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BH O-540 Cooling

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  • BH O-540 Cooling

    I had a engine cooling breakthrough on my Bearhawk with a O-540 running 10-1 pistons.

    During Airventure 2018, while walking by the Lycoming tent, there was an immaculate F-1 racer in front without the cowling with an engine similar to mine. The owner was talking to someone about cooling and how the beautiful fiberglass induction plenum did nothing to improve the cooling but the curved sheet metal transition to the firewall and tunnel did and made all the difference in the world. I did not take pictures, should have but didn’t.

    That idea incorporated to my installation significantly improved my CHT by 20 degrees. No louvres in the cowling.

    For example, two minutes after takeoff with 90 degrees F, max power, the CHT on cylinders 5 & 6 would be 422. Same conditions with the modification temperatures are 402.

    Those are worst conditions: slow speed, high power with fuel enrichment valve open. In all other regimes of flight, temperatures are nominal.

    Additionally, the exhaust was moved as far to the sides as possible and all pipes, ducting, cables were re-routed to clear the area under the cylinder cooling exhaust air.

    The transition from the firewall to the tunnel needs softening so the cooling air smoothly enters the tunnel. Anything impeding the air creates turbulence and correspondingly forms a wall stifling airflow.

    Hope this helps. I have a friend with a BH and cooling issues yet to adapt the mod. Let’s see how he does. Did not ask permission to use his name (rhymes with Smokey). 509 still for sale.

    Scott Williamson N509RF & N924PL

  • #2
    I made a similar lower fairing for my RV-7 that resulted in similar CHT reductions. Cleaning up that exit airflow makes a difference.
    Scott Ahrens
    Bearhawk Patrol Plans Built


    • #3
      I did something similar and my cht’s now stay below 400 when on floats and it’s heavy on a hot day. The down side is when the oat is 50 or below it can be hard to keep the cht’s above 300, which can be fixed by a small plate over part of the cooling air intake on the front of the cowling.


      • #4
        I also put a lip at the sharp edge of the firewall-tunnel junction, but mine was much smaller, about 1" in radius. My temps were fantastic, 300F to 350F, sometimes lower (160C to 180C). Almost too cold at times.

        So cold in fact, that I added electronic ignition and closed up the cowl outlet a lot - I probably have one of the smallest cowl outlets of any Bearhawk now, with the cowl flaps closed. You can find photos on this forum. Temps are now in the 350F to 400F range (180C to 200C), which is a smidgen hotter than ideal, but at least I know its under my control. Easy to open the cowl flaps and lower temps back to 300F to 350F (160C-180C).

        Extended full power climbs are hard though, always have been. I used to have maybe 5 minutes at full power before I hit 400F (200C), now its more like 3 minutes.


        • #5
          I made one of these. I read that the RV guys do this with decent benefit.

          Rob Caldwell, EAA Chapter 309
          Davidson, North Carolina
          BH Model B Quick Build Kit Serial # 11B-24B / 25B
          Build Log:


          • #6
            That radiused exit ramp was standard in my RV-8 kit back in the day. I suspect it is the same in all of the other RV models (well, the non-Rotax ones) as well. I never had any cooling issues whatsoever and I had a closely cowled angle valve IO-360 in there.

            I plan on making one for my 4pl Bearhawk as I have an O-540 for it.


            Maule M5-235C C-GJFK
            4 Seat Bearhawk #1078 (Scratch building)
            RV-8 C-GURV (Sold)


            • #7
              I am making an effort to clean up the exit area of my Patrol. The Vans airbox has a vertical lip which obstructs the out flow so I cut it off and flanged a piece of angle and attached it to the bottom of the box plate. Every bit should help.
              Steve W P203