what is the advantage of dual headpipes? all the preformance quads have them, so i thought it was all good. then i took a close look at the yz426 and the new cr450 engine, and noticed that they had single headpipes. i was thinking more torque with the dual and more hp with the single? with 2 smaller pipes, the gas would flow faster, so i was thinking that was the best way to go. i am so confused!

 

Look more closely at the EX and the YZ and the CRF. The EX is an air cooled engine where as the other two are liquid cooled. The dual headpipe on the EX disapates more heat quickly which is needed for an air cooled engine. The YZ and the CRF are both liquid cooled and they don't need the dual headers. This is not the only reason though. Port timing, flow, backpressure, cam lift, durration, pretty much everything is a vairable. That and I'm pretty sure that you save weight and space by combining the two ports in the head before the exit. On another note the EX has radial four valve combustion meaning the valves are all tilted so they fact the spark plug. This is for more efficient combustion and flow of the gasses. All of the engines have compelety different approaches to power which makes each unique.

 

I will have to disagree with Need4speed. It is not to disipate heat, as in the racing world we wrap out headers to keep the heat in which helps them dump it out the end (think of exothermic reaction). I is simply cheaper to make a single exaust/valve setup.

Multi valve explinations for dummies.....
The reason behind a multi valve setup is not to angle them at the sparkplug at all. Here is a test I want everyone to do. Draw a perfect circle with a small tin can or something about 1.75" across. Now pull the change out of your pocket. Take two quarters and place them in the middle of the circle, note how much space you have left over, and mind you space is needed for a sparkplug. Now pull 4 or 5 dimes out of your pile of change, fill the circle again, notice less space is left, so you can use the entire head area more effeciently than before. Five is possible as well, with odd numbers, the exaust takes the fewer. Just like on conventional head configurations the intake is always larger than the exaust, because you need to force in as much A/F that you can, and when bieng expelled it is under pressure so can do better with the smaller port. Or since the age of subcribers to this group is younger than I am used to, go to McDonells get one of thier straw, really big huh, then go to BK take two of thier straws, and notice that you get more with the BK. Also, with multiple exaust valvs you cannot run headers, but manifolds. A manifold is like a block of steel with a opening cut out in it, no exaust velocity, just a mutual dump into a common plenum. And we all know they won't use a manifold on a bike, because of weight and cost, not to mention there is better velocity with the header and a better scavanging effect as well.

I will now step off my soap box.

 

Two small headers will have the potential for greater velocity at the same volume than one single larger head pipe of an equal cross section.

If you have two headers that are for example purposes, 1" dia, and one 2" diameter single header. The cross section, area, are going to be equal. But, in the 1" dia twin headers the gas velocity is going to be kept higher.

Higher velocity is better scavenging of the cylinder, which will help with lower RPM power. The fresh intake charge isn't diluted from hot end gasses still present in the cylinder.

Header selection is just another one of the thousand different considerations when tweaking an engine for high output....