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Has anybody ever built their own exhaust? I was thinking about building one for an XT350. I have access to benders and am a pretty good fabricator. I plan on just buying a muffler probably an FMF and building the rest. I was kinda wondering what i should keep in mind concerning bend, size and length of the header.

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Do you have a stock pipe available to measure? I would start with about 10% diameter increase. Generally speaking, longer header-to-muffler pipe moves the power band toward low end, shorter pipe moves it higher. Again, I would not make a change (in either direction) greater than 10% to start.

You're probably going to be stuck with most of the OEM design, simply due to mounting requirements. Not much room on a motorcycle to re-route the exhaust.

Finally, the best way to objectively see the results of your work is on a dyno. You can try acceleration testing if you have a way to accurately record speed/time, but that still can be affected by the operator. Seat of the pants testing is notoriously unreliable.

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No I don't have a mandrel bender. I was gonna build few designs with a hand bender and once I got one that fit nice I'll take it to a shop and have them duplicate it with a mandrel bender. Thanks for the info I'll give it a shot and see what happens.

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i just did my own and you will be surprised at how easy it actually is.... good luck and feel free to ask any questions as i am subscribed.

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I built two identical systems back in the early '90s. One of them was for an '85 XR350 w/single carb, mild cam, mild compression increase w/+1mm hc piston, ported head and 36mm RS-later 41mm FS carb. The Honda responded with a great Strong power band from just off idle all the way through redline. His riding buddy was envious because he was getting left behind big time on his TT350 Yamaha... I had had the specs on the Honda and we tore that engine down and rebuilt it. I didn't have ANY specs on the Yamaha and we didn't have a local Yamaha dealer at that time to get the info from. The web wasn't real helpful so I figured that I would duplicate the design that I used on the Honda for the Yamaha... Not!! I made the pipes fit but the result was different. Understand that the Yamaha owner who was the Brother-In-Law of the Honda owner did not want to open up his engine. So no cam or porting and stock carbs. Basically just added the pipe.

Well the result was that the Yamaha became a Torque Monster but felt like it flattened out on the top end. This feeling may have been because the bottom end came on so strong, but it did not have the top end that the Honda did.

After we were all done and the owner was really happy with the results I managed to find the engine specs for the TT350. The bore and stroke was fine, but the valve sizes and carbs were basically the same sizes as a stock Honda XR250. In other words, they were small compared to the 350 Honda. This accounted for the lack of a strong top end pull.

I have just one photo of the Yamaha system. It is a bit unconventional as it used a megaphone that started next to the front of the cylinder and ran back from there. I used short equal length 1 1/4" head pipes, 1 3/4" secondary pipe and a meg that built to 3" in diameter. It was a challenge to squeeze it into the frame but it worked. Had a 1 7/8" outlet on the meg and a 2" muffler core in a 3" diameter muffler body with bored Supertrapp plates on the end. I will add a couple of pics of the Honda system to better show how it was fitted up.

YamahaTT350Megaphone.jpg

XR-Ken-Meg.jpg

XR-Ken-Meg2.jpg

All of it was built up with mandrel U-bends and I hand rolled the megaphone cone and reverse cone. Hope that this helps.

Swiss

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Figured that this might help a bit also. When you make up your primary pipes you need to make a collar that will allow you to mount the pipes to the cylinder head at the exhaust port. I didn't used to have a lathe to turn steel collars down and a pain to braze the thick collars to the headpipe so I came up with another simple way to make it work. I took some of these diameter adapters that are available at many Auto shops and cut them down to use as collars on the head pipe. If you get them to be a slip fit to the headpipe with the smaller diameter than you have the larger diameter that expands out about like a bellmouth does for a carb. Cut the larger diameter/cone at the size that you need to fit your flat mounting flange. Weld the small end to your headpipe where it will meet the copper pipe gasket and then lightly hit it with a belt sander to smooth the weld down, allowing for a good sealing surface.

Tubesizeadapter.jpg

With the piece in the photo you would cut the large end just about where the label starts and then adjust the length of the smaller diameter to fit between your head and the mounting flange. Make sure that you put the mounting flange on the headpipe before you weld the two pieces together if you are doing a twin-port header because you won't get it on after you have fitted the adapter.

Swissdecalpatch3sm.jpg

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Great advice and workmanship. In the early 70s we built expansion chambers for our Yamaha Enduros using info from Gordon Jennings book on two stroke performance, and they worked great but looked like deranged elves made them in storm damaged trees. We had access to a sheet metal shop and welding equipment. We were probably the most enthusiastic nit wits to be found anywhere. But the engines made a drastic improvement in power. Loud, too. They later vibration cracked and fell apart, so we went out and bought Bassanis, I believe. They didn't fall apart, and worked as good and actually fit.

I suggest that one is fabbed up in mild steel, try it, modify it, etc, then get a copy built of stainless of the design that works.

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In the early 70s we built expansion chambers for our Yamaha Enduros using info from Gordon Jennings book on two stroke performance, and they worked great but looked like deranged elves made them in storm damaged trees. ....They later vibration cracked and fell apart, so we went out and bought Bassanis, I believe.
Hmm, the pipes I built for my CZ's didn't look that bad, nor fall apart like that. :smirk: :smirk:
In relation to ex valve size is there a general ratio to the size of header diameter and muffler core diameter?
Yes, sort of. The general rule of thumb is that the header should be around 100-110% the size of the exhaust valve, but that was for engines with a single ex valve. For multi-valve engines, calculate the area of one valve, and multiply by the number of valves. Then find the diameter of the circle that would have that area. For example, by that a YZ450 with two 1.1" valves should have a header diameter of 1.6" or so.

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General rules of thumb are again "general". My thumb might be bigger than yours or longer/shorter etc... They are a good starting point. When you start comparing 2-v engines which many of the older tuning formulas were based on with the newer 4-v engines which came into popularity starting with the old early '70s Honda XL250/350 you find that they flow more air/make more power (generally the early comparisons were about 15% or more for similar sized engines/bore diameters).. I remember reading that the “Tuners” of the early ‘70s era determined that the Honda XL250 head flowed too MUCH air so that it only worked well with big bore cylinders and stroker cranks. HA!HA! A few years later Powroll built their “Flattrack” XL250 Race engine and pulled about 32hp from 250cc at 13,600rpms. They used a the bigger XL350 valves with a 38mm Mikuni carb and a 1 5/8” diameter 19” long pipe with a 3” Supertrapp and I think it was too small. This was in about ’77… Several Years back I gave the Powroll specs to an English Vintage roadracer and he built a copy and I recommended a shorter 1 ¾” pipe with a megaphone and he told me it ran really strong. He told me that he also got lots of comments from the “Old Timers” about how badly his little engine would run with that big pipe and the huge 38mm carb… So you have to add some additional space/diameter for that air flow to work well. Also you have to look at the newer engines and how many rpms they turn to. 250s that are turning up to 13,000rpms +, 450s that are turning around 11,000rpms +.

All of that thanks to the newer water-cooled engines that are running very short strokes to keep the piston speeds reasonable regardless of very high rpms. Years ago they did lots of dyno testing and found that the old 2-v TT500 engines liked 1 5/8-1/3/4" diameter exhaust pipes depending on how radical you built the engines... Well you can be sure that one of the modern 450cc MX bikes with the 4-v heads will flow a lot more air than the old TT500 head did. The new bikes also make more power than the old 500cc engine did even with the performance mods to the old engine.

Some ideas and theories of mine, you may or may not agree, that is your decision to make...

One of the thoughts in the past and still followed to this day is that you need small diameter exhaust pipes to maintain high velocity in the exhaust system. Now, this was not specifically to enable the cylinder to empty itself out the exhaust port before the next engine stroke started to intake fresh fuel/air mix, it was to put enough speed and force into the exhaust flow that the returning tuning pulse would not easily be able to push the exhaust back into the cylinder during the valve overlap period. Anti-Reversion designs starting with stepped port exit diameters where the port was smaller than the exhaust pipe ID and working through Jim Feuling's A-R Cone designs and up to the now popular "Chamber" designs like the new Yamaha 450, FMF Power-bomb/Mega-bomb and aftermarket chambers function as effective A-R designs. They provide an interference pulses in the pipe close to the port/cylinder that disrupts/blocks the strong returning sonic tuning pulse.

So what are you left with? Well, if you use any of the forms of A-R then you have a long tube for the exhaust gasses to flow down. The smaller the tube is, the higher the restriction against the flow. The longer the tube is, the higher the restriction against the flow. If the A-R disrupts the tuning in the wrong point along the pipe length, then you also lose any Pulse-Tuning advantage that you might build the system for. You effectively then just have that long tube again. What does happen with the various chambers is that it negates the Bad returning sonic pulses, and it negates the good ones also. The result is the same basic exhaust gas flow that you would have had with the long straight pipe but without any rpms where it specifically boosts or drops performance, which it normally would have. This typically results in a net GAIN in performance, even if it is only a little bit because there typically is more powerband negatively affected by pulsing/Reversion than is positively affected by it.

So you show some low and mid-range boost and it feels good! Please note that it is easier to physically "feel" low and mid-range boost because it is in the controllable portion of the powerband. High rpm performance is harder to find because most of us don't ride in that range as much, and it may be lost in added wheelspin etc. and you hardly notice it unless you are looking at the dyno sheet or head to head with another equal rider on a similar bike without the A-R mods...

Now, getting back to the diameter ideas... If a small diameter is restrictive (which it is) then why if we are now controlling the Reversion with Power Chambers/Bombs etc., do we need to keep the velocity high and the restriction high? Can't we just use a bigger pipe and get the exhaust gasses out of the cylinder and moving along the pipe? The A-R will control that nasty Reversion so it doesn't hurt the powerband! If we place the A-R at the head/port then we can even go back to using the length of the pipe for Pulse Tuning? Which will give us some additional boost for either our peak power or peak torque rpm!

Years ago I stopped by Powroll and spoke with Paul Olmstead who was the owner and founder of Powroll. I had built my Honda XL350/403 and was talking with him about having a strong powerband but it was flat on the top end. He looked at my bike and told me that my pipe was too long. I needed the pipe to be between 34”-36” for a tuned length. Now he also mentioned that the typical 3” diameter Supertrapp that we were using back then counted as part of the tuned length, so you tuned to the center of the plates or if using a muffler to the outlet of the muffler. So when you took about 9” off the 34”-36” pipe for the Trapp and another 2.5” off for the port length (part of the tuned length also) you got a pipe that was only 22.5”-24.5” long. I mentioned to him that the pipe that he sold was several inches longer than that and his reply was that, “We have to sell what the customer wants!”… The “standard” placement for the muffler was behind the shock absorber and that is where everyone wanted to see it if they were going to spend their hard earned cash on an aftermarket pipe/muffler assembly. Note: the standard core diameters for the 4-stroker Trapp pipes was a choice of 1 5/8” or 1 ¾” which was normally the same as the pipe diameters on the bigger bikes. That is why the length of the Trapp was considered part of the pipe length. Newer mufflers (especially custom units) often used larger cores up to 2”/2 ½” in diameter so there was a strong tuning point from the diameter change.

One thing that smaller pipes can do well is that they can be made to fit easier through small areas between frames and engine parts and air boxes etc…

Step pipes where the diameters are increased along the primary pipe length act in two ways. They function similar to the megaphone in that they create a tuning pulse where each step occurs. The lengths between the steps will determine the rpms that they affect. The secondary effect of the steps works like an A-R design in that each step offers some resistance to the returning pulse that is heading towards the exhaust valve and cylinder. So they can be effective if their diameters and lengths are right for the engine tune.

Long primary pipes can be tuned for the same rpm that a shorter head pipe can be designed for but with a lesser effect. The longer pipe tunes at a different harmonic frequency of the pipe and for each higher numbered harmonic the strength of the pulse is less and its benefit is also less.

Swiss

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Yes, sort of. The general rule of thumb is that the header should be around 100-110% the size of the exhaust valve, but that was for engines with a single ex valve. For multi-valve engines, calculate the area of one valve, and multiply by the number of valves. Then find the diameter of the circle that would have that area. For example, by that a YZ450 with two 1.1" valves should have a header diameter of 1.6" or so.

Would I be correct to assume this rule only applies to a multivalve cylinder that has both exhaust's going into a single header? And if it is a multivalve where each exhaust valve has its own header you would keep the header diameter to the 100-110% ratio of its respective valve?

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That's a good starting point. You can, of course, see what size the original pipes were by checking the size of the port exits.

When joining the two into one, go by the previous equivalent area concept, and if you're planning on using pulse tuning, try to keep both primary tubes the same length.

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When I talked with Pete (owner) from Powroll in the late '80s he said that if you run a 2-1 system with a 4-v head that the quicker you can get the primary tubes together the better the throttle response. Of course, Honda and others have used different length primary pipes on many of their bikes and some people say that using the different primary pipes widens the powerband by tuning the different exhaust valves for different rpms. That may or may not be valid, and if tuning for separate rpms for each exhaust valve, peak power will not be as high. Never have seen valid Dyno comparisons showing the validity of the different length idea.

I wouldn't place any validity on using the port area of a stock exhaust port for selecting exhaust pipe diameter. First of all, the stock ports are probably undersized to give good low end performance, and secondly you will benefit from having a "step" at the port to pipe junction that will work as Anti-Reversion so your pipe should be larger than your port. If you have a super mild cam with practically no overlap period then the reversion won't be a problem but most modern and even engines back into the '60s used cam overlap in almost all engines except maybe Trials bike engines. Honda engines are well known for their small primary pipes. I built some XR630 pipes for Baja racing back in the early '90s and used 1 1/2" primary pipes with a 2" collector. The stock primary pipes were 1 1/8". The engines ran strong and pulled hard right off the bottom end and through redline. The riders complaint, they were ripping the knobs off the back tires.

Exh-Hon-600.jpg

This photo shows the megaphone starting right at the back of the cylinder. Yes the frames were modified to fit the megaphone.

Swiss

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... you will benefit from having a "step" at the port to pipe junction that will work as Anti-Reversion so your pipe should be larger than your port.
...Assuming one buys into the whole "anti-reversion" philosophy, perhaps. Please note first that no attempt has been made by me to recommend any specific size header tubing, but rather to find a starting point for the first of what would likely be several experimental pipes.

"Anti-reversion" measures have some merit, not in increasing peak power, since they actually interfere with that, but in broadening the torque curve. The theories tend to be somewhat more useful on silencer equipped pipe than open exhausts because of the negative effects of the muffler on pressure wave reversion, but they can certainly be overdone. As I mentioned before, a muffler using the Helmholtz principal allows the use of harmonic tuning just as if the pipe were open to the atmosphere at the end.

As to the use of relatively larger header tubes, even with a muffler, there is such a thing as too big, and a practical example can be found in the '08-'09 YZ450F. That bike was originally produced with an short muffler that everyone liked the looks of. Trouble was that it was terrifically restrictive, and as a compensation, Yamaha used a considerably larger diameter header on the '08 than on either the '06 or '07 (both different exhaust designs). The cumulative effect was a pronounced dip in the power curve centered at 5000 RPM. Using almost any other muffler, including the stock '06 or '07 units, on an '08 with the '08 header produces an immediate gain of anywhere from 4-8 hp at 5K with no other modifications. However, switching to the smaller diameter '06 header, the stepped '07 header, or most any header from the aftermarket will add another 1-1.5 hp or more to the peak numbers over and above the gain from changing the muffler.

A footnote: Back when Hondas were no larger than 350cc twins, all their bikes were built with double-walled exhaust headers. They explained that there were two reasons for that. One was that it kept the chrome looking better for longer, and the other was that they felt that American buyers would be put off by the use of an exhaust pipe as small as the real correct size would have been.

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I guess that I am a Jim Feuling convert and bought into the A-R idea back in the mid/late-'80s. When he was testing on Nascar engines he didn't have restrictive mufflers in the systems. They were full race systems. Testing on the dyno and the track showed gains in the low and mid-range on engines using the high rpm exhausts, in other words the big pipes. They showed gains across the powerband, even a few hp at the top end where the big pipes were tuned for. I built my first A-R system in the mid-late '80s to fit to a Yamaha TT500 flattrack bike. I couldn't find any mandrel tubing in a small enough bend radius to fit my Honda XL350 so I built the down pipe for a local racer. He never tried it because he had his front end torn apart and then the local dealer (his sponsor) got a shipment of Mert Lawill 1 3/4" Yamaha pipes in and he grabbed one. The A-R pipe that I had made was 2" OD and included a megaphone. The "California Tuners" told him that the 2" OD was way too big and he wouldn't have any bottom end. So he gave it back to me. I ended up making about 12 pie slices on the inside of the headpipe bend to tighten it up so that I could run it as a high pipe on my Honda XL350. The result was a stong powerband from just off idle through the top rpms. Even on the smaller engine.

I haven't really made an argument against your 1-1.1 valve area idea, it generally is close to what needs to be run. However I would in theory take exception to your conclusions with the Yamaha YZF450 system:

As to the use of relatively larger header tubes, even with a muffler, there is such a thing as too big, and a practical example can be found in the '08-'09 YZ450F. That bike was originally produced with an short muffler that everyone liked the looks of. Trouble was that it was terrifically restrictive, and as a compensation, Yamaha used a considerably larger diameter header on the '08 than on either the '06 or '07 (both different exhaust designs). The cumulative effect was a pronounced dip in the power curve centered at 5000 RPM. Using almost any other muffler, including the stock '06 or '07 units, on an '08 with the '08 header produces an immediate gain of anywhere from 4-8 hp at 5K with no other modifications. However, switching to the smaller diameter '06 header, the stepped '07 header, or most any header from the aftermarket will add another 1-1.5 hp or more to the peak numbers over and above the gain from changing the muffler.

You say that the '08-'09 headers were too large as a compensation for an overly restrictive small muffler. The larger head pipe shouldn't be used to compensate for a muffler that is too small and restricts flow. I know the problems with keeping the bikes quiet, but this is an Oxymoron solution. I would agree that using a less restrictive muffler would show the large hp gain, but this is at that point where you point out the system was not correctly tuned already, the 5,000rpm spot with the dip in the powerband. Patching up a problem in design with an add on is not always the right way to fix things. Looks like Yamaha just screwed up with the entire design and adding on different mufflers didn't make it right.

I still don't agree with running long straight pipes for tuning (factory style). I prefer a headpipe/megaphone system as that is what I have had success working with. Let's just say that there are different ways to build and tune exhaust systems that get good results.

As far as the Honda footnote goes, I have heard plenty of "Tech Speak" coming from factory reps that goes against common sense, science and facts to question much of what the factory mouthpieces put out to the public. One of the good ones back in the mid-'70s was that the mid-pipe chamber on the TL250 Honda trials bike was put there to "REDUCE" the tremendous Torque of the engine! Funny, the Honda Factory bikes actually experimented with what looked like the Helmholtz/chambers in their trials bikes long before FMF or any of the other aftermarket folks built them. Look at this early Factory RTL250 system. Was it just an added muffler or a Helmholtz chamber?

EddyLejuneHondaPowerbombsm.jpg

Since you seem to have bought into the Helmholtz Resonator theory, how is it that a single Resonator chamber which is tuned by its volume/size/shape vs. the exhaust frequency is responsible for power gains across the full powerband? There should be a given frequency/rpm that the Resonator tunes for and not across the powerband. So there are other factors at work with the Resonator chambers and I feel that they are acting as A-R pulse generators that create interference pulses between the cylinder and the end of the pipe.

Swiss

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...(regarding the) Helmholtz Resonator theory, how is it that a single Resonator chamber which is tuned by its volume/size/shape vs. the exhaust frequency is responsible for power gains across the full powerband? There should be a given frequency/rpm that the Resonator tunes for and not across the powerband.
Have I said otherwise?

My interest in Helmholtz type mufflers lies in their ability to reduce sound without significant restriction to gas flow while allowing the exhaust system to "think" that it empties into the open air, pure and simple. That puts harmonic tuning back on the table, along with whatever degree of AR tuning you might choose to adopt.

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I found a never-used KTM 525 exhaust on CL for $25 and fabbed it up to fit my DR350. It works superbly, is quiet and has a spark arrestor.

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