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  #11  
Old 11-09-2007, 07:11 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default THANKS! for the INFO

Quote:
Originally Posted by jbDC9 View Post
The original reason I went with this system was noise; the usual Vetterman type exhaust is just too loud for me and has a sharp, barking kinda quality to it.

(AWI) Chris steered me towards the 4 to 1 system blending the 4 separate stacks into one collector pipe. "Great!" says I, ship it!
Well one thing, 4-into-1 is superior in performance, but I have never heard they where quieter or had a more mellow exhaust note, especially verses a cross-over.

The cracking thing freaks me out a little, but I suppose its possible that "sonic" fatigue was happening, causing the cracks. The thing that freaks me out is the belly has fairly thick skins, and it happened in such a short amount of time, 200 hours. It will be interesting to see if more cracks start. I HOPE NOT.

From your well written post & observations it seems to be a direct correlation between the pipes & skin cracks. However it's not conclusive evidence. Is it possible that the cracks where going to happen anyway for other reason? (which I have no idea, just saying). It's suspicious that they happened across the same station, like the exhaust was working that area.

I've not heard of belly cracks before, with any pipes, but that does not mean it can't happen. I have heard of complaints of more floor vibration and noise with the 4-into-1. The down-turn seems to have solved your issue dramatically, which I'm glad for.

I understand fatigue in aerospace structure from an engineering standpoint. Flush rivets in sheet metal are not as highly "fatigue rated" as protruding rivets for example. Also dimple in sheet is even less fatigue resistant. Sonic fatigue can by low stress but HIGH CYCLE. All besides stop drilling/deburing the crack, your patch should extend beyond the damage by two rows. The ones you have look fine, but if you need help with repairs let me know off-line and email me.

Good luck and thanks for sharing your experience.
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Last edited by gmcjetpilot : 11-09-2007 at 05:21 PM.
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  #12  
Old 11-09-2007, 07:54 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default A "Bunch of Hokey"?

Quote:
Originally Posted by Nuisance View Post
Recently, after changing to the ECI cold sump, I switched to the Vetterman 4 pipe exhaust. I don't believe I suffered any performance loss. Actually, I think the "tuned exhaust" is a bunch of hokey, unless you restrict your flying to one altitude.
Contraire mon frair.

Now wait a second, tuned exhaust is a "bunch of hokey"?

First 4-into-4 singles is actually a good exhaust system, with it's own merits. Not losing does not surprise me. Your new 4-into-4 exhaust is STILL a tuned exhaust. The length of the pipes and diameters are not arbitrary. I think you'll find your pipes are still all matched length at about 34"-38" long at 1.75" dia. Yes? Than they are "tuned". You are right that altitude and power does affect the ideal pipe "tuning". However you start with a sweet spot and where you will be most of the time. All pipes will be affected the same unless they have some way to vary their geometry as you climb and engine makes less power and ambient pressure goes down. Still "tuning" is valid and based on real facts and physics. On the other hand its not a holy grail just part of the big picture.

Second, many builders going from cross-over to 4-into-1 have found performance increase. The engine turns more rpms with fixed prop and they pick up speed. It has also been shown on dynos that "Headers" improve performance. Most race cars have tuned headers. On the other hand the fastest of the fast, top fuel and funny car have separate stacks. All exhaust systems have their own charm, 4-into-1, 4-into-4, cross-over and others. There is just basic physics & design. Like everything there are trade-offs however. No one thing is going to be a panacea (cure for all).

The down side of 4-into-1 on a Lyc is the scavenging is so good, that the stock Lycoming cam restricts the full advantage of 4-into-1. It's not really a disadvantage only that the exhaust is no longer the restrictive component the cam is.

The basis of saying all this is NOT a "bunch of hokey" is from Cafe Foundation dot ORG. The reports on aircraft exhaust are found here:

http://cafefoundation.org/v2/research_reports.php

They found the the 4-into-1 was BEST overall. They also found the 4-into-4 was pretty good as well. The cross over had some charm, but the balance due to pipe lengths was a negative. The worst was the "Y" pipe, dumping #1 & #3 (same side of engine) into one and #2 & #4 into one. Its the easiest pipes to make but not great for performance. No free lunch, absolute perfect everything in aerospace design.

Now the LOSS part of using down turns is open to debate. My feeling from reports like this one above, is down turns clearly give less floor pounding and noise. I would say there is contribution of exhaust thrust facing aft but it's small that its not worth it for the nose and vibrations. RV's already have a pretty well faired in exhaust system, verses a Cessna, with a stove pipe sticking out at right angles to the airflow. I don't think a 90 degree down turn is needed. A 45 degree turn is probably good enough.

PS nice work, I've followed your work for a while. Good stuff.
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Last edited by gmcjetpilot : 11-09-2007 at 08:26 AM.
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  #13  
Old 11-09-2007, 08:34 AM
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Nuisance Nuisance is offline
 
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Default

I worked with Larry Vetterman a couple of months ago on this subject. He sent me 4 straight extensions, and by measuring and cutting I made all 4 pipes the same length. I don't remember the exact length, but George is about right. I remember there was 10 inches difference from longest to shortest before I added the extensions.

Flight testing showed no speed benefit, actually a slight decrease, and the afterfiring on short final was very bad (I actually have no idea why this changed). So, I am back to what is pictured on the web page I referenced previously.

My memory of tuned pipes is that it is proportional to the rpm of interest, and that at 2700 rpm, the optimum length would be about 80". You see these 'bundle of snakes' tuned exhaust system on racing cars that run at 8-12 thousand rpm. There are plenty of variables of course, one of them being air density (i.e. now you have to pick the altitude you want to tune for). That is why it is of less interest to all around flyers, though if you want to target wide open throttle at 1000 msl for racing, you can just figure out where to put 80" of 4 pipes. The reason the 4 into 1 system is supposed to work better is that a cylinder gets a reflection from the previous one to fire, so the pipes can be shorter. I think it works better in theory than in practice.

All of this comes from a conversation I had with Kevin Murray of Sky Dynamics, who was one of the pioneers of the 4 into 1 exhaust.

I have spent plenty of time pouring over the CAFE test results, and while I think they did a lot of great work, they missed the mark alot too. I bet if they could do it again the results might change. There is a lot more available these days.

Ken Tunnel of Lycon told me that they did a lot of the testing of the Powerflow (certified) tuned exhaust system. That is the one with the collector that hangs down almost back to the gear legs on a 172. As part of this testing, he found the 4 into 1 tied with the Powerflow, the 4 into 4 next, and the crossover the lowest horsepower producer. This of course is all at one altitude (ground level in Visalia, CA).

Best, John
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  #14  
Old 11-09-2007, 09:07 AM
jbDC9 jbDC9 is offline
 
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Quote:
Originally Posted by gmcjetpilot View Post
The cracking thing freaks me out a little, but I suppose its possible that "sonic" fatigue was happening, causing the cracks. The thing that freaks me out is the belly has fairly thick skins, and it happened in such a short amount of time, 200 hours. It will be interesting to see if more cracks start. I HOPE NOT.

From your well written post & observations it seems to be a direct correlation between the pipes & skin cracks. However it's not conclusive evidence. Is it possible that the cracks where going to happen anyway for other reason? (which I have no idea, just saying). It's suspicious that they happened across the same station, like the exhaust was working that area.
Good points, but I can't really imagine cracks starting in this location for any other reason; I've read of other -8s having a crack here and there on the cooling ramp skin, mine has one small crack around a rivet head there as well... the ramp skin seems pretty light and flimsy. But I've not read about or heard of another -8 with a tendency to crack in this location. For comparison, I've looked at numerous other -8s with standard exhaust systems including Paul's ship with 600+ hrs and Danny's with 900+ hrs... and no cracks on the belly between the gear legs. That, coupled with the huge reduction in noise and vibration with the new down turned tip on my machine has me 99.9% sure that the straight pipe was causing the problem. Now I just have to put another 50 hours or so on it to see if it's a done deal.

Why, oh why didn't I just stick with my original plan of a crossover system with dual mufflers?

Quote:
Originally Posted by Ironflight
No one will ever notice them once it's painted - unless you go around pointing them out!!

Now "Get thee to a paint shop!!"
No kidding, I gotta learn to refrain from pointing down there and saying "Look at this mess!" And the paint shop?? Oh, the horror! Does that mean I actually have to decide on colors and a scheme?? Seriously, I have no real plan yet for scheme or colors; it's driving my wife nuts. I suppose that I should at least get on the waiting list at a shop, that'd pressure me to make a decision!
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  #15  
Old 11-09-2007, 02:27 PM
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gmcjetpilot gmcjetpilot is offline
 
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Default Excellent

Quote:
Originally Posted by Nuisance View Post
I worked with Larry Vetterman a couple of months ago on this subject.................

As part of this testing, he found the 4 into 1 tied with the Powerflow, the 4 into 4 next, and the crossover the lowest horsepower producer. This of course is all at one altitude (ground level in Visalia, CA).

Best, John
Interesting John good stuff as usual. I agree the optimal to make power is longer than 38 inches but than we have some practical limits. Cafe tested straight pipes of 32.25" long with diff dia. 1.625, 1.75 and 1.875, and they did OK.

I found this simple formula I plucked off the "internets".

.....850 (180 + N)
L = ------------- - P
...........RPM


where
L = length of pipe in inches
N = degree before BDC exhaust valve opens
P = distance from exhaust valve to manifold
RPM = desired RPM

Guessing at N and P and calculating for 2,500 rpm.

.....850 (180 + 180)
L = -------------- - 2 in = 122 cm = 48 inch pipe length.
...........2,500


"The diameter of the pipe needs to be calculated so that the volume of the exhaust pipe attached to each cylinder is twice the volume of each cylinder. The exact diameter of the pipe, incidentally, is not critical and should only be used as a guide in determining which standard sized pipe diameter should be used."

Volume of each cyl on a Lyc 360 cu-in / 4 = 90 cu-in

pi r^2 * L = 2 * Vol
pi r^2 * 48 in = 2 * 90 cu-in
r = 1.93 in
Pipe Dia = 3.8 in dia - a nonsense answer clearly, but working backwards using 1.75" dia and solving for L = 58", is actually pretty close.

According to Cafe the ideal header primary is 28-36” plus secondary + collector length appeared optimize at 20-30”. So the total would be in the 48" to 66" total length range. The crude formula gave me 48" and 58".

Cafe talks about fancy programs that account for "Doppler phenomenon occurring in an exhaust pipe because the sonic exhaust wave is riding on the “wind” of the streaming mass flow of fuel and air." I have no idea? As you say John, RPM and altitude changes so you can "TUNE" only so close. Induction geometry can improve with "tuning" as well of course. The 993 Porsche has a variable-length intake runners they call the "Varioram" system. This addressed the inherent compromise between high-RPM power production and low-RPM torque production. It's first of its kind to be employed on production vehicles. The first 993's did not have "Varioram" and made about 12 hp less and little less torque at higher RPM. Not a big difference, but it shows if you vary intake and exhaust geometry you make more power.
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Last edited by gmcjetpilot : 11-09-2007 at 03:32 PM.
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  #16  
Old 11-09-2007, 04:09 PM
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Hay John, would post a picture of what the exhaust looks like now with the down turn!
Thanks
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  #17  
Old 11-09-2007, 05:08 PM
jbDC9 jbDC9 is offline
 
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Quote:
Originally Posted by Adam View Post
Hey John, would you post a picture of what the exhaust looks like now with the down turn!
Well, I would, but I forgot to take a pic of it... I'll be outta town for a few days, but I'll get a shot and post it or email it to ya next week.

John
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  #18  
Old 11-09-2007, 05:31 PM
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<<Guessing at N and P and calculating for 2,500 rpm.>>

Bit long on your "exhaust valve open" (N) guess. 180 before BDC is TDC <g>

Closer to reality is 65 BBC, so:

.....850 (180 + 65)
L = -------------- - 2 in = 81.3 inch pipe length.
...........2,500

My memory about tuned pipes agrees with John's...and so does your formula.
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  #19  
Old 11-09-2007, 06:18 PM
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Quote:
Originally Posted by gmcjetpilot View Post
Induction geometry can improve with "tuning" as well of course. The 993 Porsche has a variable-length intake runners they call the "Varioram" system. This addressed the inherent compromise between high-RPM power production and low-RPM torque production. It's first of its kind to be employed on production vehicles. The first 993's did not have "Varioram" and made about 12 hp less and little less torque at higher RPM.
Nissan (VG30E), Subaru (EG33D) and Audi had variable intake length systems in production well before the 993 introduced it. Many engines use it today in conjunction with VVT. Mazda used it to win LeMans in 1991. I don't think you'll see this on an aircraft power plant any time soon with a narrow operational rpm band. Even fixed length induction runners can show major gains if made to the correct length. My new manifold is tuned for best gains in the 4000-4500 rpm range.
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  #20  
Old 11-10-2007, 12:04 AM
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Default 4-1?

I built an assymmetric 4-1 for my RV-4, and initially I tried to keep the exit pipe as long and close to the floor as possible to align with the airflow. Noise and heat beat me, so I shortened it, put a 45? turned down tip on it, and now noise is very acceptable, both inside and out. The heat under my right foot is still a bit uncomfortable on a warm day though! In applying CAFE data, most systems don't have a long or large enough tailpipe. The CAFE argument, as I read it, is that by keeping a large mass of exhaust gas moving down the tailpipe the momentum of it helps scavenge the cylinders. Most people don't like a great long tailpipe hanging out in the breeze. The skydynamics 4-1 have a large diameter tailpipe and a bellmouth exit - this gives a longer effective length, I believe. Paser looked into exhausts with his Mustang II, and found that 4 straight pipes were best for speed (tuned for half wavelength I think), and equalising EGTs, but he developed slightly more power with a crossover system.
I'll probably opt for 4 individual pipes in the next iteration.
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