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Cowl exit size

Tsabean

Active Member
Trying to figure out a starting point for the size of the opening at the bottom of the cowl. I’ve searched the archives and found multiple threads but nothing that is definitive such as a picture.
For background, Rv4, 150hp O-320 Vetterman 4 pipe exhaust. Aircraft has not flown yet. Ground runs with the cowling installed result in CHTs reaching 400F in a relatively short period. (Didn’t run timer so not sure of actual time, wasn’t expecting this to be an issue.)
Anyone have a photo of their lower cowl exit area?

Thanks, Tom
 
What I did

Been a while since I built mine, but the exit size is pretty much just the shape of the cowl as it is. Not much you can change other than closing it up with fairings and such. I have an O-320 160HP,Vetterman 4-pipe and my CHT's are way below yours. I don't know how you are getting that high on the ground. RV-4's run pretty cool compared to some other series of RV's. here are a couple of pics of mine. make sure you baffling is tight also. I have fontal baffle dams that push air up and over to the rear cylinders and will make a difference.
 

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Trying to figure out a starting point for the size of the opening at the bottom of the cowl. I’ve searched the archives and found multiple threads but nothing that is definitive such as a picture.
For background, Rv4, 150hp O-320 Vetterman 4 pipe exhaust. Aircraft has not flown yet. Ground runs with the cowling installed result in CHTs reaching 400F in a relatively short period. (Didn’t run timer so not sure of actual time, wasn’t expecting this to be an issue.)
Anyone have a photo of their lower cowl exit area?
Thanks, Tom

Aircraft cooling systems are designed to be efficient in flight, not on the ground.

Ground runs should be kept to a minimum, especially on a new or freshly overhauled engine. Ground runs are required for checking for leaks and operational characteristics only.
 
The cowling isn’t designed to function the way it’s supposed to while sitting still on the ground. There isn’t very much air pumping through there in that condition. You probably shouldn’t make any cowl modifications until after you’ve flown it and broken in your engine - assuming the engine isn’t broken in yet. Keep ground runs to a minimum and cht’s relatively cool during ground runs to avoid glazing your cylinders before the engine is broken in.
 
Tom, post a few photos of your engine without the cowl, and one showing the inside of the upper cowl.
 
My total exit area on my Rocket is 39 square inches. It cools too well, so I'm going to choke it down to 36.

I'm making more than 2X the power of the OP, so not apples to apples, but does illustrate the "art of possible"...
 
Cooling

The Dave Anders series of articles in Kitplanes are and excellent educational source for cooling information.
Search for the Mahlon Russell protocol for ground runs. Goal should be to never exceed 400 CHT on initial ground runs with new engine. Two the airplane to the end of the runway for early flights if necessary to keep CHT under control. Once the rings are seated higher temps are acceptable short term.
 
My total exit area on my Rocket is 39 square inches. It cools too well, so I'm going to choke it down to 36.

I'm making more than 2X the power of the OP, so not apples to apples, but does illustrate the "art of possible"...

That's awesome to see your numbers here on the the Rocket. I'm currently working on my Rocket cowl and making it fit the 6:1 exhaust and had it figured for 37.5 square inches in the exit area. You don't happen to know your inlet area off hand? I was figuring 50.78 on the inlet.
 
That's awesome to see your numbers here on the the Rocket. I'm currently working on my Rocket cowl and making it fit the 6:1 exhaust and had it figured for 37.5 square inches in the exit area. You don't happen to know your inlet area off hand? I was figuring 50.78 on the inlet.

My total inlet area is a touch under 45, and I'm also planning to choke it down a bit.

Big disclaimer: my outlet scheme uses augmentor tubes, so I have an advantage at low speed and high power that most dont have. I have demonstrated the ability to climb from sea level to 10,000 feet at Vx without exceeding 425 on any cylinder.

The reason I posted in this thread in the first place is because a buddy is building a -4 and we are going to fit one of my existing augmentor tubes to his airplane. The lower cowl mods will be quite simple. Based on the power I make and his (IO-360), I think we can make 19.6 inches work.
 
My total inlet area is a touch under 45, and I'm also planning to choke it down a bit.

Big disclaimer: my outlet scheme uses augmentor tubes, so I have an advantage at low speed and high power that most dont have. I have demonstrated the ability to climb from sea level to 10,000 feet at Vx without exceeding 425 on any cylinder.

The reason I posted in this thread in the first place is because a buddy is building a -4 and we are going to fit one of my existing augmentor tubes to his airplane. The lower cowl mods will be quite simple. Based on the power I make and his (IO-360), I think we can make 19.6 inches work.

Wow 45 is smaller than I would have guessed awesome to here it's even a little over cooled. I completely understand your disclaimer. I was planning one big augmentor on my exit around the 6:1 collector. It sure is alot of extra fiberglass work on the bottom cowl but I think it will be worth it I hope. Can't believe people actually build entire airplanes out of this fiberglass/ carbon ****
 
I flew my RV4 for the first time in Feb. I have a new ECI (I)OX-360 with Vetterman 4-pipe. During my medium and high power runs I got over 400 very quickly. Now flying and at 20 hrs I have OK cooling. I did have to add a duct on the back of #3 baffle and reduce the oil cooler inlet area from 5x3 to 2x3 inches to reduce and even out CHTs. I have made no changes to the stock lower cowl. I still get over 400 during full power climbs at 120kts with OAT 84 deg. Reducing power at 1000ft to 80% keeps temps below 400 with climb rates over 1500 fpm . Cruise temps are 320 to 360 depending on altitude and ambient. It would be nice to have lower climb temps but I'm content for now.
 
I fabricated and installed my new augmentor tubes and decided to choke them down to 4.5 inches. I also sealed all gaps in the lower cowl area with RTV so I am down to a total exit area of 31 inches. because my exaust pipes end completely within the lower cowl, that 31 inches supports cooling and exhaust. Flew it today in a relatively warm environment (80 ish on the ground and 75 at 8000). Immediately saw a warmer trend in the CHT (good, thats what I wanted), so did a quick turn refuel and taxed the system with a full power climb at VY to 7500 MSL. Hottest cylinder was 413, and 4 others were still below 400. Push over into cruise saw the CHT start trending cooler, and at the end of my 15 minute flight the hottest was 390 and still dropping.

I have a summers worth of data to see if this is too small on the exit or not, but it looks good so far.

Data point: 330+ HP 540 cools just fine with 31 inches total exit area.
 
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