TwinStar shoots for 100MPG

[Christ]

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)?

[Daox]

Makes sense to me.

[Christ]

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

About ideal positioning.

[Daox]

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.

[Christ]

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?

[The Atomic Ass]

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.

[Daox]

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.

[theycallmeebryan]

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.

[Christ]

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.

[Daox]

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.