SonexBuilders.net

My Onex (after 18 hours of flight time on my aerovee engine) has always had problems with high CHT's. So far I have tightened up the baffling as much as I know how. Ignition timing is perfect and the problem still exist. While flying on a 75 degree day the other day at 2950 rpm I saw the temps rise on the number 3 cylinder to 440 degrees with the number 1 cylinder close behind. number 2 and 4 were about 385 degrees. I am out of ideas! A buddy suggested checking the cht sensors which I did with an infrared thermometer and they match perfectly. I have a Rotec carb that works flawlessly and always has. My oil temps have always been good to maybe a bit on the cool side in the cooler weather (188 degrees on the last flight with the top mounted oil cooler taped off all but about 25%). I'm not too comfortable flying with such high temps and was wondering if anyone could help with advise.

Thanks
Wolfpack3
[attachment=0]IMG_1484.JPG

And you should NOT be comfortable with those temperatures, at least not anything over 400F. When I had high CHT's it was 100% the fault of running too lean. In fact I burned up 2 of my 4 exhaust valves running lean before I figured out the problem. I have the AeroInjector so not the same setup as you, but I would suspect you may be far too lean. Do you know what your fuel burn is running full rich? On my AeroInjector I'm currently at 4.4gph if I don't lean at all. That may even be a tad low, so I'm considering bumping up my mixture to be even just a bit more rich.

Mine ran hot also till I opened up the outlet----After I opened mine up, I was at Oshkosh
and seen how they opened there's up ----See there web site for there drawings
Good luck
Merle Reppert

First thing is the engine is not broken in until about 50 hours. Mine would creep up over 400 degrees even in shallow climbs for at least the first 25 hours and then slowly improved until around 40 to 50 hours. The mixture is also important. Running lean in the early hours will compound the CHT heat problem. Even after being well broken in I could get the CHT high by leaning too agressivly. Make sure you have wheel pants and any other fairings installed, they make a huge difference. Use climb attitude and mixture to keep heat down until well broken in and you will notice a big difference. The advice on checking for the right amount of cooling exit is also something to investigate. I opened mine and that helped.
After 25 to 50 hours if you see a big difference between the front and rear cylinders, I used an air dam to partially block the front cylinders and force more cooling air to the rear. This seemed to raise the front a bit but also lowered the rears, equalizing them better.

Yes, I have considered the fuel feed but my red cube fuel flow sensor reads 6.8 gph with no leaning of the mixture control at 2950 RPM. Thanks for the input. Keep those ideas flowing cuz I'm listening.

Try running on just the primary ignition after taking off and see what it does to the numbers. Timing has a big influence on cylinder temps. Although the engine is just being broken in the numbers sound high to me. The only time I have seen temps that high on my engine was one time when I had the mixture pulled back on takeoff (running mo gas) and the engine started to ping. Are you sure the bottom cylinder baffles (super tin) is installed correctly? The next time you shut down the plane try doing it with just the mixture control. Does the engine rev up higher right before shutting down? What are you seeing for EGT's? I would also stop covering the oil cooler at least for now. The vw engine relies on oil cooling as much as it does on air cooling to keep things in check.

Keith
#554

Keith, yes I have shut the secondary ignition down in flight with no noticeable change in temp at all so I know my timing is correct. Also I have made the bottom hole larger as per the Sonex revision. That seemed to cool the oil down even more than it was before. If I leave the tape off the cooler, my oil temps don't climb above 150 degrees or so. The EGT's seem to be fine except when I'm climbing extensively then the #3 climbs to 1400 but is quickly cooled when I level off. The other EGT's run about 1360-1375 in the same climb. I will remove the tape and see what happens and check the tins for correct installation. Thanks

Had early heating problems on cylinders #1&#3 in 0nex 74. Went to the 2.5 (larger) injector injector needle and opened up the lower cowling (Sonex drawing). Still have to step climb, but at 3250 RPM straight and level CHT's are in the low 300's. 95 hrs TT. It's hard to believe that at 6.8 gph you are still running lean. Does the indicated 6.8 gph match with your actual fuel consumption per hour rate?
Hang in there, it takes a while to break those cylinders in..........................Tom Ryan 0nex 74