get some help along the way, and the most helpful and knowledgeable person that I encountered was Bill Genevro (of AirflowSystems) - who’s help was invaluable.  He was generous with his time, and provided very useful advice. I am glad to have finally reached a successful conclusion to my cooling troubles. 

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Comment by Daniel Niendorff on September 20, 2023 at 3:01am

Thanks Peter.  I have not done any yarn testing on the cowl flaps.  I would suspect that adding a lip would work for them, just as you describe.  I can see how adding a lip could improve the gradient - and make them more efficient.  However since I can adjust them in flight, it’s easy enough to increase or decrease the total area as needed.  Great idea.  

Comment by Peter Eiberger on September 19, 2023 at 8:28pm

Nice work. Ever done a yarn test on your adjustable cowl flaps? I would be curious to see if you could increase the outflow with an outward lip on the edge. I had to increase the exit area on my lower cowl and add a steeper lip to keep CHTs and oil temps in check when it's in the 90s outside. 

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