how do headers work?

[grumpyvette]

I was asked about a previous post,my example of how do headers work? what I think your refering to was when I was trying to explain how headers help pull the exhaust out of the cylinders by useing the energy from the previously fired cylinder mass of exhaust rapidly moveing away from the exhaust valve causeing a negitive pressure at the exhaust valve that helps scavage the cylinder. what I said was you can demonstrate that negative presure caused by a rapidly moveing mass by getting a 2.5"x5' long section of pvc pipe hold it horizontally and insert a full coke can with some tape wrapped around it so that it just slides easily into one end of the 5' long pvc pipe, now put your hand over the end of the pipe you just installed the full coke can into and rapidly drop the other end strait down, as the coke can slides too the floor you will feel the negitive pressure on you hand caused by the falling mass. now a cylinders volume of exhaust moveing through a primary header tube works in much the same way, while it weights much less than the coke can its moveing far faster and carries enought energy that the same negitive pressure is formed at the exhaust valve by the slug of exhaust gas traveling in the header primary tube, if the header primary tube is long enought to still contain the mass of exhaust gas at the rpm that matches the cam timeing and cylinder volume, now temp. cylinder volume, cam timeing and interior pipe dia. all effect the rpm range that this effect peaks at effiency wise but in most v-8 engines a dia equal too the exhaust port and about 32"-39" will put that negitive pressure wave caused by the previous fired cylinder at the exhaust port in the 4000-6000rpm range, look at this chart,

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

 

now figure that exhaust is 1300f or 977k so thats about 25" at 6000rpm and about 39" at 4000rpm to get that pressure wave to work for you scavageing the cylinder(helping to pull exhaust out and the new intake charge into the cylinder.)hers stuff to read,

 

http://www.burnsstainless.com/TechArticles/Theory/theory.html

 

http://www.engr.colostate.edu/~allan/fluids/page5/page5f.html

 

http://www.engr.colostate.edu/~allan/fluids/page5/page5f.html

 

http://www.feuling.com/randd/articles/art11.htm

 

BTW it should be obvious why shorty headers don,t make as much power,....the primary tubes being much shourter are also much less effective at controling that negitive pressure wave timeing and strength.