SonexBuilders.net

My #3 and #5 (R center and R rear) chts are too high and was wondering if
running the plug wires outside the cooling tunnel would help by leaving
more room for cooling air to circulate. Anyone tried this?

Rick

hi Rick, what you are proposing may help, but in my view the problem is inadequate air flow to the right hand side caused by the clockwise swirl of air from the prop. The air going into the RH intake has to turn a pretty sharp bend to reach the engine. I got around the problem on my Sonex by directing about 40% of the air from the LH intake to the RH side of the motor. In fact most of that air goes just to #5. There is a deflector behind #3 to trap any air coming in on that side.

As you can see my Sonex has a more conventional plenum chamber but I have seen other builders achieve something similar with the Sonex ducts. Here's Adrian Clout's Sonex:

I would have to contact Adrian to see how well his system works but I am happy with the cooling on my Sonex.

Peter

Hi Peter, thanks for the reply! I had thought of going the full plenum route, but want to try
less drastic changes first. In your case, did you see any improvement with just the new
plenum, and how much did adding the crossover scat hose to #5 help?

I like Adrian's solution as well. That is something I may try.

Rick

Hi Rick, I have only ever had the full plenum, but fitting the cross-over hose made a big difference. I think there is another small advantage to the full plenum. I have never had to fit blast tubes to keep the ignition coils cool though I may have just been lucky to not have any problems with them overheating.
Peter

Peter were you ever able to ask Adrian about his cooling? I just changed to a prince p prop and it appears to o bite a bit harder than my sensinec does. I lost about 100 rpm across the range but it feels like it might be faster by about 3-5 mph in cruise. I think it is more of a cruise prop than my sensinec was.It has caused my egt to fall but my #5 cht is getting hotter than it used to so I was wondering if the cross tube might help me.
Mark

Mark. Do you still have a standard style plenum with internal deflectors? If you do, you could try cutting like a small letterbox type slot at the top of the deflector. It's what I did to allow a bit more air through to the rear cylinders to cure higher CHTs on both 5 and 6. I removed about the top 1/2" of the deflector.
Steve
Sonex 892

Hi Steve, yes have standard alum sonex plenum but only deflectors are the bat wings on bottom between the cylinders. Unless you mean the deflectors in the front of plenum intake that restrict how much cyl 1 cools. I did just up my blast tube to the coils from a 3/8” tube coming out of the top of the plenum to 1/2 coming out the rear above the rear heads. I tested old setup with a compressor air gun and couldn’t feel anything on the coils so put the new blast tube in both sides. I can see maybe some slight robbing of cooling air but that size tube is the recommended size by jab. Was considering trimming my cowl a little on #1 side to allow more air but would rather try less permanently visible things first.
Mark

Hi Mark. The Jabiru plenums come with a deflector at the rear. I'd forgotten I added another deflector in the middle, this is the one with the slot. See the sketch. Also have blast tubes for the coils and bat wing baffles under the cylinders.

Steve
Sonex 892

Steve, my sonex plenums are basically a square box that fits over the cylinders and goes down to about the bottom of the valve covers. There is no deflector of any kind on the inside. I could make an alum deflector that is riveted to the top of the inside. Just don’t want to disrupt what cooling air I have. should I try and aim the incoming air between the #3-5 cyl? Or direct it onto the top of #5?

I think the original jabiru deflector is set at about 45 deg with the bottom of it being between the rear spark plugs on the rear cylinders. They need to be trimmed around the heads. Here is a photo of the internal deflector in metal ones I flew with for 3 years. Replaced these in 2013 with fibreglass ones. The fibreglass ones are very close to the same dimension's as the metal ones.
Do you have a good seal around the inlets? I think that is pretty important.



Steve
Sonex 892