Honda XR650L Thumpster

Yeah, it's pretty weird. So what's the back pressure issue? If I was to keep re-working this how do I determine how long the exhuast needs to be? I'd imagine stock length is probably close to perfect (thank you japanese scientists). Can someone explain it a bit, I'd like to learn.

Heat wise: What if the headers and muffler were both pipe wrapped? would that keep the heat down enough for the oil?

What about welding an array of heat sink fins to the sides of the front oil tube? just thinking out loud here.
 
There is a right side header for sure. What if I took more disc's out of the supertrapp, or all but one? how much would that increase the backpressure?

Cuba Libre said:
Beauty is in the eyes of the beholder, so I'll keep my peace about the looks.
But it's waaay too short. chances are it'll damage your engine.
Plus where it is, it'll heat up the oil in your frame.

Is it me, or there isn't a right side header?

M.
 
It still runs and revs like it used to.
 

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compoundcycles

Others may chime in and/or correct me, but here is my take (even if a little over-simplified):

Think of an engine as an air pump. Fluids (fuel+air mixture) come in and fluids (exhaust gases) come out. What happens in between is what gives you torque.
** Let me state now that the product of an engine is torque. I know we all want "Mo Powa" but we only get power because our engines provide torque over time. The point here is that the same (normally aspirated) engine will give you more or less the same amount of torque, but you get to play with where/at what rpm this is developed - the ever so infamous "powerband". In simple terms, the higher the rpm where max torque is developed, the more power will be produced.**
There is a thing called valve overlap, i.e. when both the intake and exhaust valves are open. While you do need a free flowing exhaust to make sure your cylinder empties before the new batch of fuel+air mixture comes in (so that the intake quantity is maximized), you need back pressure in the exhaust in order to keep the new batch in the cylinder. It sounds like a contradition in terms, and it probably is. It is a compromise or, better, a ballance: You want the door open enough for the waste to leave fast but closed enough for the good stuf to stay in. Engineering is the art of compromise and there is no "big" or "small", or "a lot" or "a little". There is only "good enough" which derives form "correct" and how far you deviate from that. ("Big" and "small" are usually marketing terms...)
This should also answer your question about removing more disc's out of your supertrapp.

There is another issue with very short pipes: the heat shock. The shorter the exhaust tube, the colder the "neighbourhood" of the exhaust valves, since they are closer to the ambient temperature. So these valves have to operate in a very hot environment when the (very hot) gases have to exit and a very cold (by comparison) environment at all other times. This kind of heat cycling (hot-cold-hot-cold) can destroy the valves.

Stock length is close to perfect as long as you want a stock engine and have the same targets as the stock designers/engineers.
Designing an exhaust is not a black art, but is not an exact science either. The reason for the latter is that in practice no two engines operate in exactly the same conditions. So compromises need to be made. Have a look at this: http://www.mez.co.uk/mezporting/exhaust_length.html

Heat wrapping the exhaust pipes usually is done for two reasons: heat shielding and heat insulation (ok, and looks). The former refers to your inner thigh (not) getting barbequed. The latter is an attempt to increase the temperature of the exhaust gases so as to reduce their pressure (the Bernoulli principle) so that they get "sucked" out faster. I understand the mechanism but have some doubt about whether it works. There is also the additional drawback of the pipes running extra hot and becoming brittle (and eventually breaking off) over time.
Insulating your pipes will go some way in to protecting your oil, although it will be a remedy - an attempt to bypass a faulty design. I can't comment on whether this will be "enough".

Welding an array of heat sink fins to the sides of the front oil tube? You'd be creating more problems than you're trying to solve. Three issues I can think of here: (1) Heat stressing of an already soft frame member (2) the danger of that part of the bike falling on you in an accident and (3) cooling fins increase cooling area and therefore speed the heat exchange process. "Heat exchange process" means heat transfer from something hot(ter) to spmething cold(er). Normally you'd be trying to cool the hot oil. In this case though I don't know what will be hotter: the oil or the "cooling" air, heated by the misrouted exhaust pipe. Fins may be opening the heat transfer "door" in the wrong direction. At best you'd be doing half the job (trying to cool your oil with hot air). Anyway, that's only my gutt reaction but I can't see the point of creating a bad design on purpose and then trying to find a way around it. Better to re-route the exhaust.


I hope the above make some sense and shed some light into the subject. Pardon my command of the language but I'm not a native speaker.

M.
 
It may run and rev like it used to, in your garage, under no load, in neutral, but once you get out on the road I suspect it will be a very, very different story. When designing an exhaust for power, there is a relationship between length and powerband. I.E. a stockish length exhaust is designed to *generally* give you a broad, even powerband. When you change the length and/or the size of the pipe, you affect when and where your power comes on in the RPM range, so you might not notice it while you're just revving along in the garage, but when you put power to the pavement you WILL notice a big difference.

Also, pulling discs out of the Supertrapp would reduce backpressure, not increase it. As short as that pipe is, you might as well not even run an exhaust.
 
I assume that you are eventually going to lower the bike (or it will always look like a dirt bike and not any of your inspiration pics).
If that is the case, you will probably run into wheel / exhaust interference with that pipe. Even now, it looks like it would hit before the fork bottoms out.
 
Thanks everyone for the info. I'll take it all to heart. I wasn't necessarily going to lower the bike. I'm not trying to copy someone else's build. Just posted those bikes as inspiration. I did test the clearance using the weight of two people to compress the shocks and it barely touched. I was also contemplating a smaller front wheel. Lots to think about.
 
Cuba Libre said:
This should also answer your question about removing more disc's out of your supertrapp.

As I understand it, removing discs from the supertrapp quiet the muffler. This slows the escape of exhaust gasses therefore causing more backpressure?

Just for the record, when I get a free weekend from work I'm going to take it for a ride like this. How long do I need to ride the bike to take an accurate oil temp reading? Anyone know offhand what the stock oil temp should be?

What if the shape of a skidplate had volume to it, and it was enclosed as a muffler?
 
compoundcycles said:
As I understand it, removing discs from the supertrapp quiet the muffler. This slows the escape of exhaust gasses therefore causing more backpressure?
Yes, that's the idea. But it’s not certain that you need more backpressure (or less or whatever).

Stock oil temperature will depend on the ambient conditions and your riding style. Think around 90degC. If you go into the 100degC territory for sustained periods of time you're heading for trouble.


compoundcycles said:
What if the shape of a skidplate had volume to it, and it was enclosed as a muffler?

On the assumption that you are talking about fabricating a muffler and shaping it so as to double as a skid plate, then I suppose it could be made to work - in theory. Of course a skidplate and a muffler do different jobs, so take that into consideration. Put in different words, "skid" plates are also termed "bash" plates. Would you like your muffler bashed?
Another thing to consider is the shape that a skidplate needs to have in order to "hug" your bottom frame rails and the shape your muffler will need to fit in the same position. You could build a canister of sorts with baffles etc to fit, but think "turbulence".

BUT, my question all this while is... WHY?
Are you going to modify the engine (bigger bore, compression, cam, valves, porting etc etc)? If yes, then you'll have to consider your exhaust dimensions based on what you do to the rest of the engine. If not, is it just looks we are talking here?


M.
 
Cuba Libre said:
If not, is it just looks we are talking here?

It's not just about looks. But I'm also not trying to build a performance machine. Dirt bikes are very form follows function. I'd like to change the order.
 
Ok, putting the exhaust on hold for the time being. Started working on an electronics tray to fit under the seat. Looks like everything will fit so far, and the back wall of the tray will make a great surface to mount the tail light. Last pic is a new set of headers I picked up from the swapmeet as a backup plan.
 

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Managed to stuff all the electronics under the seat and take her for a ride around the block. I didn't really notice any performance issues other that a noticeable vibration in the front forks, probably because the muffler is only mounted at the headers, there is no secondary brace. My number one concern is now the front tire hitting the muffler when the front shocks compress. Might be time for exhaust plan B. I do like how nice and clean she's looking now with all the junk out of the way.
 

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I wanted to box in the rear of the frame but also give the passenger something to hold onto. I used two sets of bicycle handlebars for all the bends I needed to complete the top frame loop.

Started a second round of exhaust configuration using the headers I picked up at the swapmeet. I'm going to complete the headers linking up to it tomorrow, just need some more tubing for extensions. In its current position i'm still not satisfied. I think I'd still prefer something that's more inline with the center of the bike instead of hanging off to the sides. I'll finish it up and take it for a ride so I can do a direct comparison with the previous headers.
 

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Those headers are beautiful!!!
What are they off?

Also, did you relocate your battery and how/where to?

Thanks
M.
 
I'm not exactly sure what the headers are from. I picked them out of a pile of other stuff at a swap meet. The guy I bought them from said he thought they were off of a suzuki. Not bad for $40.

I did relocate the battery, if you scroll up a ways you can see pictures of the smaller battery sitting in electronics tray underneath the seat. The key was getting a smaller battery that would fit. The one I ordered was recommended by some guys over at Thumper Talk. There is a comparison photo of the two batteries back a page.
 
Just when I thought "Where am I going to find 1-1/4" stainless tubing to extend my stainless headers." Look no further than Home Depot's bathroom dept. They have a nice selection of stainless handrails pre-bent and everything. You could piece together an entire exhaust out of that stuff. Although, it wouldn't be cheap. This length ran me about $19.

Finished up the exhaust and took her for a ride. The longer pipes do perform better than the stubby exhaust. I'm still not certain I'm going to keep this setup but I'm enjoying the process of finding out what works and what doesn't. The project is starting to go....somewhere.
 

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