Aerocarb, Tins, and CHT probes

[vwglenn]

So I finally had some time to get to the hangar and play with my new Sonex.

I have a Jab 3300 Solid lifter engine and I wanted to get in there and clean some things up. My primary goal was to clean up and reroute the wiring under the hood and get an acquainted with my new powerplant. I'm concerned about a couple problems I've found.

First: The Aerocarb is mounted to the intake vertically between the mags. The mixture comes in through the top and the throttle (push type) comes from the bottom. It's mounted in such a way that the opening and closing of the throttle rubs against the plug wires. It looks like the previous owner knew this as there is a plastic sleeve on it to keep the chafing down to a minimum. I'm still concerned about the mechanism binding in the wires. My first thought was to just fab up a longer intake hose and move the entire carb towards the firewall a few inches so it would clear the top of the mags. I don't think that will work because the air cleaner would be bouncing off the engine mount. How did you guys install and/or get around this problem?

Second: Upon close inspection, I found deflector tins on the top AND bottom of the interior cylinders. This seems like it would restrict airflow between the cylinders. I found in the Sonex installation manual that they recommend using a welding rod across the top of the cylinders to safety the lower deflector tins in place. This, to me, seems like a better setup given the Jabs reputation for running hot. Is there any consensus on this?

[hickej]

I had to use some creative routing to avoid rubbing the wires from the distributor. I ended up using a standoff made from a zip tie and tubing. (the tube is the standoff, the ziptie goes around the things being separated and through the tube.

I used welding rod on top of the cylinders and safety wire to hold the bottom deflectors in place per the plans.

The key to even temperatures is the front deflectors I went through four or five different heights before I got it all balanced. The deflector in front of #1 is very high and shaped like the head profile to get air to #5. The deflector in front of #2 is less than an inch on mine. Make sure the plenum is pushed down all the way as it will bleed a lot of air if not.

Now that I have the balance right and the engine broken in I see all CHTs under 300 in cruise. One or two might climb a little above 300 during climb out but nothing approaching the yellow range. I haven't seen yellow since the initial break in and baffle tuning. (NOTE: Extensive slow speed flight like a flight instructor has you do will cause higher temperatures due to the airflow change.)

-Jim

[fastj22]

the inter-cylinder block off plates (bat wings) are critical to cooling. seems counter intuitive but the cylinders don't need the airflow, the heads do. So blocking off escaping air between the cylinders and forcing it through the heads is key to good cooling. I've not heard of using both top and bottom, but it would have the same result. All corvair and VW engines (car and plane) use similar blocking plates.

Also, I've found the recommended Sonex exit cutouts for the Jabiru to be lacking. I kept opening mine up (per recommendations from Jabiru USA) until the temps settled down, eventually eliminating the dual tunnels and making one larger one.

[vwglenn]

the inter-cylinder block off plates (bat wings) are critical to cooling. seems counter intuitive but the cylinders don't need the airflow, the heads do. So blocking off escaping air between the cylinders and forcing it through the heads is key to good cooling. I've not heard of using both top and bottom, but it would have the same result. All corvair and VW engines (car and plane) use similar blocking plates.

I'm all too familiar with the VW setup. Leaving those deflector tins off the bottom is a quick way to cook your engine. Having them on top and the bottom seems like you're restricting too much flow over the cylinders. I'm of the opinion those cylinders have fins for a reason and it's not because they don't need cooling air to draw the heat away. Still I can agree that the heads might need more air going over them and feel the deflector tins on the bottom would accomplish that.

I had to use some creative routing to avoid rubbing the wires from the distributor. I ended up using a standoff made from a zip tie and tubing. (the tube is the standoff, the ziptie goes around the things being separated and through the tube.

Any chance you have a pic of that?

I used welding rod on top of the cylinders and safety wire to hold the bottom deflectors in place per the plans.

Think I'm going to go that route.

The key to even temperatures is the front deflectors I went through four or five different heights before I got it all balanced. The deflector in front of #1 is very high and shaped like the head profile to get air to #5. The deflector in front of #2 is less than an inch on mine. Make sure the plenum is pushed down all the way as it will bleed a lot of air if not.

I've read that somewhere and my front deflectors look like they've been fine tuned. Guess I won't really know until I get everything sorted out. The previous owner said he had no cooling problems with the engine at all in spite of living in Texas. It didn't stop the engine from eating a cylinder and requiring a rebuild a few hours ago though.

On the subject of CHTs...The wires from my plugs run across the top of the cylinders and then down behind the last cylinder on each side. They appear to be in decent shape but the routing is not gentle to them and they're rubbing pretty hard as they come out of the back side. Is there any reason not to run them through the same hole as the spark plug wires?

[DCASonex]

I put the bat wings on bottom, as that makes sense to wrap the airflow around the cylinders. Cylinders do need the cooling, especially at the top end. I made custom bat wings that better prevented airflow that did not pick up heat, any that gets by without passing through head or cylinder fins just creates problems with too much air trying to get out the bottom of cowl. Also, for what it sis worth, my new Camit engine came with the bat wings on the bottom.

David A.

[Bob]

Make sure any zip ties you use in the engine compartment are "heat rated". A RV builder in our EAA group didn't have the paperwork and had to buy another batch to appease his inspector.

Xenos #59 builder Bob

[vwglenn]

So I was looking at replacing my CHT probes. Evidently Sonex sells the K-type and says they can be installed on the early engines by just using the bolts on the valve covers. This seems like a better way to go to me. Have any of you tried it and noted a significant difference in temps?

[Pickleman]

Spread the word and find a new inspector . . . This kind of idiocy is why we have separated government inspectors from the government with the DAR. An appropriate response would have been "you know these need to be so and so type zip ties? Are they? If not, change them." This is EAB. We can make the darn things out of limp spaghetti noodles dried in place. I am NOT advocating for shoddy construction--or spaghetti zip ties--but reason . . . Sure. Did the inspector examine EVERY piece of aluminum in the plane to make sure it was 6061? Even the parts the were polished and painted with no marks? Coulda slipped some 5052 window channel in there by accident . . . . I've personally seen that one happen.

This is NOT a flame against the poster, just a cry for (un) common sense in a crazy litigious world. I Would have told the inspector to pack up and leave. I have a friend who builds prodigiously. He is an engineer educated at one of the nation's better aviation oriented schools. He works in aerospace. His favorite airplane hardware store is Tractor Supply. He determines the job of every nut and bolt on the plane, and chooses accordingly. When I questioned the practice of using grade Five bolts and jamb nuts to hold bushings in place on a control system, he walked me through the process. When one has a load factor ten to a hundred times less than the hardware stands a chance of failing in . . . So many places on an airplane, a bolt size is determined not by the load it carries, but by CONVENIENCES based on the size of the bushing, wrench to be used, or hole that can be drilled. The old SOB has one more infuriating habit too. He can use a slide rule faster than I can frame the question and use a calculator. . .
It is a joy to watch him break in a new DAR . . .