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  #11  
Old 06-11-2018, 07:38 PM
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hgerhardt hgerhardt is offline
 
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I just bought a pair of clamp-on turndowns from Clint at Vetterman a couple months ago, so he does indeed still make those.
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  #12  
Old 06-11-2018, 08:40 PM
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sirlegin sirlegin is offline
 
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Quote:
Originally Posted by hgerhardt View Post
I just bought a pair of clamp-on turndowns from Clint at Vetterman a couple months ago, so he does indeed still make those.
I have a call in to Clint. Thanks
Can you tell any differences?
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  #13  
Old 06-12-2018, 07:50 AM
vetterman vetterman is offline
 
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Default Turndowns

Let me add my opinion as I designed and tested the systems being discussed here. Compare apples to apples and keep this subject related to the -6 and -7 only. I placed pictures on the web site for proper tailpipe angle etc. that angle is 25 degrees in relation to the belly skin. Why 25? You either get the benefit of a little free thrust at that degree or add turndowns and drag the exhaust plume along. The difference as tested was 2-3 mph difference in cruise speed.
My comment on the original poster is the sub cowl as currently viewed is too shallow to allow proper mounting and engine cooling. The quick fix is to cut 2 round openings to allow the tailpipes to be turned to the proper angle.
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  #14  
Old 06-12-2018, 09:10 AM
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Larry, from the standpoint of exhaust system design and fabrication, what do you think about switching to dual outboard exits on an A-model?

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  #15  
Old 06-12-2018, 04:08 PM
vetterman vetterman is offline
 
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Default Exhaust design

I fully tested an exhaust that exited on the sides of the subcowl and it was all positive. There was ample room to mount heat muffs or mufflers. It was easier to install a tailpipe mounting kit and there was a slight power increase due to less bends bringing the pipes into the center to exit area. The CHTs decreased @ 10-15 degrees also.
With that said, I did a poll with quite a few builders and most of them did not want to take the additional time to fabricate exhaust exit ramps-(for some reason RV guys donít like fiberglass), and modify the bottom cowl fastening system. So the end result was to leave the systems as is and place them in the overcrowded center exit area. I mentioned the change to the factory and was told that to change a cowl design with a large number of existing cowlings out there was not a good idea. Maybe itís time for someone to come up with a modification kit, as it wouldnít take much to change the exhaust.
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  #16  
Old 06-12-2018, 10:29 PM
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Carl Froehlich Carl Froehlich is offline
 
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Have side exhausts on the six cylinder Franklin RV-8. A lot of power, looks good, sounds good and hard to get CHTs above 360.

Carl
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  #17  
Old 06-12-2018, 10:42 PM
crabandy crabandy is offline
 
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Quote:
Originally Posted by DanH View Post
Larry, from the standpoint of exhaust system design and fabrication, what do you think about switching to dual outboard exits on an A-model?

Perfect for a horizontal induction nose dragger, Iíve thought about it for my 7 but the carb/airbox hangs so low already the Center exit seems a natural. So much room in the corners for curves/fairings/exhaust pipes etc!
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  #18  
Old 06-13-2018, 06:07 AM
dtw_rv6 dtw_rv6 is offline
 
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Dan,

I installed this in the lower cowl of my -6 last fall in an attempt to block off excess cooling air into my cowling. I was consistently running well below 350F on all cylinders prior to this. Now on the hottest days, I have to be careful to stay below 400F. The plug wasn't originally made removeable, so I don't have good speed data to draw any conclusions about true drag reductions.


I just finished a mod to be able to remove / modify / replace this plug to optimize the drag reduction and determine if it has any measureable effect on speed.

My intent here is an incremental step toward what you sketched up. I wanted to try to learn if the type of extensive modification you've suggested has potential.

Quote:
Originally Posted by DanH View Post
Larry, from the standpoint of exhaust system design and fabrication, what do you think about switching to dual outboard exits on an A-model?

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  #19  
Old 06-13-2018, 04:16 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by vetterman View Post
I fully tested an exhaust that exited on the sides of the subcowl and it was all positive.
Good report. I suspect freestream pressure is lower at an outboard exit, which may explain the lower CHT.

I've tried to encourage a few guys to do it, but they do seem to be a wee bit reluctant about the necessary glasswork. Sheesh. Glass is so easy

Quote:
Originally Posted by crabandy View Post
Perfect for a horizontal induction nose dragger, I’ve thought about it for my 7 but the carb/airbox hangs so low already the Center exit seems a natural.
The fat part of cowl housing the airbox can be made smaller, to fair with a small bump for the nose gear structure. It would require a different airbox. Take a look at Chris Zavatson's Lancair airbox: http://www.n91cz.net

Quote:
Originally Posted by dtw_rv6 View Post
I just finished a mod to be able to remove / modify / replace this plug to optimize the drag reduction and determine if it has any measureable effect on speed.
I wouldn't expect much effect. The plug shrinks exit area and thus increases exit velocity, but a lot of that velocity will be lost in turbulent flow behind the excess frontal area.
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  #20  
Old 06-13-2018, 07:10 PM
dtw_rv6 dtw_rv6 is offline
 
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Would it capture the intent of your design if I were to shell out the existing outlet bump? That would eliminate the large flat area on my exit plug. Or are you implying that the exhaust tubes need to be much further apart (as in straight down from the cylinder ports)? My glasswork isn't necessarily pretty, but I'm working on it. The real issue for me would be custom exhaust fabrication.

However, now that I'm thinking of it, that might make for a very clean exit path for the cooling air -- straight back and down.

Don
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