| Author | Message | ||
     Danger_dave |
have to be the same length? | ||
| Sleez |
found on the web, FWIW; "The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector." good info here; http://forum.concours.org/index.php?topic=45090 "Equal Length Exhaust The effectiveness of equal length exhaust is widely debated. Assuming that an exhaust system is otherwise properly designed, equal length pipes offer some benefits that are not present with unequal length pipes. These benefits are smoother engine operation, tuning simplicity and increased low-to-mid range torque. If the pipes are not equal length, both inertial scavenging and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult to achieve optimal tuning. Equal length pipes eliminate these exhaust-induced difficulties. "Tuning", in the context used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of mechanical components to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance with a combination of parts that must work together in a complimentary manner. In an exhaust system that is properly designed for it’s application, equal length pipes are generally more efficient. The lengths of both the primary and main section of pipes strongly influence the location of the torque peak(s) within the powerband. In street and track performance engines with longer pipes typically produce more low-to-mid range torque than shorter pipes and it is torque that moves a motorcycle. The question is... Where in the powerband do you want to maximize the torque? Longer pipes tend to increase power below the engine’s torque peak and shorter pipes tend to increase power above the torque peak. Large diameter pipes tend to limit low-range power and increase high range power. Small diameter pipes tend to increase low-range power and to some degree limit high-range power. "Balance" or "equalizer" chambers between the exhaust pipes tend to flatten the torque peak(s) and widen the powerband. Among the more astute and responsible exhaust builders, it is more-or-less understood that pipe length variations should not exceed 1" to be considered equal. Even this standard can result in a 2" difference if one pipe is an inch short and another pipe is an inch long." | ||
| Danger_dave |
Cool - thanks - what about using different diameter pipes to compensate for different lengths/back pressure - is that done? | ||
| Sleez |
i recall reading about that exact scenario somewhere, don't have any refs, but believe that can compensate to a degree. | ||
| Sleez |
more interesting reading; don't know how valid it is. http://www.piteraq.dk/flight/exhaust.html | ||
| Old_man |
I don't know how important same length pipes are in an engine like ours, with uneven firing impulses. | ||
| Just_ziptab |
What's really wild is the theory behind two stroke expansion chambers to "get it right". | ||
| Lovedabueller |
"with uneven firing impulses." what do you mean by this. | ||
| Mr_grumpy |
This thread reminds me of a lab scene from "Police Squad" . Undoubtedly the best cop show ever. "Well Jimmy, that's a really interesting question, let's have a look at this lady here & see if both pipes are equal......" | ||
| Spiderman |
Cool - thanks - what about using different diameter pipes to compensate for different lengths/back pressure - is that done? The Duc 999 had a larger diameter rear pipe to 'fool' the motor into thinkin it had equal length pipes. | ||
| Spatten1 |
Cool - thanks - what about using different diameter pipes to compensate for different lengths/back pressure - is that done? That won't fix the pulses getting kicked back with the same timing. I've seen chopper guys use an obstruction at the same point on both pipes to create the same timing for the reverse waves/pulses, without sacrificing the all important aesthetics. Don't know how well it really works. | ||
| Ourdee |
"with uneven firing impulses Do you mean, because of the 45 degree offset of the cylinders? | ||
| Ratbuell |
The other issue with different diameter pipes is velocity. While you can get the same *flow* with a larger diameter, the flow *rate* can change and mess up the equilibrium. That's the same reason, when porting heads, that a bigger port isn't always better. Think of it this way. You've got a gallon of water. Pour it into a 6' long garden hose and how does it come out the other end? Now, pour that same gallon into a 6" diameter piece of PVC plumbing pipe, the same 6' long as the garden hose. How does it come out now? | ||
| Bluzm2 |
Stepped headers can also be used for different length headers. Exhaust design is part science, part black art and a dash of instinct and experience. | ||
| Old_man |
Do you mean, because of the 45 degree offset of the cylinders? Yes, If the firing was evenly spaced, opposite the other, I would see a big advantage to scavenging through same length pipes. I don't know about our engines. | ||
| Lovedabueller |
"our" engines are "opposite" each other. when one is on the power stroke the other is on the intake stoke and vise versa. and to go to the original question the pipes should be within about 3/4" of each other in length. the reason this is important is to maintain the proper flow and back pressure per cylinder. you can gain hp and/or torque by different lengths. so you want each cylinder to run as close to each other as possible performance wise. is this understandable. | ||
| Old_man |
"opposite"-I was referring to complete opposite, firing equally opposite the other. Which ours' do not. | ||
| Lovedabueller |
you mean like a "boxer" of horizontally opposed. like a goldwing. the same effect is involved in each, | ||
| Old_man |
Or a horizontal twin. triple, or four. They can have their firings evenly spaced. Ours cannot be evenly spaced because of the 45 deg. angle and a common crank connection. |
