TwinStar shoots for 100MPG
 

Looking for motorcycle guys' thoughts on exhaust cross overs and scavenging from 2 cylinder inline engines?

I was thinking that since I have to redo the exhaust on a fairly low-end anemic 180cc engine, maybe I'd add an x-pipe to it to help get some more low-end torque out of it?

Also, since this engine doesn't have any stepped Y collectors in the exhaust, rather 2 single small diameter head pipes which lead into mufflers, where would the X be ideal (if at all)?

Makes sense to me.

Crap that was fast... Did you read the part I added after I posted it?

About ideal positioning.

The longer your primaries, the lower rpm torque boost you'll get. For cars and header design, I think they recommend at least 34" for street applications.

So basically, putting an X-pipe right in front of the engine just isn't ideal at all... I'd have about 12-14" of pipe before the x-over point.

This is a part of exhaust that I'm not totally familiar with, if you can't tell... the dynamics of changing pulse properties elude me, to say the least, and I haven't found a good study or read on it that would suggest optimal placement on such a configuration.

I have some space under the bike that I could use for a cross-over, but that would mean tighter bends in the later sections to get into the mufflers, although I could also make the tighter bends from slightly larger diameter pipe to ease flow restriction, I guess.

Does any of this seem vaguely correct?

An easier route would be to redesign for single muffler. If the particular bike you're working on has an aftermarket exhaust option, examine it, if possible, to design a similar setup around your existing stock parts.

I'd agree with Atomic Ass. I'd also stay with stock pipe diameter. Larger diameter pipe has a more drastic effect on torque than an x-pipe will and will likely increase peak torque rpm.

The diameter and length of the primaries is the most important thing. A Larger diameter primary will shift the torque curve higher in the RPM range, so in your case you'll want to keep the diameter small. A collector 32-34" from the head (as daox mentioned) is the general practice.

The bike has 1.5" primaries. With this pipe size, I think I have enough room to literally tuck the pipes under the bike with a collector, and still have the collector and muffler under there, never actually leaving the space under the frame. This will free up alot of space down the side of the bike's frame, leaving more room for bags and possibly helping the aerodynamic profile?

That would give me about 3.5 feet of total exhaust length, and the muffler should still keep the bike fairly quiet due to it's proximity to the ground.

Even straight pipes are quiet when they're aimed at the ground.

Anyone have any suggestions further than that?

I'm thinking basically to keep the 1.5" pipe diameter, running into a 2.25" collector so I can use Larry Widmer's straight through muffler design. The pipe will be angled slightly out from the frame so that it doesn't exhaust directly on the tire while stopped idling or at low speed, where it could cause some damage to the rubber of the rear tire.

So I'll end up with a single muffler, blowing out the right side, 1.5" primaries about 30 inches into a 2" pipe (the 2.25" collector is to make up for the space of the muffler packing) which will make up the remainder of the exhaust pipe, vented slightly to the right.

That is a bit larger than I'd go. How much horsepower does this engine put out? What are the normal cruising rpms? What rpm range are you targeting to increase torque?

From the literature I have, a 1.5" primary is good for 26-38 hp per cylinder. 1.25" would be plenty (17-22 hp per cylinder) with a 1.5" collector outlet (which emphasizes low rpm torque).

Edit: I just checked the outlets on my 82 CM400, and outlets on the mufflers are only around 5/8" and that engine puts out roughly 43hp.

Yeah, that's the muffler outlet, though. The primary pipes are 1.5" off the head.

The OEM exhaust on my CM185T has 1.5" primaries leading in to 1" mufflers with expansion chambers in them. I just figured on staying with the 1.5 primaries because I already have them.. ya know, using what's there. I can just re-bend the pipes to the shape I need, after I heat them up.

If I go with smaller primaries, I have reversion issues, because the holes in the head are 1.5" as well. It's a 180cc engine, according to the jug casting.

I'm trying to get more torque in the 2000-4000 range, the engine's HP is like 15@9500... I wish I had a dyno graph of it, but I don't. It's got a relatively high RPM at 55-60MPH, but I can't tell exactly what it is b/c of the lack of a tach thus far, not to mention that it's non-operational at the moment.

I just figured since I had to redo the exhaust anyway, I might as well do it something closer to optimal.

I think that a primairy pipe size of max 26 mm internal should be used. This size is used on some Honda 500 -4 engines. Sufficient to support 15 HP per cylinder. The length of the primairy is longer for two into 1 muffler than when each pipe has his own muffler. This comes because the temperature is higher in the first case leading to a higher velocity for the pulses.

To get good data on exhaust pipe length use one of the calculators.
Here's one: Exhaust Length,exhaust pipe,induction
And another: http://dairally.net/daihard/chas/Mis...s/DaiPipes.htm
Read ALL about it with this book: http://www.bentleypublishers.com/aut...ry-1483-2.html

For your engine and the RPM you want, it's gonna be pretty darn long.

If you cut the calculated length in half you'll get somewhat less of the same effect due to friction as the pulse travels back and forth twice, but it helps for compact packaging.

Take a look at a GS500 or Ninja 500/250 to see what they do for the crossover.

I'd have to agree with janvos.... for a 90cc cylinder, the primary should have an inside diameter of no more than 1" in my opinion.

The Stock Ninja 250 headers have an "H" pipe crossover about a 6-8 inches out of the head, FWIW. The 87-07 Ninja 250 have a dual exhaust and didn't merge into one. Most aftermarket exhausts for the bike have a 2-into-1 design. The 08-present Ninja 250's have a 2-into-1 design in stock form.

theycallmebryan - I'm sure that 1" would be better for me, but I simply can't put 1" primaries on it. It's not going to help at all, because I'll end up with reversion. The diameter of the pipe that's already on it, and the hole in the head, is already 1.5". If I were looking to re-map the exhaust port in the head, that would probably be an option, but it's really not right now, and most likely never will be.

beatr911 -

Thanks for the calculator, though I actually have the harmonics formulae written down somewhere. I do alot of exhaust work on cars, but obviously, there is much more room on a car to get desired effects than on a bike.

What I'm looking for is to extend the torque at the 3000 RPM range, because later, I want to gear down so I'm not running so high in the RPM range at 55-60MPH. (I have no idea what it actually runs right now, but it's audibly "up there").

The problem I find with alot of flow calculations is that they do not take into account the bends of the exhaust. While 1" pipe might be optimal for 15HP per cylinder, it's not optimal when the exhaust gasses aren't flowing in a straight or mostly straight line, because the dynamic flow effect of the gasses in the pipe basically makes the cross section of flow smaller than the pipe's diameter. This is the reason that lots of exhaust systems which require bends (like on smaller bikes and such) use aftermarket "whale penis" exhaust pipes, which have a huge blown out section in them. It prevents flow restriction in the bend, although sacrificing velocity a bit to do it. (Some of them also use that space as an expansion chamber, to quiet down the exhaust note.)

For all those reasons, and the fact that I'm not really going to spend weeks fabricating my exhaust for this, I think I'm going to stick with the idea up there ^^^, using the OEM primaries with a collector. Collector size will depend on what size PVC I have available, though.

Later on, I'll probably switch the intake around as well, to run two smaller carbs instead of one carb w/ the 180* manifold. Then I can add in a runner for each carb to help atomize the fuel more, and also to increase harmonic intake resonance at the low band.

This guy has a nice calculator that works, simple too www.rickwrench.com/bunchofcalculators.html

X pipes don't work on twins as the pulses are too far apart to mesh as they do in a V8.
The small pipe on the Ninja250 H layout is called a balance pipe. It is too small for scavenging but I can't remember if it was for noise or emissions ......

Beatr911 that link at dairally looks deep, i cant wait to try it out !

On the carb switch, 1 single carb usually gives better low end torque while separate carbs perform better at higher RPMs.

I thought about that after I posted... it's also more expensive to use/tune 2 carbs, so I'm going to stick w/ the single carb, but try to lengthen/smooth out the manifold so I can at least catch the 4th region harmonics... cuz man, that thing is "dirty" right now. It's a log-type intake w/ the carb right in the middle, so the immediate circumstance is that the flow path has to make 2 90's just out of the carb, and I'm certain I'm getting fuel pooling in there somewhere.

Pics!
 

Here's something to give you all an idea of the exhaust that I'm going to end up with.

By the way, there is a correction to be made. The head pipes are actually either 7/8" or 1". I hadn't accounted for the fact that head pipes are double wall, stupid me. :thumbup:

That means I'll bring my head pipes down into a collector that only picks up to 1.75" or so, which will give me an overall collector ID of 1.5" by the time I'm done, maybe a little smaller.

Unfortunately, there is only about 24" of usable primary space. There was only 26" of primary pipe on the original exhaust anyway.

The pipes pictured are the OEM pipes moved to where I want them, mufflers broken off at the rusty bits.

http://ecomodder.com/forum/member-ch...8-dscf2182.jpg
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http://ecomodder.com/forum/member-ch...6-dscf2179.jpg

Pay no mind to how dirty the engine and assorted parts are... I'll be taking care of that when I get to actually taking the bike apart and cleaning it all up to get ready for the final build.

You can't see it in those pics, but the carb is almost tight up against a section of the frame, but I can make two pipes that go around the frame section, remove the airbox, and place the carb with a cone in the spot the airbox used to be.

This will also afford enough room for a standard 12V motorcycle battery to be placed under the seat where the airbox was, saving more room and further slimming the bike's profile.

So - Can we just change this to my project thread? Darin? Anyone?

How about "TwinStar shoots for 100MPG" for a title?

I've worked a few things out... no new pics, though.

To add the points that I've been playing with in my mind:

Removed under-seat airbox.
Removed passenger pegs and OEM exhaust hangers.
Relocated rectifier and condenser/ground wire to areas where they're not seen as easily or in the way.
Adjusted front and rear brakes.

Measured open area under the seat for battery possibilities. 1" tall, 3.5" wide, 7" long. Could use 2x6V cells in that area, then wire them in series later for 12V upgrade.

Thus far, the widest parts on the bike are the foot controls and handle bars/forward controls.

Want to make bags and a trunk/backrest, possibly tapering at the rear for aero consideration.

Still have to make y-pipe collector and shorty muffler.

Have to consider intake designs.

Have to stop getting side tracked on other crap all the time!

here's to your first 100 mpg tank! :thumbup:

I don't think I'll have 'er on the road until Spring, but I'm shooting for 100MPG right out of the cage! (With any luck, I'll get it. I doubt I'll hit it right away, with no gearing/jetting changes, though.)

So what happend with this bike? Get her running? Any problems?

Here's mine: Full description - Generation Vibe Discussion Forums: star_deceiver - 1979 Honda CM185t twinstar

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