First flight / cylinder temperature raisers?

[Paul Eastham]

Hi VAF,
My RV-9A, N42PE flew for the first time yesterday under the command of local airplane guru Robin Reid. I haven't yet flown it myself due due to some other commitments and a single problem that needs solving...I'll save the fancy writeup until then! I do have a few pictures here: link

The problem: a big CHT difference between the front and back cylinders. 370's in front, 460's in back. He kept the RPM down to 2400 to keep the back cylinders from getting any hotter.

The baffle directions talk about temperature risers, but I didn't receive such a part nor do I see one on the BOM or plans. (O-320 baffle kit here). I found a picture with it installed on Dan C's site:



My first thought was to temporarily install some aluminum tape along the length of the front two cylinders, overlapped with the baffle floor. My neighbor's RV-7 has been flying like that for a long while.

The local A&P didn't like that idea and thought I should rivet in some plates right now. I'm a bit hesitant to do anything permanent before the engine is broken in. Besides, how would I know how big and wide to make the plates without parts from the baffle kit, and without experimenting with some tape first?

Any similar experiences out there? RV-9A, O-320, 1 mag, 1 lightspeed.
Photos of your final configuration would be most appreciated.

Paul

[Radomir]

Congrats on first flight!

I'd try the tape first... by the way, it works just fine... and it'll be easy to play with it until you get the right height/width.

OTOH, I've seen a plane apparently fly with that metal piece clecoed to the ramp... so, I wouldn't get too concerned...

[pierre smith]

Hi Paul,
Congratulations for sure. We Georgians appreciate visitors from California .

Take a look at a Cessna 172 or similar. A friend and I discussed this same point recently. I've also seen silver foil tape used and it stays reall well.
Regards,
Pierre

[dan]

Congrats! Imho, don't try to chase down uneven CHTs until the engine is well broken in. You'll just be rechasing things later.

[vlittle]

Originally Posted by Paul Eastham

Hi VAF,
My RV-9A, N42PE flew for the first time yesterday under the command of local airplane guru Robin Reid. I haven't yet flown it myself due due to some other commitments and a single problem that needs solving...I'll save the fancy writeup until then! I do have a few pictures here: link

The problem: a big CHT difference between the front and back cylinders. 370's in front, 460's in back. He kept the RPM down to 2400 to keep the back cylinders from getting any hotter.

Paul

Paul: In my experience, you are chasing down two problems here:
- High CHT's at full power (take-off and climb).
- High CHT's in cruise.

Although you have a new engine that is expected to run hot, I found that I had to re-jet my carburator in order to have a rich enough mixture to prevent CHT spiking during climb out. I sent my carb back to AeroSport and they did it under warranty. Since break-in requires high power settings, this is an advisable adjustment.

As for cruise, I used dams screwed to the front baffles as temperature risers to perform CHT balancing in cruise. This changed as the engine broke in. Currently, #1 and #2 cylinders are slightly hotter than #3 and #4, and this changes with summer and winter configurations (I tape off the oil cooler in winter).

The biggest effect I had in both climb and cruise temperatures was to remove the flashings on the cylinder fins between the spark plugs to improve airflow around the cylinders. This dropped indicated temperatures by about 35F! I provided all of the details to AeroSport who passed it to ECI.

From having consistently high climb and cruise temperatures, I now have to work to make sure I keep enough power on to keep the CHTs above 300F in economy cruise. That's at least 50F to 75F cooler than with a new engine before all of the adjustments, tweaks and improvements.

In summary, your problem sounds more like a mixture issue than a baffle issue, but I may not have all of the relevant information. Good luck.

Vern

[osxuser]

Make sure there is a 1/4"-1/2" gap between the back of the rear cylinder fins and the rear baffles. Air needs to be able to flow in some volume down that backside.

[vlittle]

Originally Posted by osxuser

Make sure there is a 1/4"-1/2" gap between the back of the rear cylinder fins and the rear baffles. Air needs to be able to flow in some volume down that backside.

My experience is quite the opposite. My #3 has always been the coolest cylinder. I suspect every engine or cylinder design is slightly different, so adjust the setup as necessary for your engine. I've seen only a 2F variation in cruise between CHTs when everything was tweaked-- but this does change with mixture seetings and OAT somewhat. Vastly better than certified aircraft, I think.

I have a 'redline' at 435F for my cylinders, and I prefer not to exceed 400F. With all of the changes, I've never reached redline, but beforehand I could.

V