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Air Box Diverter / Block / Grid

Date: Early October, 2007

So after purchasing a Dynon Engine monitor to more fully understand what my engine was doing, I discovered that my cylinders were not peaking at the same time when I leaned them out, and this led to higher CHT's on cylinders 3 and 4.  I was informed that I needed a "diverter" inside the air filter box in front of the air filter, to ensure that the Ellison Throttle Body would get a more even air flow into it, and even out the leaning.

Following the advice of our esteemed brethren, Sirs Miller, Schubert, et. al., I spent a weekend building and installing a diverter for my airbox, which houses a Fram air filter leading into an Ellison Throttle Body. Pics at:

This is what I installed:

View of someone else's diverter just in front of the air filter - one of the examples I used to build mine

View of someone else's diverter just in front of the air filter - one of the examples I used to build mine

Here's my diverter, before installation in the air filter box

The air box is upside down with the diverter installed and ready for microing

View of the diverter from the mouth of the air filter housing

I used the two pictures on the right sent to me by other folks as examples of what I was trying to do.

So Saturday morning I went up for a flight to see what the effect of the diverter was/is. The first thing I noticed was that my RPM's on takeoff seemed to be about 50 RPM or so below what I normally get on my takeoff roll and climb, and my climb rate seemed a bit anemic (by COZY standards) as well. I climbed up to 8K ft, leveled off, and then slowly leaned the engine with the Dynon monitor in "Lean" mode, so that I could track which cylinder peaked first and how lean of peak each cylinder was.

Well, by gosh, it DID seem that the cylinders peaked much closer together, and when fully leaned out, the differnces between the EGT's were around 50F at the most. OK - the gizmo seems to have a positive effect in this regime, and once fully leaned, I could see what my CHT's were, and that 3 and 4 (the front cylinders in the rear facing engine) need a bit more air, even when all the cylinders are reasonably equally ). Good info, for sure.

However, in all flight regimes, it was clear that I was down in power - 30 to 90 RPM, depending. This was NOT acceptable, even with the better air distribution. I landed, removed the diverter, and went back up for a test flight. The power was back up, RPM's back to where I expected them, and the EGT's were back to being more spread out.

So I'm thinking that maybe I just need to make the diverter smaller - it may just be blocking too much airflow.  Another weekend's project...


Date: Mid-October, 2007

So I tried the diverter, and while it seemed to even out the EGT's a little bit, it stole RPM, so I took it out. For the last few flights, I put in a sheet metal block (about 3.5" wide) that blocked air from ramming into the front of the air filter, but didn't restrict airflow to the rest of the filter at all. This didn't change the RPM a whit, which is good, but it made the mixture distribution worse, or at least no better than with nothing.

OK, so that's coming off before the next flight (it cost about $0.10 in aluminum and silicone - no big loss). I bought the Ellison throat air straightener from ACS (P/N 08-00814), which they claim helps with this mixture distribution business. We'll see...

Aluminum Plate siliconed to front of air filter

Closer view


Date: Late October, 2007

So yesterday, I removed the air filter block and installed the air straightener grid that ACS sells for Ellison Throttle bodies. I was a little concerned, since the diameter of the grid is slightly smaller than the bellmouth throat of the Ellison where the air filter housing attaches, but a phone call to Ellison confirmed that yes, it's OK that the diameter is a bit smaller as long as the pressure sensor probe is visible from the bottom, which it was.

Upon takeoff today with Deanie and full fuel, 5K ft. DA, about 65F, all RPM's were nominal - between 2490 and 2510 in climb, depending upon mixture setting (richer for lower CHT's gives a bit less RPM), so the grid wasn't stealing any power - that's good. The CHT's seemed to stay a bit lower and more reasonable range in climbout, although that could just be me learning how to read the engine monitor and adjust the mixture better in climb to keep them low - hard to say. Upon leveling off at 7500 ft., I put the monitor into "Lean Mode" and leaned the engine.

It was DEFINITELY better than with the air filter block - much more even EGT's. I think it was better than with the diverter as well - #1 was NOT the first cylinder to peak, and they all peaked within 50-75 degrees of one another, which I don't think I've seen before. On the way home, the experience was similar. Obviously, I'll keep an eye on it, but I was able to lean the engine to 8.5 gph at essentially full throttle at 7500/8500 ft., at about 2590-2600 RPM. Previously, the best I'd seen was about 8.9 - 9.2 gph. Clearly, two short flights is not enough info, but the trend looks good - I'll keep an eye on it and report in in a while. SEEMS like the grid does something, for $75 including shipping from ACS. You could make a composite one for $1, at the cost of a few hours. P/N 08-00814.


View of Ellison Air Grid looking straight up into carb. throat

Oblique view of grid, showing bottom of air filter housing

View of grid looking rearward from below

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