Exhaust specs for midrange

I talked with MMM for a bit yesterday while ordering new chain, sprockets, and some other bits and pieces. I always enjoy ordering from 650 Central because I end up learning so much during the process.

I am currently running stock 17/34 with just the headpipes. I mentioned to MMM that I was about to build the exhaust. He advised designing a 38-40" long dual pipe exhaust out of 1 1/2" pipe, necked down to 1 3/8" after the bends in order to maximize the midrange for streetability with 18/30 or 18/32 sprockets.

My pipes will likely follow the frame in a similar to stock configuration. Is anyone running something similar? Do you have pics? I'd like to be able to visualize a 40" pipe and I am away from my bike right now.

As another person that is about to build new pipes, has weak midrange, and wants optimal in-city streetability, I am extremely interested in this discussion.

Also interested in hearing other opinions.

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Bear in mind, I am still learning here, and am just regurgitating what I've read. Check this out : http://www.physicsforums.com/showthread.php?t=594484

The last post is very interesting to me. In automotive race exhaust tuning, there are claims of up to 20% peak hp increase by creating a suction wave to assist in scavenging exhaust gases from the exhaust port, and depending on cam characteristics, the cylinder. However, for streetability, we don't want a "peaky" powerband. We do, however, want a decent amount of power in a lower, broader powerband.

So, for maximum midrange, can we take advantage of a suction wave design (increasing diameter in the exhaust at some point by no more than 2:1) and tune to the desired range by varying the length of the primary pipes? This would, of course, change the timing of that suction wave.

Obviously, a truly tuned exhaust would be built to match cylinder head flow characteristics and valve timing. For most of us, those factors are unaltered from stock... For now.

The old "1.5 in by 32 in long" formula that I believe was from the original Minton mods list is probably great for a drag bike or bragging rights for peak hp numbers. However, for those of us who ride more up and down and round - about than fast and straight and level, what would be ideal? There must be some formula to plug in exhaust port size and find ideal primary I.D. From there, it seems powerband tuning would include a combination of primary length prior to expansion or reduction (causing reflected suction or pressure waves respectively). Is this correct? If so, is this due to changes in the pulse velocity based on rpm? If that is the case, would the formula then include figuring the length of time it takes for the exhaust pulse to travel down the pipe and return to the exhaust port, then changing that length based on target rpm?

Heck, I don't even know what I'm saying at this point. Hopefully someone who knows what they are talking about will come along and correct me!

MMM is right, of course. Diameter impacts torque peak--the fatter the pipe the higher it moves up the rpm range. Length impacts power peak--shorter pipe moves it higher up the range. This is not a matter of opinion.

Another thing to consider is that a straight pipe narrows the power band. Best street performance will be achieved with a free-flowing straight-through glass pack muffler. Your bike can look bad or it can be bad, it's your choice. If you need proof that noise and horsepower aren't closely related, take a ride on a straight-piped Hardly Ableson, then twist the throttle on a whisper-quiet Suzuki SV650 or Gladius 650 twin; just make sure you have a good grip on the bars before you hammer the Zook.

For a look at what professional race tuners with big bucks to play with used on the XS twin, buy yourself a copy of Craig Weeks' performance manual; just click the link at 650 Central. In the real world, tuners match length and diameter to the track--longer, smaller pipes for TT and short tracks, fatter, shorter pipes for mile and fast road race tracks. If you want to try to get smarter than guys like Axtel and Lillie, hey, have at it!

MMM shared a trick with me a few years back. Exhaust length is specified as the center line length from the exhaust valve face to the outlet of the pipe--you measure the pipe length inside the bends, the length outside the bends, average the numbers, and add the distance from valve face to pipe inlet. What Michael found is that in the XS650, the result will be so close to the outside line of the pipe that you can use that number and skip the rest.

Hey, Firerunner, this is always a fun topic. Friend of my father tuned pipe organs, gave me a lesson or two on this fascinating world. Wish I could time-transport you back to the `70s bike shops when this was bantered around, and odd designs surfaced. One day at the strip, a fellow showed up on a bike with, for lack of a better term, a 'trombone' exhaust. Somehow, he was able to lengthen/shorten the pipes on demand while riding. Never heard anything about it since. Here's a thread that may interest you:

http://www.xs650.com/forum/showthread.php?t=27223

Also try forum search 'reversion' for more fun...

Perfect! I knew someone with experience would show up soon! Michael is of course correct, I'm not doubting that a bit! What I'm trying to do is understand the physics behind it, to learn a bit.

I too wish I could be a fly on the wall of the old bike shops, but us young guys will have to learn from you (ahem... Old) guys who were there.

So:
1- larger diameter moves tq peak up
2- shorter moves powerband up
3- straight pipe narrows the powerband

What effect does muffler placement (assume free-flowing straight-through glasspack) have?

I'm certainly not aspiring to reinvent anything, but I'd love to learn as much as I can of what Axtell, Lilley and the like were able to discover and pioneer.

So, the question remains, why? What is happening to create the above effects?

Hoo-boy... This would be like trying to explain the rise and fall of the Roman Empire in 5 words or less. I wish I could explain it as I see it, but good foundation in wave theory, thermodynamics, properties of the atmosphere and gases, wind (musical) instruments, aerodynamics (both compressable and non-compressable flow), reynolds numbers, sound and vibration, would help bring things in focus. Then, there's older difficult to find books and articles on racing engines, flow work, exhaust design, 2-stroke expansion chamber design, camshaft profiles, some of which were coveted secrets, others with solid theoretical basis, needing experimental confirmation, all buried in a quagmire of street talk and urban myths. Then, there's the modern publications, which put much of this fantasyland in it's place, and establishes a better methodology, but uses terms that sometimes rely on knowledge of the older work, or obscure relationships that send you off on a tangent to research them instead, like stroke/rod ratios, and what's that got to do with exhaust design.

Forums of the future may eventually include a 'vulcan-mind-meld' button, which could really help here, but until then, all I can say is get a library card from your local university, pitch a tent in the science/engineering section, and suck the life outta them books. That's what I had to do...

That gearing recomendation is questionable unless you're a 100 pound jockey or something. 18/30 will give you the slowest accelerating bike possible. That's the long haul, accelerate once then cruise for two hours gearing. If you're looking for around town acceleration and punch, the stock gearing is fine.

I think a lot of people drop the gearing when they get one of these because they're not used to the vibration and it makes them nervous, but that's how the thing is designed and how it's meant to run. Creeping around at low rpm and shifting at 4k cause you're scared of the vibes just turns a 43 horsepower bike into a 23 horsepower one. Cruising at 5k isn't hurting it. If this thing was an airplane it would be expected to cruise at 6k (using common rules of thumb for mean piston speed)

YES, GreasyC!!!! I usually don't "yell," but this is a bit of sanity I've been abused more than once for offering and I'm grateful for the company. Gents, you will do more damage overgearing and lugging these motors than you will cruising them at Interstate speeds with stock or near-stock gearing. Those who don't like the vibration should for crying out loud go buy something else!

The exhaust while a simple device, accomplishes a difficult task, and it's one that still isn't fully understood due to the dynamic nature of the internal combustion engine.

What you're doing by adjusting length is changing the timing of a negative pressure wave at the exhaust valve. This pressure wave causes the cylinder pressure to decrease even more than it is already, which draws in more air/fuel. This is partly the reason why you can have volumetric efficiencies higher than 100%.

The more valve overlap you have, the more critical and harder it is to get this right. Bikes with stock cams and little overlap are less sensitive to this. This should be fairly intuitive to understand.

While diameter has some effect on the tune length, it's more a velocity issue. The exhaust pipe is an extension of the exhaust port.To paraphrase a NASCAR and NHRA header designer/builder, "isn't it convenient that the port wants to step up in diameter where the exhaust port ends, always?"... he was being sarcastic .

You can think of it in terms of blowdown. In primary exhaust pipe at high rpm, the exhaust gases are in the pipe while the valve is still open. To preserve velocity and get those exhaust gases moving out as fast as they can, it is common for modern header designers to use a first primary diameter the port size or "up to 10% smaller". It is very rare they use a pipe larger than the port diameter.

This is why you often see steps on stepped headers in the 10" range from the port flange (dependent on port length)... this corresponds with where the blowdown length ends within the header.

By preserving this velocity, you're removing most of the exhaust gases before the piston reaches BDC, so during the exhaust stroke, you've reduced pumping losses.

Granted this can work too well and you have what they call over-scavenge. This is where intake air/fuel is sucked out the exhaust valve during overlap. To get around this, top engine builders either use a cam with less exhaust duration, or they shrink the exhaust valve (which in cylinder heads with inline valves, lets them use a larger intake valve).

Steps, best I can find from both academic and the top builders in the field, are more of a velocity thing than having to do with wave refraction. That being said, any quick change in diameter is going to cause a return pressure wave. This can bee seen using exhaust pressure analyzers, which some have been kind enough to share elsewhere on the internet. I have access to modeling software that shows you this as well.

People have to realize that the exhaust design is a compromise, as is intake length. It's only going to be in tune for a certain rpm range. This isn't an issue for race bikes as their range is fixed. On a street engine, going in and out of tune, there are going to be flat spots in the power curve, this is especially noticeable the more aggressive your cam is. Certain designs do a better job of eliminating these flat spots... a 2-1 with the stock crank phasing should see improvements. The Harley guys have made this very clear and there is good demonstrations of this online elsewhere. On a twingle type engine, it's going to have to be a 2-2 as everything would be arriving in the collector at the same time.

If you use PipeMax, or pay Burns to run it through their program, or use a high end modeling program like EngMod4t, you're going to find the first recommended primary diameter for a XS motor is going to be 1.5" od and less. Length will vary, but you're going to be looking for something about 50" in length from the valve seat to atmosphere for a 2-2 set-up on a stock-ish motor.

Depending on how big the exhaust port is, that will dictate what you run. I'd find the pipe that matches your exhaust port, step up about 12" from it, and then carry it out to the above mentioned length.

My friends are car racers and that's the world I grew up in. 1.75"od pipes are WAY too large for any XS motor. Even 450cc single cylinders are using 1.75" pipes as the initial diameter and they're revving over 10,000rpm and making 70hp (per cylinder). NASCAR is using the same primary size as 40 years ago and making double the hp. The trend is smaller primaries and getting everything right. I know of someone who modified a Jemco on a different application and saw good things come from it.

Another to keep in mind, and this is important when using the smaller primaries. The larger the bend radius the less flow loss. Tight bends would require a larger pipe diameter. This is why if you look at a real high end headers on a car, you'll notice they make a lot of effort to use 6" bend radii. Look at the big bends on the Harley XR-750 flat track bikes.

Last important thing. Carry the pipe out the cylinder head at the same angle the port leaves the cylinder head, the centerlines of the port and header should match. This is something the professionals take great care in doing.

This sums up some of which I've read and collected over the years from top level professionals and academic texts.

Hope this helps and excuse any grammatical errors... I quickly typed this up and haven't looked over any of it.

Cheers,
Bob

Simmer down there, gentlemen. There are a lot of factors at play here.

Sure, the stock XS was and is a great machine, but even Yamaha sought out improvements. Brakes changed, TCI replaced points, clutches went through model year changes.

We spoke earlier of Axtell and Lillie. They didn't leave well enough alone. They changed things. Thanks to a lot of guys in this community such as Punkskaller, Pamco Pete, MrRiggs, and the sadly departed XSJohn, we are still developing what can be done with these engines.

There is a level of irreducible complexity in a build formula. One must think not only of gearing, but of what exhaust for what gearing with what size tires, and what ignition settings with what compression and what carbs and intakes and last, but not least, what purpose! Let us not be simplistic. Everything works together. What we are talking about is moving that precious powerband down into a more streetable range which will allow the new gearing to allow transmission ratios to be broadened, and allowing for engine temps to drop, gas mileage to increase, and hopefully engine life to be extended.

We all know the stock powerband is high... But is that the best option? Is there room for improvement in this old 1950's german technology engine?

Hoffman9000, thank you. That is the type of discussion I'm after.

Firerunner,

It's a good topic

Here is the set-up on my bike. It's a little on the longer side, but I need it to clear the kick starter. I gave away filling in a 3500rpm hole and making more on the overrev by running it this long, but according to the sims, it picks up a little more in the 4000-6500rpm range over a 50" pipe (which is where the math puts it at).

Without the muffler:

On the bike with the muffler:


Megaphones are a totally different animal. Note: a megaphone muffler doesn't tune like a megaphone, the baffling messes with the wave. To really get a megaphone to tune you need a set-up like you see in my pictures. Look at real high end motorcycle race systems and you'll that's how they do it. A set-up like that makes some really cool pressure pulses behind it, kind of like those air cannon guns you can buy. You can feel air from 8' back.

This is a well written article on exhausts:
http://victorylibrary.com/brit/mega-c.htm

IMO, it's the best write-up online without having to dig through many different sources and sifting through it.

This is an important quote from that article and easy to over look:

Only engines with sufficient overlap built in to their cam design can benefit from this form of tuning. The overlap threshold (mimimum) will vary with combustion chamber design, compression ratio, valve sizes, peak power RPM, port efficiency, &c. If the cam is too mild (has a brief overlap period) this will not work, and the most efficient exhaust system is the biggest valve and the biggest pipe you can fit, since the resultant reduced pumping loss is the only remaining benefit.

Click to expand...

Basically, with little overlap, you gain little from tuned exhaust length. This means the exhaust is being mostly pumped out, and in doing so you need to minimize pumping losses which means bigger exhaust valves, etc. . With more overlap and carefully designed exhaust system, you remove most of the exhaust gases before the exhaust stroke, so the exhaust valve can be smaller, etc.