Tuned Exhaust

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Flyhighdave

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I have been working on the design of exhaust pipes for my Barracuda project - IO540. I have a pvc mock up kit from one of the custom exhaust companies and am trying to achieve equal lengths to the
3 into 1 collectors on either side of the engine. This is a tedious and I believe impossible task given the limited space available for wiggling the pipes around in an effort to get them to be the same length.
The guy at the fabrication shop said that I probably can't get them to be truly equal & that seems to be correct.
This leads to my question of whether it's a worthy undertaking at all. Is the potential increase in power worth it? Will there be a benefit if the pipes are not quite equal in length? Maybe I should just keep it simple & go straight out on either side. It seems that all the extra hot pipe bending around will also add a lot of heat to the engine compartment.
What say ye fellow aeronauts?
Thanks!
 

TFF

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You want them long. Perfect length is only worth it in racing, but long easy flowing exhausts are always worth it over a compact manifold for power.
 

lake_harley

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Along with what TFF said......the "perfect", equal length also depends on what the RPM range is you're trying to get good scavenging/power increase. Just being equal length doesn't mean that the length is "right" for the RPM you're working with. Maybe you know that already so if you do I'll quote Rosanne Rosannadanna and say "never mind" :)

Lynn
 

rv6ejguy

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Tuned length (full wave) for Lycoming primaries will likely be in the 65-70 inch range for 2700 rpm. Doubtful you have enough cowling room to fit 6 of those in there.
 

Flyhighdave

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Thanks for the input. These responses lead me to believe that long & smooth (straight) all coming together into one is better than a lot of bending around to achieve equal length. Am I understanding correctly?
Again thanks!
David
 

TFF

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BJC

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If the bends are not smooth then yes, but equal length or close to equal are done all the time. Ford "Bundle of Snakes" is not worth it on an airplane, but "1/2 wave" length with each close to an inch or two will be good and look good too. http://www.aircraftexhaust.net/lycoming.html http://www.skydynamics.com/index.php/products/exhaust-systems for ideas and you can look for the CAFE exhaust articles in Sport Aviation archives if an EAA member.
SkyDynamics was an excellent company to do business with some years ago (personal experience), and I still hear good things about them.


BJC
 

Toobuilder

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The CAFE articles and testing indicates the 360 (and by extension, the 540) likes 38 to 44 inch primaries. Its worth about 8 HP on a 200 hp 360. In my mind that is a significant gain for nothing more than a few more feet of tube under the cowl.

Generally, the collector is a tuning aid which helps extend the power range of an engine. This is important for cars and bikes because of their need to pull from idle to redline, but not so much on an aircraft. You can get a very strong resonant signal at a specific RPM from a single tube if sized correctly. I'm planning on a 6 into 6 with 44 inch primaries on the Rocket to start. I will add or trim the length to find the sweet spot. I'm looking to hit resonance at 2300 RPM and 9500 DA.

Sharp bends are ok, but kinks are not. The only thing you need to worry about with a "bundle of snakes" system under the cowl is the heat concentration. Those tubes are glowing orange for hours at a time and that's a lot of surface area to radiate heat.
 

wsimpso1

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If they are all the right length, you have smooth flowing absolutely tuned pipes at one rpm. Get more than a few % of design RPM away from design rpm, and the results of all that work on the long pipes fades. At one time the car racers went way overboard getting big specific power, but very peaky. Then they figured out they needed torque across the top 25-35% of the possible rpm, and they started doing things like running some induction stacks short and other long, and varying the lengths of the exhaust pipes. Reduced peak power a little, but gave good torque across the intended rpm range. The bike racers found similar things and ended up with tuned expansion chambers of several different volumes on each engine to fatten the useful torque range.

If you do not have enough room for pipes the right length, you are shooting for smooth flowing, and same length is a good idea, but hardly essential. Get 'em close and you will still have good flowing, more power than stock, smoother running, better induction, etc. Get close if you can, but do not sweat it.

Billski
 

wsimpso1

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Hmm, trying to remember my engines class (it was 1978), but with equal length pipes and a good collector, isn't the reflection point the exit end of the pipe, not the collector? That detail is lost... And don't we get reflections from all of the exhaust pulses that are plumbed out that pipe? As much as I have done in my life with resonance and vibration, and I am having trouble on this one, the first one I learned about? Now you have me wanting to get the books out and searching the internet...

Billski
 

Rockiedog2

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how bout let's say an O-200 cruising at 2500 rpm. what would be the "correct" length stacks for that one? (4 individual stacks) I see the racers running little short ones but they're turning way up there 4K or so I think
 

Victor Bravo

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We ran 3900-4200 on BooRay (my old F-1 racer), and we only had short stub stacks. My first crew chief felt that the extra weight, heat, and volume taken up by a tuned system would not be worth it. Probably because our airplane was pretty heavy.

Later on, another engine builder told me that he thought the pipes would be worth it, but he also was really spooked by the heat being that close to a fiberglass cowl. We figured that a whole new cowling would have to be developed to make all that possible. So we started building a new cowl, which turned into a new front end fairing for the whole airplane, which grew into integral wing root fairings, which became a very large and unwieldy one piece mold. And then 'life gets in the way' showed up and tore the whole mess down, taking a couple of friendships, my racing "career", and a good chunk of my sanity down with it.

I understand that several of the racers have gone to tuned exhausts and they believe it works well enough to justify their existence. But those are definitely single purpose "point design" airplanes of course.
 

Himat

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Hmm, trying to remember my engines class (it was 1978), but with equal length pipes and a good collector, isn't the reflection point the exit end of the pipe, not the collector? That detail is lost... And don't we get reflections from all of the exhaust pulses that are plumbed out that pipe? As much as I have done in my life with resonance and vibration, and I am having trouble on this one, the first one I learned about? Now you have me wanting to get the books out and searching the internet...

Billski
I did never have the engine exhaust course during my engineering studies. On the other hand, it sound like pipe acoustics, something that is somewhat to the side of hydro acoustics that I am into.

Anyway, I would say much the same, only that there is a reflection both at the collector and at the end of the pipe. The general rule is that at every junction the pipe changes cross section area there will be a change in acoustic impedance and a partial reflection of the propagating sound wave. Only when the pipe taper and not to sharp there is no reflection due to impedance change (cross section change).
 

TFF

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There are calculators like this http://www.mezporting.com/exhaust_length.html Remember too that when talking max horsepower it is only good at sea level takeoff. At whatever altitude you will have a different need. Short stacks are simple and have a crackle ,that if you love noise is heaven. If you are trying to get a quiet cabin, and have to minimize power loss or really have some good data long will be the way. Way better than some certified log exhaust.
 

Mcmark

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It's been noted in one of Kitplanes mags in recent months and as noted above, good for certain HP.
The tube size formula is: (Peak torque RPM x 1 cy displacement) 88,200=PPA. And you can adjust for peak by the length of the tube. Longer is more torque at lower RPM or short for torque at higher.
Everything I've read/seen indicates the 1/2 wave reversion length is somewhere in the 36-46" length. Individual tubes is easiest to route and according to Kitplanes take on the CAFE report, almost as effective for these engines.
One of the guys on the Biplane Forum used stainless bath hand rails cut and fit for 4 to 4. It looked nice.
 
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