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Discussion Starter #1
Just because I think its gonna get funny:

At the risk of personal injury and altering existence as we know it, I pose this question.
Is there an advantage of having a 2 into 1 exhaust system over a 2 into 2 system? More importantly, why do you say so?

Go ahead - have fun.
 

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For the most part it depends on the motor in the bike. Is there a particular bike you're talking about?

Two cylinder two stroke motors tend to need two separate exhausts.

Craig
 

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For the most part it depends on the motor in the bike. Is there a particular bike you're talking about?

Two cylinder two stroke motors tend to need two separate exhausts.

Craig
 

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Supposedly the 2-2 makes more power on a 180 degree crank twin, and the 2-1 makes more power on a 360 degree crank twin. I've never seen any conclusive evidence to support either. I built 2-1 systems for 175 twins because they were lighter and less prone to crash damage and sounded really cool. At the state of tune of most vintage bikes, even race bikes, I'm not sure it makes enough difference to worry about.

I will say that 2-1 pipes on a 360 crank twin seem to make a noticeable difference in mid-range power on the race track, on a small displacement twin (which always need help in the mid-range). But I've never dyno tested them back to back with a 2-2 system.
JohnnyB

PS. I'm not sure if my reply was humorous, but everyone has a different opinion of what's funny.
 

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Supposedly the 2-2 makes more power on a 180 degree crank twin, and the 2-1 makes more power on a 360 degree crank twin. I've never seen any conclusive evidence to support either. I built 2-1 systems for 175 twins because they were lighter and less prone to crash damage and sounded really cool. At the state of tune of most vintage bikes, even race bikes, I'm not sure it makes enough difference to worry about.

I will say that 2-1 pipes on a 360 crank twin seem to make a noticeable difference in mid-range power on the race track, on a small displacement twin (which always need help in the mid-range). But I've never dyno tested them back to back with a 2-2 system.
JohnnyB

PS. I'm not sure if my reply was humorous, but everyone has a different opinion of what's funny.
 

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Why does what happen? Mid-range power?

Theory is that with a 360 degree twin with a 2-1 system exhaust pulses from one cylinder create a negative pressure wave in the system that helps evacuate gases from the other cylinder, and vice versa. The effect varies in efficiency with rpm, cam timing, primary header length, collector pipe length etc.
JohnnyB
 

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Why does what happen? Mid-range power?

Theory is that with a 360 degree twin with a 2-1 system exhaust pulses from one cylinder create a negative pressure wave in the system that helps evacuate gases from the other cylinder, and vice versa. The effect varies in efficiency with rpm, cam timing, primary header length, collector pipe length etc.
JohnnyB
 

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With a 360 degree crank, all of the pulses are 180 degrees apart, or even firing, so one pulse can help scavenge the next, and the next, and so on. With a 180 degree crank, the lead pulse may scavenge the following pulse a little, but then both cylinders have a big wait as they both have to intake on top of each other. JohnnyB is right. It can work real well, as long as all of the other factors are optimized too. That usually means getting really lucky{not likely} or spending a boatload of time with a dyno, and lots of tubing.
Think of it this way. I have a fairly simple brain, so I have to do things like this sometimes.

360 degree crank twin = 1 bang-2 exhaust, 2 bang-1 exhaust, 1 bang-2 exhaust, 2 bang-1 exhaust,,,, See the symmetry?

180 degree crank twin = 1 bang-2 bang, 1 exhaust-2 exhaust, 1 bang-2 bang, 1 exhaust-2 exhaust,,,,,
If I wasn't sooo lazy, I'd do it out with drawings and proper degree spacings.
 

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Discussion Starter #11
Very impressed!!
Very serious and direct answers, thank you!

So how about design, pipe length, resistance, etc........
cooments?
 

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There are some very complex formulas out there to calculate the various head pipe, mid pipe, collector, length, ID etc. I've yet to see one that was basic enough that the average person, or even above average could use them. Major inputs are usually, displacement, stroke, crank timing, compression, cam timing, valve size, average port ID, etc. I've seen some software you can buy that supposedly does such things, but often is auto specific which makes it all but useless for bikes. I've seen one small engine simulation package but I thought it sucked.

Usually it's easier to just find out what has worked in the past on a given engine and use that as a baseline.
JohnnyB
 

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Chuckle, I've seen some of that math, and smoke started to come out of my ears. D-ooh!
 

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Discussion Starter #14
"Usually it's easier to just find out what has worked in the past on a given engine and use that as a baseline."

Yeah, I hear that. But you know whats true. Someone puts new upholstery on their bike, and swear its faster! Who really hs a dyno in their garage and run it every time they change only one thing to measure change?

If they do exist, whats their address? Got a bunch of things I'd love to try out!
Otherwise, I'll be the guy with no upholstery on his seat.....

-R
 

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Discussion Starter #15
"Usually it's easier to just find out what has worked in the past on a given engine and use that as a baseline."

Yeah, I hear that. But you know whats true. Someone puts new upholstery on their bike, and swear its faster! Who really hs a dyno in their garage and run it every time they change only one thing to measure change?

If they do exist, whats their address? Got a bunch of things I'd love to try out!
Otherwise, I'll be the guy with no upholstery on his seat.....

-R
 
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