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Exhaust Exit

Zazoos

Well Known Member
Vetterman Crossover Exhaust on an 0320 with a 0360 scoop. In the following 3 pictures you can see the exhaust tips point towards one another. The closer the tips point towards one another the greater clearance I have at the exit from cowling. The picture with the tips furthest apart have about 1/2 inch clearance to the cowling at the exit.

I was thinking the exhaust would point more straight back. Am I missing something or does this look correct?

I appreciate the input.
ex2.JPG

ex1.JPG

ex3.JPG
 
I have the same cowl on my 2004 RV7 with an IO-360. Although the pipes do point towards each other somewhat, they are much more straight back than yours appear. I have the Vetterman crossover exhaust and can adjust the outlets a little in the swivel joints.
 
Your cockpit will be a lot quieter and the belly will be a lot cleaner if you put down-turns on those pipes.
 
It would seem obvious that you are using the wrong cowling for your installation. If changing to an O320 cowling is not feasible, then you will need to modify (enlarge) the lower cowling outlet to allow sufficient clearance above and below your pipes. Yes, turndowns will significantly reduce cockpit noise and vibration.
 
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It would seem obvious that you are using the wrong cowling for your installation. If changing to an O320 cowling is not feasible, then you will need to modify (enlarge) the lower cowling outlet to allow sufficient clearance above and below your pipes. Yes, turndowns will significantly reduce cockpit noise and vibration.

With the amount of normal movement the pipes have, I'd be concerned that they'd contact the bottom of the firewall - they look pretty close there. The pipes on my -6A naturally point back and down - they don't point towards each other at all. It doesn't look as though that is possible with the cowl as shown however. Good news though is that it's fiberglass, which means it can be changed without a lot of drama :)
 
pipes

First, I can't tell from the pictures if your cowl is cut parallel to the firewall, any trimming in that direction will increase pipe clearance as the pipes are angled.

Second, rotating the pipes at the ball joints will show you different clearances. It is a delicate balance of inner cowl clearance and exit clearance. Seems Vetterman has an "angle" they are designed to be at. So once you rotate to acheive the angle you will be close.

Don't feel alone with this issue, I have worked with several cowl setups and worked and worked to achieve alignment, even calling Clint at Vetterman for help, which he gladly gave me.
 
First, I can't tell from the pictures if your cowl is cut parallel to the firewall, any trimming in that direction will increase pipe clearance as the pipes are angled.

Second, rotating the pipes at the ball joints will show you different clearances. It is a delicate balance of inner cowl clearance and exit clearance. Seems Vetterman has an "angle" they are designed to be at. So once you rotate to acheive the angle you will be close.

Don't feel alone with this issue, I have worked with several cowl setups and worked and worked to achieve alignment, even calling Clint at Vetterman for help, which he gladly gave me.

Here is what my 320 looks like. As Bob Martin said, rotate pipe in ball joint and trim back the bottom of the cowl. Another post talked about the angle down turn of the pipes to lower noise. Yes it does help a lot. Purchased exhaust system direct from Vetterman and asked for the turn downs. They use to sell a bolt on turn down that I asked for when I ordered and they offered the built in turn downs for less money than the bolt on parts.

2018-06-11_10.08.36.jpg


2018-06-11_10.08.45.jpg


They are NOT perfect parallel to each other but do not point to each other as much as yours.
 
Here's how I did mine. The crossover exhaust was made by Custom Aircraft Parts in El Cajon, CA and I've been very happy with it. There's about 1" of clearance top and bottom. Those down-turns could be oriented however I wanted then locked into place.

xEh5wQ.jpg
 
Here's how I did mine. The crossover exhaust was made by Custom Aircraft Parts in El Cajon, CA and I've been very happy with it. There's about 1" of clearance top and bottom. Those down-turns could be oriented however I wanted then locked into place.

xEh5wQ.jpg

Any ideas where you can get the add on turn downs that fit the Vetterman exhaust pipes?
 
I just bought a pair of clamp-on turndowns from Clint at Vetterman a couple months ago, so he does indeed still make those.
 
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.
 
Larry, from the standpoint of exhaust system design and fabrication, what do you think about switching to dual outboard exits on an A-model?

n64x76.jpg
 
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 don?t 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 it?s time for someone to come up with a modification kit, as it wouldn?t take much to change the exhaust.
 
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
 
Larry, from the standpoint of exhaust system design and fabrication, what do you think about switching to dual outboard exits on an A-model?

n64x76.jpg

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. So much room in the corners for curves/fairings/exhaust pipes etc!
 
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.
oEn.jpg


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.

Larry, from the standpoint of exhaust system design and fabrication, what do you think about switching to dual outboard exits on an A-model?

n64x76.jpg
 
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 ;)

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

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.
 
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
 
Exhaust

The intent of the original question was directed at A models whereas the nose gear A frame and exhaust pipes occupy much of the exit area-plugging up the exit area. By moving the pipes outward which is only @ 4 inches it opens up that crowded area. The redesign of the current crossover system for vertical sump setups reduces both crossover bends from 105 degrees to 90 degrees. Simple to do during fabricating a system.
One more note of interest. The turndowns offered as an option for the RV-8 only do to the tunnel design and exhaust pulsing on it.
 
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.

Larry and I think so too.

It's not really a design, but rather a concept. If developed further, yes, I would shrink the inlet scoop body as much as possible, and remove all of the outlet "coal shovel", leaving the least possible bump fairing for the nose gear.

One more note of interest. The turndowns offered as an option for the RV-8 only do to the tunnel design and exhaust pulsing on it.

That would be another case of your design desires being compromised by stock airframe considerations. Swapping the aluminum ramp for a one piece SS ramp, then damping the backside of the ramp and the first belly panel allows a slash cut tailpipe. Best way to damp the rearmost rivet in the floor stiffener (the one that cracks the skin) is to drill a .25" hole in the floor just above it and inject a small blob of Firebarrier 2000 into the enclosed space.
 
Vetterman Crossover Exhaust on an 0320 with a 0360 scoop. In the following 3 pictures you can see the exhaust tips point towards one another. The closer the tips point towards one another the greater clearance I have at the exit from cowling. The picture with the tips furthest apart have about 1/2 inch clearance to the cowling at the exit.

I was thinking the exhaust would point more straight back. Am I missing something or does this look correct?

I appreciate the input.
ex2.JPG

ex1.JPG

ex3.JPG

Here's my aircraft - I0-320 with Vetterman x over. (the centre pipe is the Christian "overboard" pipe)

41529042760_9a4a98bb06_k.jpg
[/url]_MG_0715 by WIGY TV, on Flickr[/IMG]
 
Okay, finally getting around to making my prior plug a little better. I haven't downloaded my EFIS data for hard data comparison, but I am definitely feeling a difference in climb performance with the plug I made versus without. This modification should cut down on the frontal area drag also.


Concept:
otj.jpg


I just started on the actual modification today. I ran out of water and time today, so I'll do layups tomorrow night. Hopefully it won't be 100F outside then :p

Cut the existing cowl:
otS.jpg


Inside view:
ot2.jpg


Roughing in the formers:
otB.jpg
 
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I'm seeing a minimum of a 5 knot increase with my modified cowl exit:
oeL.jpg


Next step is a set of aft exit ramps to re-attach the air to the fuselage:
oeA.jpg


Now for the part that makes me dizzy. I'm plotting about 2 hours worth of data from my EIS - manifold pressure on the x-axis, and TAS/RPM vertically:

Pre-modification:
oec.jpg


After lower cowl mod:
oeo.jpg


This is just from me flying around tuning my autopilot and some local burger flights, so I didn't get enough samples for all manifold settings. All I did was sort the data (high to low) for MP, and then for RPM. The large spikes are the periods of climb and descent, while the gentler sloped areas give me a rough idea of the airspeed for level cruising.

For the curious, I'm running a Catto designed for a 190hp engine on my 180hp Superior. Note how the RPM and airspeed numbers go flat after about 24" on the old cowl, while they tend to increase with MP in the second graph. Less drag / more speed. I'll likely send the prop for a re-pitch this fall if I can't get enough RPM with drag reductions alone.
 
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I can't find any useful conclusions in the data as presented. Don, got anything that compares TAS for same MP at same altitude, in level flight? Forget RPM.

Note how the RPM and airspeed numbers go flat after about 24" on the old cowl, while they tend to increase with MP in the second graph.

The flat look is just graph scaling. Two-thirds of the Y axis only covers a MP spread of 0.7".
 
I'm seeing a minimum of a 5 knot increase with my modified cowl exit:
oeL.jpg


Next step is a set of aft exit ramps to re-attach the air to the fuselage:
oeA.jpg


Now for the part that makes me dizzy. I'm plotting about 2 hours worth of data from my EIS - manifold pressure on the x-axis, and TAS/RPM vertically:

Pre-modification:
oec.jpg


After lower cowl mod:
oeo.jpg



This is just from me flying around tuning my autopilot and some local burger flights, so I didn't get enough samples for all manifold settings. All I did was sort the data (high to low) for MP, and then for RPM. The large spikes are the periods of climb and descent, while the gentler sloped areas give me a rough idea of the airspeed for level cruising.

For the curious, I'm running a Catto designed for a 190hp engine on my 180hp Superior. Note how the RPM and airspeed numbers go flat after about 24" on the old cowl, while they tend to increase with MP in the second graph. Less drag / more speed. I'll likely send the prop for a re-pitch this fall if I can't get enough RPM with drag reductions alone.

Don,

I think the graph that you have pictured is really showing yourself hooked up to an EKG. I?m seeing some ST segment elevation and I think you may be having a heart attact due to you being so excited flying your airplane. My recommendation is that you slow that thing down and go get checked out by your Dr. LOL!!
 
Looks like I was fooled..

Dan,

I had to look through lots of engine monitor files to find matching altitudes and manifold pressures, but I did manage to pull it together. No fancy graphs for us this time, but I'm no longer convinced that there is a measurable difference in speed between the stock cowl exit and my modified version. CHTs, absolutely hotter. TAS, not so much.

I''ll be cutting it back off next weekend. :rolleyes:

Don
 
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