exhaust port size?

hotrod · Post by **hotrod** » Thu Nov 08, 2007 5:19 pm

Late response here to Moper, --- sorry I missed your question when originally posted.

What I'm asking is, the negative reflection..wouldnt that only be present at the exact spot of the size change, and not extend or travel back up the smaller tube? Seems that if gas temperature were removed as a factor (for discussion anyway) that the "depression" would be extremely localized, and very difficult to harness.. How far out am i now?

No like all waves it will move both ways. The negative pressure wave will run back up the trough (exhaust pipe) The original pressure pulse will move out into the larger body. The best analogy of this effect is the example I gave of two different size ropes tied together. If you do that experiment you will see two wave fronts moving away from the knot junction, one moves on down in the original direction and a second reflected wave goes back up the smaller rope toward the point of origin.

Exhausted:

I have considered doing the same thing in the past, sort of a stepped header with the first tube gently flaring out to the normal header tube size.
Some of the motor cycles are using tapered headers now and I wonder if that is the reason (increasing flow velocity). They have been discribed as doing it to maintain flow velocity as the boundary layer builds up inside the tube as it moves away from the exhaust port.

Are you willing to discuss approximately how long that smaller diameter section can/should be before it begins to have negative effects due to is internal drag?

Larry

exhausted · Post by **exhausted** » Thu Nov 08, 2007 6:27 pm

Larry,
If by motorcycle you mean PS in NHRA, that does not surprise me much. I have played with some of them over the years.The rpm range and small port size on the 4cyl's puts them in a weird place. They probably do not need to taper the entire tube, too small a taper angle, they only need to taper the first 4-6" of tube and then just find the right length and collector size from there. The problem they have is the headers are confined and the Harleys start looking really good from that point, eh?
The "blowdown length" of a header is about 7-10" (8-10k 2valve engine)from valve and a seriously overlooked reality in the engine tuning world!
Thanks

Ed-vancedEngines · Post by **Ed-vancedEngines** » Thu Nov 08, 2007 6:33 pm

Carl,
We verified the view count numbers of this thread. Don said it is an old thread that everytime it pops back up it gets a lot of hits. It has always had lots of hits as this subject is interesting to most all motorheads. lol

He also tells me this thread has been #1 on the Google Search for Exhuast Tubing Sizes and similar searches.

Ed

hotrod · Post by **hotrod** » Thu Nov 08, 2007 8:08 pm

If by motorcycle you mean PS in NHRA, that does not surprise me much.

No I have seen references to tapered exhaust pipes on multiple bikes, including production and racing superbikes. The problem is that it is a very subtle taper and is difficult to see in the illustrations.

http://www.totalmotorcycle.com/photos/2 ... X450Fc.jpg

The KX450F comes equipped with a tapered titanium exhaust pipe that expands in diameter from 38 to 41mm,
contributing to its broad powerband, and the silencer features new, long-fiber packing.

http://www.totalmotorcycle.com/photos/2 ... KX450F.htm

http://www.interbike.be/nieuwemodellen/ ... 6MY_KX.pdf

http://www.arcfabrication.com/ss.html
http://www.arcfabrication.com/images/DSC000152.JPG

http://www.motorcycle-usa.com/Photos/10 ... ail_03.jpg
http://www.motorcycle-usa.com/Article_P ... 042&Page=1

http://www.mototuneusa.com/kawasaki_sup ... clutch.htm

Notice the welded aluminum gas tank and the tapered exhaust design. The Suter dry clutch is a beautiful piece of work. This is one trick race bike !

http://www.mototuneusa.com/sweden.htm

http://www.bikeland.org/story.php?storyID=26564

At the moment, I cannot find the illustration I am thinking of, but the above will give an idea. Instead of a conventional megaphone at the end of the pipe to produce traditional tuning effects, they are applying a small taper over the first 6" - 12" of the exhaust header as it comes out of the port. As mentioned in one of the quotes above a typical amount of tape is 38 mm to 41 mm = 3 mm or about 1/8 inch increase in tubing diameter in that first part of the header.

The first time I saw it discussed from a technical point of view the text mentioned that the taper compensated for a build up of boundary layer inside the pipe as the exhaust flows away from the head. There is essentially no boundary layer just outside the port due to the short distance of travel and very high velocity, then as the exhaust moves down the pipe it (was asserted) that it slowly chokes its flow with a thicker and thicker boundary layer so that the effective diameter of the pipe is actually smaller than the physical ID of the pipe when you get several diameters down the header.

I have no way of knowing if that was speculation or informed analysis but it seemed to be a reasonable working theory that happens to merge in well with the topic of this thread.

It also implied that a stepped header accomplished the same effect in a slighly less elegant way, but was much easier to fabricate.

Larry

Post by **wfolarry** » Thu Nov 08, 2007 8:18 pm

exhausted have you done any work with the Harleys? They have had a 1-3/4" pipe for as long as I can remember. There are some step versions out there but I've never seen a smaller one. The reason I ask is because in another post you mentioned smaller pipes for heads that had ports too large. I work on Harleys [motor & headwork] & one of the toughest things in a performance build is getting the exhaust right. It's more about looks than power for a lot of people. I have used torque cones in the past to help out some bad systems but the exhaust systems you buy now are a lot better. I still think there is more to be had in tuning the exhaust system. Some guys are experimenting with the Supertrapp muffler on their 2 into 1 pipes with good results. But that pipe sizing you mentioned has got me thinking.

exhausted · Post by **exhausted** » Sat Nov 10, 2007 7:59 am

Hot Rod;
Wow, thats a lot of references! Thanks! It will take me some time to go over them. I want to point something out. Motorcycles are a whole different world to me because they are almost all an even fire engine. People who work in that world I think see a lot of things different than us (uneven firing engines.) I admit I have not spent enough time there and would like to because they see different things in their systems then we do. I have done some 4cyl auto engines but not in conjunction with anyone seriously looking at it. I see them being a lot more sensitive to exhaust. It is easier to miss?
Then go and throw in 4 valves per cylinder and independent runners on the intake and things get real sensitive. I have worked with 4 valve V8's with a bent crank, and the flow characteristics change everything. I am not saying we can not learn from motorcycles, it is just that crossing from one to another is difficult. A taper is a small megaphone. It also would attenuate the pressure wave sending back mini negative waves during the blowdown while still helping with this boundary layer issue. I would also decidedly say that a system needs to get larger the longer it gets, but there are limits. Steps do that just fine, but I agree that a taper close to the head is needed, from what size to what size is the tricky part.

exhausted · Post by **exhausted** » Sat Nov 10, 2007 8:09 am

WFOLarry;
I have done some chopper pipes and know some people deeply involved but other than that I only have opinion. If you keep the tubes separate you just have to go to all the books on single cylinder engines exhaust theory and go from there. You can get the system to help for a certain power band and that is going to be it! The pipe stays empty for 2 crank revolutions, tuff to deal with. probably why diffusers can offer some help,
I really do not know.
If you bring them together, you can definitely broaden your power curve over single tubes. the firing timing is funny but properly sized primaries and venturi collector can make a real improvement.

larrycavan · Post by **larrycavan** » Sat Nov 10, 2007 4:53 pm

Rob Muzzy used various headers on Eddie's Superbike back in the early 80s. One such system was made up of individually welded sections that tapered all the way from the port to the end of the megaphone. As I recall, Rob experimented with different header designs that included the tapered header and another setup with a collector that had the 4 pipes talking to each other rather than just entering straight in. Each variation had it's strengths. The benefit came from applying the correct header to the correct track in order to maximize the bike's performance for a given application.

It appears as though the switch to 4 strokes in Moto X has led to revisiting tricks from the past in order to broaden the power.

Seems to me that most of what I see is a new version of an old trick that's been hauled back out of the archives and reapplied.

What I'd like to see is a formula for calculating the required taper for a given application.

I can totallly appreciate the efforts a header designer puts into their work. They optimize an exhaust system for an application and then someone changes part of the engine combo....doesn't the exhaust system need to be revisited to optimize it once again for the new combo?

JMO

Larry C

exhausted · Post by **exhausted** » Sat Nov 10, 2007 8:01 pm

larrycavan:
Tapering a exhaust pipe has a effect on both the pressure waves and the flow of particulates. They have been used everywhere in exhaust systems at one time or another.
Keeping with this thread which pertains to the very first part of a system, I think I stated before that a tapering port might have some advantages in certain apps. but I would be very concerned about getting too large too soon. The motorcycle stuff mentioned earlier are using a very gradual taper and while difficult to manufacture could well offer something.
When a motor is cammed for a given intake and exhaust system and then a change is made, it is always appropriate to go back and recheck headers and intake adjustments to make sure things are the way they should be. Exhaust loops in Cup usually involve changing secondary lengths on headers,(421's). If they notice more of a change then normal it might draw attention and they might try a diiferent collector choke or header. The same for intake, spacers and whole manifolds, they have hundreds!
"The one's that throw the most parts at a motor in the least amount of time are the winners"! Stumble technology rules!

williamsmotowerx · Post by **williamsmotowerx** » Sat Nov 10, 2007 8:05 pm

i developed a true tapered header for moto x applications. i was able to get away with a smaller than normal start, 1.25" then taper it to 1.625" over 15". i have built it both ways... cone sections like a 2strk pipe for R&D and a true tapered tube for production, including tapering through the bends.

all of the other manufactuers and oems that claim a tapered exhaust just have short pieces of a taper welded in between their respected bends. not a true taper.

exhausted · Post by **exhausted** » Sat Nov 10, 2007 8:31 pm

HotRod
Just got to look at the links about the bike stuff. A couple of points.
Single cylinder engines are much easier to quantify these minute attention to detail.
I also think that (I think I mentioned before) a multi cylinder even pulsing motor will also be more sensitive to minute changes.
I am somewhat aware of bike systems and I look at them longingly, wishing bent crank v-8s were so simple.
I hope that some day I can employ a small taper in the high velocity part of the headers, for now I know a tube step (which is 1/8") is necessary every 10-12" and we see an effect. I know that tapered headers do not run any better than a stepped header in any current 2 valve US engine! But maybe some day we can justify the effort!
Thanks for the links!

exhausted · Post by **exhausted** » Sat Nov 10, 2007 8:49 pm

williamsmotowerx wrote:i developed a true tapered header for moto x applications. i was able to get away with a smaller than normal start, 1.25" then taper it to 1.625" over 15". i have built it both ways... cone sections like a 2strk pipe for R&D and a true tapered tube for production, including tapering through the bends.

all of the other manufactuers and oems that claim a tapered exhaust just have short pieces of a taper welded in between their respected bends. not a true taper.

Wow!, once again a single cylinder but the principle is obviously valid.
The part I like best is "smaller than normal start". This is what I am trying to point out in this thread, our ports start out too large!
We can taper headers through the bends, it is just a lot of work and can not be seen in a 1400hp PS engine.

hotrod · Post by **hotrod** » Sat Nov 10, 2007 10:11 pm

The Motorcycle guys have a huge advantage in the cost of parts to try things. For the same effort a V-8 has to expend to try one concept or improvement, the motorcycle guy can try 2-4-8 different things depending on how many cylinders he is playing with.

It would be interesting to build up a one lung engine which you could play with to develop pure concepts for things like combustion chamber shape, piston crown, ring packages, honing finish, rod length and cam timing etc.

There would obviously be some areas where the multiple cylinders in a real engine interact like in the intake manifold and in some cases the exhaust manifold but a single lung engine would let your try a boat load more "tweaks" like piston design etc.

Back in the 1940's when NACA was doing fundimental research on air craft engines they frequently did exactly that building up a single jug engine to play with.

For a V8 setting up 2 cylinders 180 deg out and running 6 dead cylinders would cut the development costs for one off parts by a factor of 4.

Larry

exhausted · Post by **exhausted** » Sat Nov 10, 2007 10:24 pm

hotrod wrote: There would obviously be some areas where the multiple cylinders in a real engine interact like in the intake manifold and in some cases the exhaust manifold...

Larry

You are absolutely right, but when do multiple cylinders not interact in the manifold?

williamsmotowerx · Post by **williamsmotowerx** » Sat Nov 10, 2007 10:43 pm

it just doesn't work like that hotrod... yamaha had a straight four cyl 2 stroke awhile back... basically 2-twins bolted together. we did our initial testing on the twin and then tried it on the four... just didn't add up.