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Cowl exhaust tunnel - extending vs cutting

prkaye

Well Known Member
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)?
 
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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.
 
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?
 
... 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.
 
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.
 
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
 
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.
 
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
 
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?
 
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Yes, I did lots of reading about break-in procedures and talked to Bart at aerosport, all before first flight. Basically for break-in you want to run at the highest power settings you can while keeping CHTs in check. I'm developing quite high Man Press at 2400RPM (29" at 2400RPM... it's a little suprising actually, I'm not sure if that indicates a problem) so I'm pretty sure this is high enough power to be effective for break-in, but I'm concerned my CHTs are in the marginal caution zone. I don't want to run for much longer at those temps and risk damaging my cylinders (if I haven't already). I didn't mean for this thread to be about break-in procedures... but about how to reduce CHTs which are uniform but high. My first attempt is going to be higher airspeed via pants and fairings, look for and remove any casting flashing, look for any gaps in baffling, and possibly the "washer trick" to move baffling away from places where it is in contact with cylinder fins. With all these mods hopefully my second flight will have lower CHTs.
 
Yes, I did lots of reading about break-in procedures and talked to Bart at aerosport, all before first flight. Basically for break-in you want to run at the highest power settings you can while keeping CHTs in check. I'm developing quite high Man Press at 2400RPM (29" at 2400RPM... it's a little suprising actually, I'm not sure if that indicates a problem) so I'm pretty sure this is high enough power to be effective for break-in, but I'm concerned my CHTs are in the marginal caution zone. I don't want to run for much longer at those temps and risk damaging my cylinders (if I haven't already). I didn't mean for this thread to be about break-in procedures... but about how to reduce CHTs which are uniform but high. My first attempt is going to be higher airspeed via pants and fairings, look for and remove any casting flashing, look for any gaps in baffling, and possibly the "washer trick" to move baffling away from places where it is in contact with cylinder fins. With all these mods hopefully my second flight will have lower CHTs.

Phil,

Take a look at the back of #3 baffling at the Cylinder wrap. There is a small tab that the builder needs to bend under and many forget to do so. I think the plans even forget to tell you to make this bend... ask me how I know. After fixing mine, I've found this issue on three other RV's flying with CHT issues. Once the baffling is installed it is almost impossible to bend the tab, I impregnated fiberglass cloth with red RTV and layed it in from the top to seal the hole up.
 
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Take a look at the back of #3 baffling at the Cylinder wrap. There is a small tab that the builder needs to bend under and many forget to do so

Hmm... I don't recall this. Do you have a pic? Is this tab, and how it should be bent, shown in the drawings?
 
Hmm... I don't recall this. Do you have a pic? Is this tab, and how it should be bent, shown in the drawings?

I don't have the drawing here with me, probably in the hangar. The tab is bent inward towards the base of the cylinder just behind the cylinder fins. The hole the tab fills is about 3/4" wide and 1" or so deep. Sealing this large hole (air leak) dropped my temps about 30 degrees.

If you look from the back there is a relief cut and a pre-drilled hole for the bend line in the cylinder wrap piece. If this tab is still vertical it did not get bent. You can also look down from the top of the engine at the far back side of cylinder #3 (firewall side) and if you see a gapping hole, the tab was not bent up. :)
 
Little tab not bent until recently . . .

Funny thing - in taking a final look at my FWF and RTVing the baffle, I noticed the large gap behind #3 - had to remove the oil tube, but could get some 'tools' back there to bend the tab in - then added RTV - it is sealed nicely - haven't flown yet, but glad to see I did the right thing. Baffle instruction really need to be updated to include this . . .
 
Funny thing - in taking a final look at my FWF and RTVing the baffle, I noticed the large gap behind #3 - had to remove the oil tube, but could get some 'tools' back there to bend the tab in - then added RTV - it is sealed nicely - haven't flown yet, but glad to see I did the right thing. Baffle instruction really need to be updated to include this . . .

Yup... If I had to guess, I'd bet over 1/2 of the RV's flying that use the prefab baffle kit have this tab not bent over. It does make a big temp difference... mainly on climb-out. I'm also running a plenum so the temp difference/spread before and after was huge. :)
 
I don't want to run for much longer at those temps and risk damaging my cylinders (if I haven't already).
Phil, you didn't damage your cylinders. Are they Nickel/Carbide or Chrome? I would guess Nickel. I think they fair much better in the break in process, especially when not in ideal conditions.

Check to make sure you have the correct oil sender programmed into your EFIS. There are 2 styles available, if the wrong one is inputted you will have potentially higher indications.

Don't sweat it. The temps will come down.
 
Thanks Tony. My oil temps actually look good and appear to be what I would expect during break-in: 212-215 or so. It's those high CHT's I need to work on. I've got a few things to try now before 2nd flight to improve cooling.
My cylinders and Nickel Carbide I think (ECI).
 
Don't have my plans here at the house, but I seem to remember the instructions from Van's suggested cutting off as little of the exit area as possible. I took that to heart and just barely cut enough to clear the exhaust pipes. I have had no CHT or oil temp issues since break in. CHT's usually run in the low 300's while cruising LOP or the high 300's ROP. Oil temp will get to about 205 during a long climb to altitude in hot Florida summer weather, but will settle down to about 185 in cruise.
 
Phil,
I had talked to ECI about break in and maybe some of the VAF experts will correct me, but I was told that nickel/carbide cylinders break in within minutes and don't really have the "glazing" troubles of chrome. Neither are they as heat intolerant during break in.

I actually thought your Oil temps seemed a bit high. Yours are where mine were at when I first flew. Still, it'll take about 30 hours (IMO) for temps to normalize.

Back to your original post about cowl exhaust trimming, I'd wait. I have a set of uninstalled Louvers to show for my own concerns. In the end, I am pretty happy with my temps right now. My #1 cylinder still gets hot in climb (440 or so).
 
Oh yes...

back to your thread, no extra cutting at all on the bottom of my cowl from in line with the bottom of the firewall.
 
but I was told that nickel/carbide cylinders break in within minutes

Bart said something similar... that most of hte break-in would have already happened when they did the 1.5 hour "run-in". But if that's the case, why would it take hours for temps to stabilize? My understanding is that the high temps during break-in are caused by the metal on metal contact as the cylinder rings are pushed against the cylinder walls and etch themselves into a perfect match. If this is done within minutes, then shouldn't the temps come down immediately?
 
Paul, the tolerances in the engine are tight at first. Those will open up as the moving parts mate with each other. Excess heat is the by-product. The seating of the piston rings to the barrel is of concern in the critical first few hours. That's why it's important to run at a high power setting. The rings need to "grind" into the barrel early on to stop blow by and oil consumption.
 
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?

If you have new cylinders, you shoul check for the flashings. The cooling fins should have slots that are unobstructed between the plugs. I don't have pics handy, but there are some on the internet if you search.

As for re-jetting, you should determine if you need to do it right away. When in flight, try leaning the engine. If you can't find a lean peak and are then able to go 100F rich of peak, you are already too lean and need the carb rejetted. All of my RV buddies have had to do this in order to get lower CHTs at takeoff and a distinctive lean peak.

V
 
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