What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Question on drag (closing the tunnel)....

Reflex

Well Known Member
After "messing" with the stock vans 4 into 1 exhaust, I opted for a Vetterman aftermarket exhaust. The standard exhaust just didn't fit like I thought it should. Example, it actually came into contact with the #4 cylander intake tube. I've got a IO-390 Thunderbolt NOT the new EXP-119 215 HP version. The Vetterman version is a dual exhaust. Since I don't have the EXP-119 cowl, it looks like I've got some fiberglass work to do. Kind of a bummer since my cowl is done, but no big deal.

What I'm scratching my head about is the tunnel. Now...I don't need it. It's just a big hole running down the middle of my airplane. My questions are:

Should I close this hole/tunnel?
What impact of cooling will it have?
What impact on cruise speed will it have?

The "standard" RV-14 cowl has a nice outlet in the center where it intersects the tunnel, I'm thinking I could leave it open and add a couple of blisters for the exhaust. So why close it? I read the post by Dan Horton in Kitplanes (link below) regarding where the EXP-119 version is getting much of it's speed. Looks like closeing the tunnel might be a good way to add some knots.

You'll need to scroll to the bottom of the article to see Dan's writup called Speaking Truth to Power

Link to Dan's writeup here

Thoughts?
 
The "standard" RV-14 cowl has a nice outlet in the center where it intersects the tunnel, I'm thinking I could leave it open and add a couple of blisters for the exhaust.

Fred, if I were to add dual outlets for the tailpipes similar to the -119 cowl, I would eliminate the frontal area of the center outlet (i.e. remove the reverse coal shovel) and keep the tunnel. From there, adding the tunnel closure and variable area exit door could be an easy future option.

New Lower Cowl and Tunnel Cover.jpg
 
After "messing" with the stock vans 4 into 1 exhaust, I opted for a Vetterman aftermarket exhaust. The standard exhaust just didn't fit like I thought it should. Example, it actually came into contact with the #4 cylander intake tube. I've got a IO-390 Thunderbolt NOT the new EXP-119 215 HP version. The Vetterman version is a dual exhaust. Since I don't have the EXP-119 cowl, it looks like I've got some fiberglass work to do. Kind of a bummer since my cowl is done, but no big deal.

What I'm scratching my head about is the tunnel. Now...I don't need it. It's just a big hole running down the middle of my airplane. My questions are:

Should I close this hole/tunnel?
What impact of cooling will it have?
What impact on cruise speed will it have?

The "standard" RV-14 cowl has a nice outlet in the center where it intersects the tunnel, I'm thinking I could leave it open and add a couple of blisters for the exhaust. So why close it? I read the post by Dan Horton in Kitplanes (link below) regarding where the EXP-119 version is getting much of it's speed. Looks like closeing the tunnel might be a good way to add some knots.

You'll need to scroll to the bottom of the article to see Dan's writup called Speaking Truth to Power

Link to Dan's writeup here

Thoughts?

I have a 14 with the non EXP engine and cowl and used the Vetterman exhaust. I modified the lower cowl to allow exhaust to exit but did not close up the exit space between the pipes. I installed the EXP tunnel cowl flap. I have found that the cowl flap is not that effective because of the larger exit area that I left open between the pipes. I have plenty of exit area with the cowl flap closed. If I were doing it again I would really have to consider just closing the tunnel area and not putting the cowl flap in or closing some additional cowl exit area. Currently I get about a 2 degree drop in oil temp with cowl flap closed versus open. CHT's are never an issue. I only use the cowl flap on landing in attempt to keep under cowl temps lower after landing. Also not sure I reduced much drag with the exit area being so large, certainly don't see any speed change with cowl open versus closed.

IMG_7543 - Version 2.jpg
 
Last edited:
I originally installed a Superior XP400. Due to the cold air sump, Vans crossover exhaust would not fit and I went with a Vettermen 2 x 2-in-1 trombone exhaust, like a Jetmart (Glenn). My pipes exit outboard of the tunnel but through the “coal shovel” cowl exit (pic below). When the Superior was recalled, I moved the sump and exhaust to my Thunderbolt 390 (also NOT EXP119). I installed the tunnel cooling flap mod, also like Jetmart, but I appear to be happier with the result. While my initial idea was less drag and more speed, I am a poster child for sticking exhaust valve on cyl #2 and what I ultimately hoped to gain was higher CHTs as valve therapy. As for results, my somewhat crude before (no tunnel cover) and after (tunnel cooling flap installed and closed) comparisons yielded increased cruise from 1 to 3 knots depending on altitude, or slightly decreased fuel consumption, increased CHTs of 10 - 20° F (now low 300s vs high 200s although I’m also now running the engine harder hoping for better valve behavior), and increased oil temp of 5 - 10°. I fly in Texas and only use the flap open in long high power climbs in hot summer conditions - otherwise it just stays closed. An issue for post-flying install of the tunnel cooling flap was the cable activation, requiring an additional firewall penetration and cable mounting. I opted to use an electric linear actuator with great results - I’ll use this on my new build. (https://vansairforce.net/threads/tunnel-cooling-flap-electric.211661/)
 

Attachments

  • version=1&uuid=00000000-0000-0000-0000-000000003C84&mode=compatible&noloc=1.jpeg
    version=1&uuid=00000000-0000-0000-0000-000000003C84&mode=compatible&noloc=1.jpeg
    1.8 MB · Views: 49
Last edited:
Gentlemen,

Sincerest thanks for your responses and thoughts. Nice to have specific data and pictures.

A couple of questions:

Dan - I'm assuming I'd have plenty of cooling if I eliminate the reverse coal shovel as I'd effectively have an always open exit door plus the open area of the tunnel.

Glen - Do I understand correctly that you covered the tunnel and added the exit door but left the reverse coal shovel in place?

Keith - Great data. Did you flatten out the reverse coal shovel or leave it in place?

Again, thanks very much for the responses.

Fred
 
Gentlemen,

Sincerest thanks for your responses and thoughts. Nice to have specific data and pictures.

A couple of questions:

Dan - I'm assuming I'd have plenty of cooling if I eliminate the reverse coal shovel as I'd effectively have an always open exit door plus the open area of the tunnel.

Glen - Do I understand correctly that you covered the tunnel and added the exit door but left the reverse coal shovel in place?

Keith - Great data. Did you flatten out the reverse coal shovel or leave it in place?

Again, thanks very much for the responses.

Fred

Yes I left in what I believe you are calling the reverse coal shovel and added the cowl flap.
Here is how mine ended up. I have plenty of cooling with this exit area being open. So much that opening the cooling flap has very little effect on temps.

IMG_4309.JPG
 
I'm just starting to install the heat shield. I've been leaning towards making a pattern out of paper before I start hacking on the shield. Decisions, decisions!
 
Dan - I'm assuming I'd have plenty of cooling if I eliminate the reverse coal shovel as I'd effectively have an always open exit door plus the open area of the tunnel.

I'm cooling a 390 with about 46 sq inches, variable exit door open, and 30 or so when closed. The areas were derived iteratively over a few years of flying with steadily shrinking exits. Generally speaking, the door is closed below 50F OAT.

It would be hard to go wrong if you simply copied the -119 arrangement.

Marvin and I have an experiment going on his 14, an afterfairing behind the coal shovel with rear hinged door. When the door is closed, the only exit area is the aft portion of the tunnel. So far the results seem to be good control of CHT, but no speed increase when closed.

Outlet v1 Inverted.jpg

exit.jpg
 
Keith - Great data. Did you flatten out the reverse coal shovel or leave it in place?
I made 2 parabolic cutouts for the exhausts but left the scope otherwise intact. This is shown better by this pic taken before paint and without deflectors on the pipes. it appears my pipes have less downward angle than Glenn’s.
 

Attachments

  • version=1&uuid=00000000-0000-0000-0000-0000000034AE&mode=compatible&noloc=1.jpeg
    version=1&uuid=00000000-0000-0000-0000-0000000034AE&mode=compatible&noloc=1.jpeg
    1.6 MB · Views: 19
  • version=1&uuid=00000000-0000-0000-0000-0000000034B9&mode=compatible&noloc=1.jpeg
    version=1&uuid=00000000-0000-0000-0000-0000000034B9&mode=compatible&noloc=1.jpeg
    1.7 MB · Views: 19
Would you guys please share your oil temp with the respective OAT that you are seeing. I know 14 has a better cooling then 14A, probably due to lack of nose gear blocking some of the exit air but I can't imagine to be of that much better cooling.
I have flown multiple 14A with both the standard 390 and the EXP119 and they all have had a higher oil temp then I like to see. None are getting anywhere close to the red line but 210-215F when the OAT is 75+ is not uncommon. My cowl flap gets closed only when I am flying higher up and LOP.
 
Only anecdotal data, but if I see oil temps running consistently high in cruise (say OATs in 80s-90s, oil temp 200+), I can open the tunnel cooling flap and drop 5° in 3 minutes, CHTs double that drop) - it really works. I can also “crack“ the flap door (say 1/2“) and hold a lower temp.

As a side note, if the engine is running hot (TX summer, lower altitudes, higher power), and you keep the flap cooling door closed, the tunnel itself heats up (no air flow so it rises to FWF temps and warms up inside the cabin. After 20-30 minutes, you notice it but I crack the flap door (same 1/2 inch as above) returning flow through the tunnel and it cools back down.
 
.it appears my pipes have less downward angle than Glenn’s.

Mine are turned down more than most I have seen. I did nit change them from what Vetterman delivered. They fit perfectly. This is a 390 Non-EXP set up.
 
This is pretty typical oil temps with multi grade oil. I did notice a drop of 8F to 10F in oil temps when I switched from Aeroshell W100 to Phillips 25W50.

IMG_8918.JPG
 
I found this comparison interesting: https://rv-14a.blogspot.com/

Typically, my oil temps run 180 to 200 F even in Florida summers. Early 14A build with the non EXP engine. Did not close up the tunnel. 4 into 2 Vetermann's exhaust. Kept it simple and minor cutting of the stock cowling. I also have an oil damper I use even in Fl.
 

Attachments

  • 20231031_110235.jpg
    20231031_110235.jpg
    324.4 KB · Views: 17
  • 20231031_140433.jpg
    20231031_140433.jpg
    280.5 KB · Views: 17
  • MUFFLER.jpg
    MUFFLER.jpg
    394.5 KB · Views: 3
Last edited:
I found this comparison interesting: https://rv-14a.blogspot.com/

Typically, my oil temps run 180 to 200 F even in Florida summers. Early 14A build with the non EXP engine. Did not close up the tunnel. 4 into 2 Vetermann's exhaust. Kept it simple and minor cutting of the stock cowling. I also have an oil damper I use even in Fl.
Interesting data from the E's blog. To add a bit more data, recently I was flying my friend's RV14A with the EXP119 engine and other than the avionics, everything is setup very similar as my own. The effect of closing the cowl flap was somewhat different than my own plane. His oil temp initially would go up by as much as 5F and then shortly later come back to about what it was with the flap open but his CHT would go up and stay up by as much as 30-40F
Closing flap in my plane will cause the oil temp go up and stay up but little rise on the CHT. Here is a sample data from my plane. I can also report that my EXP119 is both faster and thirstier than my old 14A which had the standard 390.
 

Attachments

  • IMG_3101.jpg
    IMG_3101.jpg
    939.1 KB · Views: 8
If I were converting a standard 390 to dual exhaust, and adding blisters to the cowl for the outlet, I might seriously consider purchasing the parts from Vans for the cowl flap. It will make for extra cooling when you need it, and low drag when you dont.
 
Back
Top