Cowl exhaust tunnel - extending vs cutting

[prkaye]

I've been doing lots of reading on old threads related to addressing cooling problems. I'm a bit confused by two seemingly contrary mods people do to help with CHT cooling. The lower cowl exhaust tunnel as it comes from vans is cut off squarely in-line with the firewall. Some people suggest cutting the edge of this tunnel back (towards the nose) to open up the exit area. At least one pic I saw showed the cut angled back, so instead of being parallel to the firewall the is cut angles down and forward (so that the bottom of the tunnel is forward of the firewall).
Others talk about extending this exit tunnel back behind the firewall under the airplane, so smooth out the exit airflow - the theory being that air spilling down the firewall is getting in the way.

Am I understandign these two options correctly? Cutting the tunnel is certainly a *much* easier mod and can be done incrementally. It also provides more room for the exhaust pipes. I don't care if I lose a knot's worth of speed... is there any otehr reason not to cut this exhaust tunnel on the lower cowl a bit shorter (so it terminates forward of the firewall)?

[f1rocket]

I've cut the bottom of my cowl back on two airplanes. However, I would concentrate on smoothing the airflow into the engine first. That's where a vast majority of the cooling problems are found.

[prkaye]

However, I would concentrate on smoothing the airflow into the engine first. That's where a vast majority of the cooling problems are found.

Yeah, I'll look at this, but I'm not sure what more I could do. I already glassed-in the sides of those ramps when I was building. I have the dams in front of the front cylinders, but I don't want to cut these down because my temps are extremely uniform between all cylinders. THey're just all too hot. At higher power settings the aft ones do heat up more than the forward ones, so this is another reason not to cut those dams down.

What else could I do to smooth airflow into the engine?

I've cut the bottom of my cowl back on two airplanes.

Did you get much improvement in cooling by doing this? Would the Louvers be a better idea than cutting?

[hydroguy2]

Originally Posted by prkaye

... is there any otehr reason not to cut this exhaust tunnel on the lower cowl a bit shorter (so it terminates forward of the firewall)?

Don't do it...yet.

The reason is: it's too early in your engine break in to have info quality info to go hacking away or extending. Do this instead: Fly when the OAT is cooler and fly higher, get your fairings on and speed up to get some more cooling right now.

[f1rocket]

Cutting things back didn't help much because the problem wasn't getting the air out, it was getting more air in.

As far as things I would look at, the biggest thing you can do it open up the space between the top of the cylinders and the ramps. I cut back the ramps on my Rocket and my temps went down almost 60 degrees. I know you have them glassed in but make sure you can fit your fist up through the opening.

Also as others have suggested, speed up while you are breaking in your engine. Speed equals more airflow and your engine needs to be run WOT. Once you finish break-in, then you can systematically attack the cyl temp issue.

[yakdriver]

What temps are you seeing on the CHT. You can expect low to mid 400s in the first few hours. Make sure you run with the mixture full rich until you see the CHT drop. This usually takes 5-10 hours. You need to build up a nice coat of carbon in the cylinder heads that aids in cooling. Its part of the break in. Don

[prkaye]

What temps are you seeing on the CHT.

I posted this data on my first-flight announcement... reproduced below. The reason I'm worried is that when i talked to Bart from aerosport before first flight, he advised me it was best to keep CHTs below 400. I could not acheive this while keepign the power up. It seems I have good MAP though, so hopefully break-in is not being compromised...

**FIRST FLIGHT DATA**
Overhauled O-320D2A from AeroSport with new ECI cylinders. The engine was run-in for about 1.5 hours at aerosport, and my initial ground runs did not take more than 0.1 hours total. Fixed pitch catto prop.
Flew at about 1500ASL (~1000 AGL) for about 1 hour. It was quite warm outside. Ran full-rich the whole time.
Oil pressure and temperature steady at about 70 and 212-215 respectively.
With power around 2300RPM, the CHTS were all very close in the 425 range. Whenever I ran it at 2400RPM or higher, the CHTs on all the back to cylinders crept up to around 430. Manifold Pressure was about 29".
If I powered back below 2300RPM, the CHTs would fall off to around 405.
I had wheelpants and leg fairings off and was only making about 120-130 kts.
I didn't like the high CHTs but I felt it was important to keep the power up for hte break-in. So I ran it with the CHTs in the 420s, and when the temps crept much over 430, I powered back to cool them back down closer to 400, and then increased power again until the CHTs crept up over 430 again. I ran like this for a total of about 1 hr.

[vlittle]

Hi Phil, I have a bit of experience with this.

First, I found that my CHTs would spike on take-off and stay high during my initial fly-off period. The fix for that was to have Bart re-jet the carb. This increase fuel flow (richness) at high power settings and reduced temperatures substantially at WOT.

Second, my ECI cylinders had casting flashings in the air fins between the spark plugs. Removing these (a tough job) reduced my cruise temperatures by about 35 degrees F. In total, I've seen a 100 F reduction in CHT as a combination of these changes and the natural reduction due to break-in.

During the winter (40-50F ambient), I will see low 300's and high 200's depending on power setting. In summer I see mid 300's, with high 300's at takeoff/climbout.

I hope this gives you some hope before radical cowl surgery.

V

[prkaye]

Thanks Vern. I can't get out to the airport again to inspect things until mid next week, but I will look for the flashing then. My engine is an overhauled O-320 with new ECI cylidners. Would the new cylinders include new cooling fins (that might have this flashing stuck between them), or would the cooling fins be the original ones? When they replace cylinders, I'm just not sure what this typically includes.

Can you describe what to look for regarding these flashings? What do they look like? Would they be under the engine paint?

I think it makes sense to wait until after break-in to determine whether I need to get my carb re-jetted... do you agree?

[Gerald Clabots]

Have you read ECI breakin recommendatios See here
http://www.eci.aero/pages/tech.aspx#breakin.