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Exhaust system performance

scsmith

Well Known Member
Larry DeCamp's recent thread about finding intake leaks contained an interesting reference provided by Kyle Boatright on exhaust system performance, written by the CAFE foundation:

http://acversailles.free.fr/documen...ur/Echappement/Aircraft_exhaust_system_IV.pdf

I don't see a date anywhere, I don't know how long ago this was printed in Sport Aviation.

I found this article very informative, but I think there is an error in the discussion of cross-over exhaust systems. The author states that cylinders #1 and #2 fire 180 degrees apart, and similarly cylinders 3 and 4. Thus he describes a crossover system that connects the exhaust headers from cylinders 1 and 2, etc.

However, using Lycoming cylinder numbering convention, the firing order is 1,3,2,4 and so cylinders #1 and #2 fire 360 degrees apart, not 180 degrees. Same for 3 and 4.

I am trying to recall how my Vetterman cross-over works without going out to pull my cowl. I think it does connect cylinders 1,2 and then 3,4. This would mean that the pipe lengths would have to be adjusted to provide some beneficial scavenging with 360 degree delay. But it does mean that the benefit would be symmetrical, i.e. the benefit from #1 reflected wave helps #2 in the same way that #2 helps #1.


If the cross-over worked with 180 degree delay, then it would not be symmetrical -- For example if you connected #1 and #3, you would have 180 degree delay for #3, but then 540 degree delay for #1 after that.
Anyone with more in-depth understanding of crossover exhaust systems care to comment?

Also, am I the only one that gets a blank page with nothing but a /**/ in the upper left corner when I try to go to vettermanexhaust.com?
 
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Hmmm....

My Vetterman Crossover connects #1 and #2 - Left Stack, and #3 and 4 - Right Stack...
 

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The Vetterman site works for me. But your link to the exhaust report doesn't. Go figure. ;-)

I fixed the link to the CAFE article.

On Vetterman's web site, I checked a Mac, an iPad, and a HP windows machine. It must be my ISP blocking because of an unrecognized security certificate or something?
 
Another error! "this results in the valves opening twice during the four-stroke cycle" is wrong. Because the cam turns at half the crank speed, each valve opens ONCE for each four-stroke cycle, which occurs over two crank revolutions.

Honestly.....

One intake valve opens and one exhaust valve opens.... The valves open twice...:D

He did not say EACH valve opens twice...:rolleyes:
 
At ease. I think the error come from the way "cycle" is used in engine speak.

A cycle generally means a circle back to where one starts such as a yearly cycle of spring, summer, fall, winter.

In the same way, an engine would have a "cycle" of one CAMshaft rotation which includes the intake-compression-power-exhaust events. These event are not cycles in the usual sense of the word cycle.

So, I believe in this case the error is a brain stumble using 180 deg to mean "exactly opposite" and thinking in terms of the circle of engine events. But, yes, definitely 360 deg crank rotation between 1 and 2 cyl, and 360 deg between 2 and 1. Same for 3 and 4. I accept the error as a notational one, not a conceptional one, though confusing. One could be prone to this error if your thinking is the 360 deg of camshaft cycle. Peace.
 
At ease. I think the error come from the way "cycle" is used in engine speak.

A cycle generally means a circle back to where one starts such as a yearly cycle of spring, summer, fall, winter.

In the same way, an engine would have a "cycle" of one CAMshaft rotation which includes the intake-compression-power-exhaust events. These event are not cycles in the usual sense of the word cycle.

So, I believe in this case the error is a brain stumble using 180 deg to mean "exactly opposite" and thinking in terms of the circle of engine events. But, yes, definitely 360 deg crank rotation between 1 and 2 cyl, and 360 deg between 2 and 1. Same for 3 and 4. I accept the error as a notational one, not a conceptional one, though confusing. One could be prone to this error if your thinking is the 360 deg of camshaft cycle. Peace.

It is pretty clear, it says "A crossover exhaust joins the headers of cylinders whose firings occur 180 crankshaft degrees apart. " which is incorrect. I'm wondering if the majority of exhaust-tuning experts are 2-stroke specialists and they just have that locked into their heads.
 
It is pretty clear, it says "A crossover exhaust joins the headers of cylinders whose firings occur 180 crankshaft degrees apart. " which is incorrect. I'm wondering if the majority of exhaust-tuning experts are 2-stroke specialists and they just have that locked into their heads.
Ha. Or like a tuner that keeps calling a guy named Homer, Henry. Maybe may exhaust fumes? :)

I did the exhaust sys on an original 180 Lyc acro. It was 4 into nothing with 1 5/8 tubing. It sounded good and seemed to pull pretty hard. Simple. No collector.
I think most of the exhaust configs could benefit from a little bit of nozzling. It will reduce the noise some and increase the exhaust thrust. Most engines are OK with a little back pressure. Actually, the engine does not "feel" back pressure until the cylinder pressure drops quite low near the exhaust clearance phase. The article mentions exhaust thrust near the end. Puts it at 5 or 10% of total thrust and 3 knots extra.
 
The only experience I have with nozzles was on a turbocharged engine. A nozzle at the exit raises the turbo back pressure, which raises the turbo inlet pressure for the same boost (the turbo was sized to produce excess boost and the waste gate was bypassing some exhaust.) The higher turbine inlet pressure obviously raises the back pressure at the exhaust flange.

I overdid the nozzle a little, and the higher pressures showed up as higher CHT, which was already marginally high (w/67 in hg MAP) so we opened the nozzle a little, and the CHTs dropped. No idea how to find the optimum other than by experimental flight test measuring top speed.
 
I wonder what the results would be on a 4-1 if you use a 'merge' collector that they have now.............


:)
 
I wonder what the results would be on a 4-1 if you use a 'merge' collector that they have now.............


:)

Yes, the article did seem to make a good case for 4-into-1 systems. There are other advantages too. DanH is thorough enough that I imagine he had seen this article in 1997 and may have played a role in his selection. If you look at how DanH did his cowl exit, this is the cleanest way to manage the cooling exit flow, facilitated by the 4--1 pipe.

I didn't evaluate their description of the 4--1 collector geometry closely enough to know how it would compare with the Vetterman 4--1. I was too preoccupied by the obvious error in their discussion of cross-over systems.
 
It is pretty clear, it says "A crossover exhaust joins the headers of cylinders whose firings occur 180 crankshaft degrees apart. " which is incorrect. I'm wondering if the majority of exhaust-tuning experts are 2-stroke specialists and they just have that locked into their heads.

Headers, or tuned length exhaust systems work on the principle that cooling exhaust gas increases their velocity in the tube. If multiple tubes are interconnected, the pressure from this velocity increase in one tube can create a slight vacuum in the interconnected tube. That vacuum helps to pull more air/fuel into the cylinder during the in/ex valve overlap period, helping to reduce the amount of exhaust gas that remains and replace with more usuable air/fuel. There is some science involved in spacing the cycles, as well as managing indvidual tube length before the interconnect point, but it is not limited to 180 or even 360 degrees. I believe the Nascar guys have the length matched to the valve timing to maximize the vacuum produced.

This is purely a power thing and doesn't really affect efficiency. Also, all modern cars actually ADD exhaust gas into the mixture when not under full power or heavy load, as they have found it lowers certain emission levels (EGR valves).

WHile I have never seen a cam profile map for a lyco, I don't believe they have a lot of overlap, so the effect here is less than more performance oriented engines. Ever notice that lumpy exhaust sound on a hot rod? That sound is from a cam profile with a very large overlap. It ensures full fuel charge in the cylinder at the expense of sending some of that fuel out the exhaust before ex valve closure.

Larry
 
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CAFE did many flight test articles on exhaust and electronic ignition published in EAA Sport Aviation magazine. If you are an EAA member you can go back in the archives and see all of CAFE's tests. It is well worth the time.

It is pretty clear, it says "A crossover exhaust joins the headers of cylinders whose firings occur 180 crankshaft degrees apart. " which is incorrect. I'm wondering if the majority of exhaust-tuning experts are 2-stroke specialists and they just have that locked into their heads.
I read all these articles a long time ago but recall all references are per CAM timing not CRANK rotation.... The firing order for a Lyc is 1-3-2-4. Cyl 1&2 share the same intake cam lobe, or 180 degrees apart of cam timing (360 crank). Same with Cyl 3&4, which share the same intake cam lobe and are 180 degrees apart in cam timing. Each Cyl has it's own exhaust cam lobe. The crank turns 720 degrees for the 4-stoke cycle, while Cam turns once. Vetterman cross over does connect Cyl 1&2 and 3&4 , which is 180 degrees of cam timing or 360 degrees of crank Rev... I think the article is OK. Read it and look at the graphs. I believe there were several articles on exhaust not just one. They tried many exhaust configurations and sizes.

https://images.app.goo.gl/DEZBF5FXeiVRpt3V6


This is purely a power thing and doesn't really affect efficiency.
Well I would say both. Making more power at altitude is more efficiency. Improved exhaust (4 into 1 headers with tuned equal length runners of proper length) is almost a free lunch (almost read below). It adds more power without much down side. For me when building my RV7 4 into 1 was not available off the shelf, so I mocked one up and had it made. Now you can buy them ready made. You can throttle back and be more efficient and make the same power due to efficiency in your exhaust OR make more power. You have more efficient combustion and less "pumping loss". When ever you make it easier for the engine to breath, it always adds efficiency and power.

Note Vetterman did some test comparing 4 into 1 (which he does not make) to his cross over which he does make. He found little difference. I would argue Vetterman used a set of 4 into 1 pipes for comparison that were NOT optimal in both primary pipes and collector pipe diameters and lengths (see below). BTW the 4 into 4 Vetterman pipes he makes for the RV4 are very efficient and better than cross over in many flight and power conditions. Cross over does not really help or hurt, it's a compromise. What it does is simplify the exhaust pipe installation, and it gives you a place for cabin and carb heat muffs. A 4 into 1 is harder to fit in RV's, especially ones with bottom mounted carb/FI or nose landing gears (but can be done, just asymmetrically). That big single pipe at center line for the 4 into 1 collector is not as easy to fit as two smaller pipes on the L & R of a cross over, but the cross over is not for performance. The worst are Y pipes, where the two Cyl on left #2 & #4 are connected in a Y, and on right hand side #1 & #4 are Y'ed together. There you get no scavenging, maybe even backpressure, but it makes installation super easy with least amount of pipe.


Also, all modern cars actually ADD exhaust gas into the mixture when not under full power or heavy load, as they have found it lowers certain emission levels (EGR valves).
Not relevant to airplanes but EGR is recirculation of some exhaust gas into the intake, lowering the oxygen going into the engine to control NOx. It is of course for emissions not power (as it lowers power and causes deposits in the intake). EGR has never been used on planes. EGR in cars started in late 60's early 70's with the pollution control and safety bumper laws So older technology cars hard EGR's.


WHile I have never seen a cam profile map for a lyco, I don't believe they have a lot of overlap, so the effect here is less than more performance oriented engines. Ever notice that lumpy exhaust sound on a hot rod? That sound is from a cam profile with a very large overlap. It ensures full fuel charge in the cylinder at the expense of sending some of that fuel out the exhaust before ex valve closure. Larry
The CAFE foundation tests on exhaust and electronic ignition have graphs showing the overlap, where exhaust and intake valves start to open and close, with other data as well as such as exhaust pressures. Overlap is when both intake and exhaust are partially open at the same time. The Lyc is a low Rev'ing engine but has a very large bore, with 360 Ci with 4 jugs not 8. Because of the large volume of each jug you need significant overlap. Lyc does have a "lope" at idle due to this overlap. Is it as radical as a hot rods racing cam? Don't know, but theLre is overlap and exhaust does benefit a Lyc as much as a Chevy 350 V8. The comment you make does beg the question, can a CAM grind (more lift, longer duration overlap) improve performance more and take more advantage of exhaust. Sure but there are issues of cam lubrication, cam angles to tapped angles... The CAFE article using my dusty memory does mention this. That with improved induction and exhaust the cam is limiting. Old hot rodder's learned everything had to match, carb, intake manifold, cam, exhaust. With an airplane engine I am not willing to be radical. Exhaust is pretty benign and as I said really a free lunch (with the price of a more expensive harder to fit exhaust). However if you play with exhaust, especially on an O-320 with a Carb, you better be ready to RE-JET the carb because you will run lean. Our Lycs in our RV have much better induction airboxes and exhaust than certified planes that used similar engines. I recall a C-172 with 160HP engine was lucky to make 138HP due to the airbox and exhaust restrictions choking it....

Overlap at TDC with both valves slightly open allows the intake to start being pulled in as the exhaust is being evacuated , pushed or sucked out. That is the scavenging. The goal is exhaust from the previous connected cylinder creates a lower pressure in the exhaust pipes, helping pull the exhaust out which helps pull air/fuel in. A 4 into 1 has a tuned COLLECTOR pipe diameter (much larger than the primary pipes) and it's length is critical and the optimal dimensions varies with altitude and power. Since all four pipes are collected you have the potential for more savaging, but this requires all the primary tubes to be of the right length (about 37" for a Lyc) and all equal for all 4 cylinders. So the back cyl pipes require same extra curves to create more run than the pipes for the front two Cyl 1 & 2. This is the down side. The header or 4 into 1 is more complicated to make and fit.

A good collector (at least 2.25" to 2.5" I recall) is going to extend aft well past the firewall, about 19" to be optimal for overall performance. That is a chunk or pipe, and many builders cut the collector length down.... for looks mostly but compromising the full potential of the header.
 
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CAFE did many flight test articles on exhaust and electronic ignition published in EAA Sport Aviation magazine. If you are an EAA member you can go back in the archives and see all of CAFE's tests. It is well worth the time.

I read all these articles a long time ago but recall all references are per CAM timing not CRANK rotation.... The firing order for a Lyc is 1-3-2-4. Cyl 1&2 share the same intake cam lobe, or 180 degrees apart of cam timing (360 crank).


AGAIN...I quoted directly from the article:
it says "A crossover exhaust joins the headers of cylinders whose firings occur 180 crankshaft degrees apart. " which is incorrect.
 
Well I would say both. Making more power at altitude is more efficiency. Improved exhaust (4 into 1 headers with tuned equal length runners of proper length) is almost a free lunch (almost read below). It adds more power without much down side.

I was referring to fuel efficiency. Well designed exhaust will help pull more fuel into the cylinder. The extra power produced comes from additional fuel pulled into the cylinder. The extra power does not come without extra fuel, therefore not more efficient. Unlike, CR increases or proper ignition timing, which can produce more power from the same fuel charge and therefore are more efficient.

Larry
 
AGAIN...I quoted directly from the article:
it says "A crossover exhaust joins the headers of cylinders whose firings occur 180 crankshaft degrees apart. " which is incorrect.

"AGAIN" No need to yell. Good gosh. It's a typo... Did not say you are wrong did I? No. I said I read them long ago, and I confirmed it is cam degrees not crank degrees. No big deal SCSmith. Sorry to upset you. Have a nice day. :)
 
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I was referring to fuel efficiency. Well designed exhaust will help pull more fuel into the cylinder. The extra power produced comes from additional fuel pulled into the cylinder. The extra power does not come without extra fuel, therefore not more efficient. Unlike, CR increases or proper ignition timing, which can produce more power from the same fuel charge and therefore are more efficient.

Larry
That is what the little black and red knobs are for (blue if you have it) are for. Pull them back. You can go the same speed or miles across ground on less fuel with more efficient exhaust. Yes you can burn more fuel at high power if the induction (carb/FI) are not limiting.

I understand where you are coming from. Hot Rod power adders headers, bigger carb is to make more power to go faster and thus burning more fuel. However for example in an airplane going from Carb to FI is for efficiency. We run at 75% or less so we can lean. FI allows more even fuel distribution and better LOP operations than Carb. However a CARB can make as much power as a FI, just not as efficient. I do understand what you meant...
 
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