More on so-called Tuned Exhaust

[N1478D]

My plane is parked outside in the Texas sun daily on weekdays all year long, and I fly alot of weekends too! Have used the inside covers all these years and people still ask about my new windshield. It was in the plane in 98 when she was purchased, and had been in there for a while then. IMHO, that's hogwash about a double dose of sunlight. Was reading an aviation maint rag about how mechanics should deal with the tuned exhaust such as from Power Flow. Power Flow had their 2 cents in about how a tuned exhaust works. This is a paraphrase from Power Flow - when an exhaust valve opens on a stock exhaust the gas has three options, leave out the tail pipe, stay in the muffler, or go up the other pipes. When it goes up the other pipes it causes pressure when that valve opens. So, our engineers designed long pipes so that the harmonic balance is in tune so that at 2450 RPM the waves of energy are timed just right so that the gas is actually sucking out the other gas. Some manufactures feel like they have to say something to market their products. A double dose of sun and one gas sucking out another gas are right there in the same arena as TV Wrestling. It's just promotional. Another engineer could be found that would say the long pipes on a tuned exhaust provide an opportunity for back pressure, and there would be an engineer who could find fault with exterior covers. They all do some good and maybe some bad. Not trying to knock exterior covers, like the interior ones are being knocked, just saying that the interior ones have certainly worked very well for me and others.

[Bruce Fenstermacher]

This is a paraphrase from Power Flow - when an exhaust valve opens on a stock exhaust the gas has three options, leave out the tail pipe, stay in the muffler, or go up the other pipes. When it goes up the other pipes it causes pressure when that valve opens. So, our engineers designed long pipes so that the harmonic balance is in tune so that at 2450 RPM the waves of energy are timed just right so that the gas is actually sucking out the other gas. Some manufactures feel like they have to say something to market their products.

Well Joe this is way off the subject but what Power Flow says is right. It is old proven technology. How effective it the Power Flow system is on our engines or how well Power Flow accomplishes what they set out to do has been hashed out elsewhere and I don't want to re-ignite that here.

As for the side window replacement. Those that used pop rivets I have a few questions. Did you use aircraft pop rivets or Ace Aeromotive type? Where the rivets countersunk? If not countersunk did the head interfer with the window closing?

And finally how exactly did Cessna buck the rivets?

[GAHorn]

So....does that mean the PowerFlow exhaust is NOT in tune at say....2330...like the beginning of takeoff roll? And also NOT in tune at 2500? ...with wide-open throttle while needing as much power as possible? (In other words....What a load of B.S. IMHO. If the other cyls happen to have their ex. valve closed then what possible harm is there in having exhaust gas pressures in the riser? And if the other cyls ex. valve is open....then it's piston is also overwhelmingly shoving it's own exhaust gases out the pipe...with far more pressure than any ambient exhaust gas waiting in the riser. Another kudo for marketing types, maybe.....but pure B.S.
How 'bout THIS: If it takes a looong exhaust to be in tune so as to meet their claim....then how do they explain how all that claimed exhaust-gas pressure is sitting there waiting around to cause detriment to other cylinders...in a SHORT system...where the so-called pressures have already exhausted to atmosphere?)

[Bruce Fenstermacher]

George I didn't say it worked or not in our application I was just trying to tell Joe that what Power Flow was saying they did is what they are trying to do. It isn't a bunch of garbage in all applications.

Now how about commenting on my window replacement questions.

[jrenwick]

Here's some info on tuned exhausts: http://auto.howstuffworks.com/question172.htm

Best Regards,

John

[GAHorn]

Here's some info on tuned exhausts: http://auto.howstuffworks.com/question172.htm

Best Regards,

John

Here's what the pertinent paragraph says:" The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector. "
IN our airplanes ....each cylinder also gets it's own exhaust pipe....called a "riser". These pipes come together in a larger pipe called a collector (in our airplanes a muffler. It doesn't really muffle a whole lot....it doesn't have all the sound-deadening (back-pressure creating) internal paraphernalia that cars have....it's primary purpose is to extract heat from the exhaust gases. Very little reduction in flow or delay in exhausting the gases occur here.) Lastly, "By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure ..." IF you look at our risers you'll also see they are practically the same length except to accomodate fitment to the collector/muffler.
(And auto exhausts are typically 10 feet long or longer....while aircraft exhausts are very short where the gases have excaped to atmosphere far before any "backpressure" becomes significant. But throw your money away if you wish.) There goes my 2 cents.

[jrenwick]

OK, the howstuffworks explanation was a little naive. It didn't say that not only is it important that each cylinder have the same length of pipe into the collector, but the length of the pipes is selected to work with the engine working at a specific RPM and power range. That's what the tuning part means.

Here's a better explanation from http://www.amtonline.com/publication/ar ... =1&id=1502:

"Now consider a tuned exhaust. The main goal is to more efficiently evacuate the exhaust gases from the cylinders. This is mostly controlled by the length of tubing. "There are multiple waves of both pressure and heat that are going to come out when the exhaust valve opens," Tilman shares. "Length dictates whether the pulses are going to benefit you or not. The key thing about tuning is if you send those pulses through a long enough tube, so that they are not interfered or influenced from another one at the time the valve is open, then you can utilize that to create essentially a mirror image -- a negative wave or a suction at the time the valve opens."

"So the engineers working on a tuned exhaust system determine what the optimum length of tubing is. A tuned system is technically 100 percent optimized for one given set of conditions. Typically, an exhaust is tuned for a peak boost down at sea level at 2,450 rpm.

"The four tubes then join at the four to one collector. When they meet at the collector, they are the same length. They then go on to the muffler and out the system.

I always understood the tuning as creating a standing wave in the header pipe, so that there is a stationary low pressure point just downstream of the exhaust valve. This makes it easier for exhaust gas to get out of the cylinder. It's like when a trombone player moves the slide to adjust the length of the instrument, while buzzing his lips at the frequency of the note he wants. The note won't really sound until the length is tuned to resonate at that pitch.

I googled to a more technical description of a different kind of system at http://www.piteraq.dk/flight/exhaust.html. Given the amount of effort racing engine designers put into the design of extractor systems, it's hard for me to see writing it all off as B.S.

Best Regards,

John

[N1478D]

How did we get from side windows to tuned exhaust?

The sunlight reflected off of the tuned exhaust gas and fogged up the side windows!

Race engines are probably run at a certain RPM range most of the race. Maybe at an aircraft engine test stand, a test at a certain RPM, say 2450 would show an improvement with a tuned exhaust. And, if you could fly all of the time at 2450, then maybe you could achieve some benefit from an expensive modification. It sure sounds hard to believe about pulses, waves, etc working out for all types of flying - takeoff, formation flying, lazy looking around flying, etc.

[zero.one.victor]

As long as we're talking off-topic about tuned exhaust, I remember back to my dirt-bike days in the early 70's when designing an "expansion chamber" exhaust system for 2 stroke engines was quite a black art. The sorcerers who were big into this could tailor-make an exhaust system to optimize either horsepower or torque at whatever rpm range you specified. I think a wee-gee board, a pair of dice, and some chicken bones were required equipment for the design phase of these projects. You could substitute a "magic eightball" in certain circumstances. No doubt things are a little more scientific these days with computers, etc....

Eric

[GAHorn]

John, I'm certainly not intending to disparage anyone who wishes to subscribe to the theories about exhaust tuning.
But let me suggest that racing engines run at rpms quite a bit higher than aircraft engines (due to the need to stay within propeller efficiency ranges.) Aircraft engines almost always run well below 3500 rpm with most around 2500 rpm unless they are geared. Auto/race engines run 9,000 rpm or above where exhaust systems, other than the very short ones like we have in aircraft, are a different "tune".
Salesmanship, however, works very well anytime the buyer can be caused to doubt his own judgment, regardless of the product.
Should I seperate these exhaust posts into their own category? What do you guys think? My judgment is affected.

[blueldr]

Re: Tuned exhaust systems on aircraft.

If it was that easy to pick up all that extra horsepower, why do you suppose the aircraft manufacturers don't do it????

[zero.one.victor]

Why not find out what the boys who race at Reno do about exhaust systems? Specifically, the formula one racers-- their engines are most like our 145's: (highly modified) Continental O-200's. Of course, from what I understand, they turn somewhere between 3000 and 4000 rpm instead of our 2700 max. For the Lycoming-powered among us, check out the biplane exhaust systems- they usually run O-360's.
That'd be a lot more germain to us than cars, motorcycles, etc.

Eric

[mvivion]

Well, the formula one guys do run very specifically tuned exhausts, but George will argue that they are running high rpm as well (3500 for most), so you'll have to discount that as well, I guess.

There is so much evidence from so many sources that exhaust tuning works that its fascinating to read the opinions of "experts" here, who argue that since it hasn't been tried on a Continental O-300, it can't be true.

Several companies have DOCUMENTED, via independant testing, that they can pull more horsepower from a given engine with a carefully designed exhaust system. That's a fact. Its been done in a lot of venues, automotive, aircraft, snowmachine, and probably lawn mowers.

Furthermore, while such an exhaust system will be optimized for a specific rpm range, it certainly won't hurt performance in other rpm ranges. Clearly the most benefit from one of these systems would come in an installation with a constant speed prop, where rpm is adjustable, and can be set for a specific rpm if desired. But the point of having a 180 hp engine isn't to run 180 horsepower ALL THE TIME, fer cryin out loud. The point is to use 180 hp for takeoff, then back the power off for climb, cruise, etc. So, if the gain is only optimized for takeoff rpm, then it will do precisely what we want it to do: Make more horsepower when we need it most. Granted, aircraft with fixed pitch props might want to have the hp gains optimized at some different rpm, as in that rpm typically achieved at takeoff. So?? That's where you'd optimize it, I'd guess.

Having been somewhat skeptical about Power Flow's claims my own self, but for other reasons, I asked representatives from both Lycoming and Continental what they thought about those claims. Both responded that there is no question that additional power can be had by optimizing an aircraft's exhaust system, and that they have no doubt of Power Flow's claims, though their respective factories have not verified such claims. They were in agreement, however, that most stock aircraft exhaust systems are very inefficient (note that the engine manufacturers DO NOT build the exhaust systems that are typically installed, that comes from teh airframe manufacturer).

So, why don't the manufacturers start installing these things from the factory? Certification hassles, proprietary manufacturing rights, cost, etc, etc. There are LOTS of legitimate improvements to aircraft that don't appear on factory airplanes. Good grief, is someone actually suggesting that Cessna has their airplanes totally up to date?

Here's a little analogy for you: The new Cessna 206 is not available with a factory float kit. Oh, you can order it with what Cessna calls a float kit, but then you have to go to WipAire and buy a used seaplane rudder and bolt it on your brand new airplane, to the tune of $18,000 last I heard (oh, they'll paint it to match). So, I'm a little skeptical when someone suggests that Cessna (or other manufacturers) is right on top of design or engineering.

Mike Vivion

[GAHorn]

... its fascinating to read the opinions of "experts" here...,
Mike Vivion

Mike, you may consider yourself an expert, but I don't claim that for myself. It'd be appreciated if you wouldn't derisively assign the title to those who don't claim it.

The mfr's of magic exhaust systems PAY those so-called independent laboratories for their results. Which independent labs are quoted by them that did NOT make the conclusions they desired? Independent? I don't think so. They're on the payroll.

In one of the lastest posts, another source is quoted to say, "The key thing about tuning is if you send those pulses through a long enough tube, so that they are not interfered or influenced from another one at the time the valve is open, then you can utilize that to create essentially a mirror image -- a negative wave or a suction at the time the valve opens."

I can't help but wonder why anyone would lengthen an exhaust system if they are worried about back pressures (or added weight for that matter.) It seems rather obvious to me that the longer the gases are held captive in the tube the longer they are present to influence the feared backpressure. After all, it does take energy to propel the gases along the tube due to resistance, and the sooner the gases are given an exit the sooner they are not present to influence the engine.
As for a "suction at the time the valve opens"... I find that statement so ludicrous and obviously aimed at marketing it's laughable. I don't suppose the atmosphere would make any attempt to fill that imagined "suction" would it? Nor would any of the gases from the adjoining cylinders which are connected to the same "collector".

Under what circumstances would I subscribe to a tuned exhaust? I would endorse a tuned exhaust when there are independent intake manifolds and carburetors for each cylinder, in order to provide equal breathing for each cylinder to enhance smoothness. It's difficult to syncronize multiple carbs/inductions unless they are all equi-distant and unless exhausts are equally complimentary to the intakes.
That's a major reason tuned exhausts on most airplanes are not particularly beneficial to anyone except the sellers. Our intake manifolds are not equi-distant, nor are they connected to individual carbs. They are all connected to the same carb thru a manifold system designed to fit under the cowl with a minimum of installation interference. A tuned exhaust will not benefit such a system...especially an exhaust which is actually longer than the OEM system. IMHO

[N2255D]

Anybody up for a discussion on auto vs avgas or maybe MMO ?