Resolved High CHT's, But....

[N787KV]

I have a 7a with an ECI IO-360, duel Pmags and a 4 pipe exhaust now with 61 hours. I have had my CHT's wanting to blow passed 400 deg. on take off and a hard time keeping them below 380 deg. in cruise. Oil temp is 180. My baffling is very good and run the A curve on the Pmags.

What I've found from looking at some local tail wheel 8's, is that I have much less cowling exit area because of the four pipes and the nose gear. And no, I did not measure, but it's obvious. After talking with Allan from Antisplat, I placed nylon spacers on both sides of the exit opening the lower rear cowl and created a wedge like gap on either side. The spacers lowered the cowl next to the exit 3/4". I went for a test flight and I could leave the throttle wide open and the highest CHT was 381 deg. and the oil temp also came down causing me to close the butterfly valve to the oil cooler to almost full closed. To me, it is clear that I need to increase my exit area and I don't believe louvers are the answer.

These are what I see as my options:
1. Open the exit area by lowering the trailing edge (the easiest)
2. Open the exit area by widening
3. A combo of the above
3. Adding a cowl flap in conjunction with one of the above for takeoff

Thanks

[Kahuna]

Originally Posted by N787KV

These are what I see as my options:
1. Open the exit area by lowering the trailing edge (the easiest)
2. Open the exit area by widening
3. A combo of the above
3. Adding a cowl flap in conjunction with one of the above for takeoff

Thanks

Im not sure what you mean by lowering the trailing edge. But take a 1/2" cut off the fiberglass off the opening and go fly it. Only if your 100% convinced you have a clean and tight top plenum area

[LifeofReiley]

I took 3/4" cut off the fiberglass exit and it solved my heat/temp issue.

[alpinelakespilot2000]

Originally Posted by Kahuna

Im not sure what you mean by lowering the trailing edge. But take a 1/2" cut off the fiberglass off the opening and go fly it. Only if your 100% convinced you have a clean and tight top plenum area

Originally Posted by LifeofReiley

I took 3/4" cut off the fiberglass exit and it solved my heat/temp issue.

So when you guys talk about cutting off the opening, are you saying that you cut 1/2-3/4" off the trailing edge of the cowl scoop so that it is no longer even with the firewall but actually forward of it?

[LifeofReiley]

Yes... and you can cut it at an angle forward too. That is what I actually did.

[BillSchlatterer]

What does that really mean ? Did you take 3/4" off the outlet lip and did that make it even with the firewall. Mine comes back of the firewall maybe an inch and I have wondered if taking it off might help a little bit.

[Kahuna]

Originally Posted by alpinelakespilot2000

So when you guys talk about cutting off the opening, are you saying that you cut 1/2-3/4" off the trailing edge of the cowl scoop so that it is no longer even with the firewall but actually forward of it?

That is exactly right. Just take enough off that your happy with the temps in your worst case scenario. Remember that You could take out many inches and get lots of cooling. But your giving up speed. You only need to take enough off to make you happy and NO MORE.

[RVDan]

Anyone have pics of the outlet after modification? I have a -6A with O-360 and in the summer high CHT's and Oil temp are a problem. My upper plenum area has good baffle sealing.

[DanH]

Originally Posted by RVDan

Anyone have pics of the outlet after modification? I have a -6A with O-360 and in the summer high CHT's and Oil temp are a problem. My upper plenum area has good baffle sealing.

Before you reach for the saw, read what Mike said one more time. Every increase in mass flow will increase drag. If that's cool with you (pun intended), then yes, increasing exit area will reduce temperatures.

Quick sketch..cut back the cowl exit chute as shown:




Variable exit area allows mass flow control. Note that variable area does not mean standing up some sort of wall or spoiler in the airstream. It merely means installing some kind of system to vary the size of the exit hole(s).

Ken Kopp and I did slightly different versions. His is hinged at the forward end, much like a Cessna, while mine is center hinged to reduce required force at the actuator. Both worked very well. It is nice to flow as much cooling air as desired for unlimited climb, then transition to cruise, toggle the door shut, and watch the speed roll up a few more clicks.

http://www.vansairforce.com/communit...63&postcount=1

http://youtu.be/LetuLfkR4Vo

http://www.vansairforce.com/communit...&postcount=174

http://www.vansairforce.com/communit...&postcount=175

Think you have good upper plenum sealing and cylinder baffling? Measure your cowl outflow temperature and see the truth. The goal is to (1) reduce leakage and (2) increase heat transfer, the result being hotter exit air.

Air which leaks past rubber seal strips never gets near the hot parts, so it doesn't pick up any significant quantity of heat.

Most aluminum baffle wraps allow air to leak out the sides. Air allowed to flow in the general vicinity of fins at low velocity doesn't pick up nearly as much heat as air forced entirely between fins, for as much length as possible, with enough velocity to be turbulent.

[flightlogic]

Thanks for the pictures. Have been following this, as I am sure many readers have... and I have some difficulty translating the text into a diagram in my head.
Something interesting I noticed this week. Had removed the vacuum system and plugged the firewall hole temporarily with aluminum tape. During the first flight, I noticed a blast of air below the panel that rivaled my shop vac.
Sure enough, the tape blew off both sides of the firewall right away. What surprised me was the positive pressure in the area. The vacuum inlet was in the standard postion on the firewall... which is a fair distance north of the cowl outlet. It is now plugged and sealed... but interesting that I got to see some evidence of airflow I had not expected.
Considering cutting back the outlet to increase outflow as shown above. Cylinder repairs are pricey, and I can sacrifice a bit of speed for lower CHT's.