crysalis_01
Well-Known Member
I see its been attempted a couple times but not the whole story has been told. The reason why they were there has more to due with the way the air pulses (because as you all know air charge doesn't move smoothly in the manifold, it "bounces" back an forth off the intake valves) "flow" in runners of different lengths at different rpm. Long runners flow better and produce better power at lower rpm and shorter do the same at higher rpm. In the old B-head 4v's due to the two intake runners of the intake manifold per cylinder being of a substantially different length shutting the short runners at low rpm force the air down the long runners which allow the air pluses to hit the opening intake valves at a more optimal time. Likewise with the long runners allowing a better pulse at higher rpm.
Basically its about playing with the "harmonics" of the air and the mass and velocity of airflow in the runners. Every engine parameter has an RPM where it works best which is why its important to match them together. As rpm increase, the velocity of airflow will increase in the runners. Once it reaches a certain velocity (actually around 300 mph) it will start becoming restrictive. So, as the velocity increases, the more inertia it carries with it. Once the piston reaches the bottom, the column of air will use its inertia to cram a little more air in there before the valve closes.
A long skinny runner will hit that peak velocity early in the powerband and use its inertia to cram extra air in the cylinder down low in the rpm where torque is favored. Above those rpm it starts choking off flow and restricting high rpm power. A big short runner will reach that peak velocity later in the rpm band, meaning you have to rev the engine higher to get it to get any inertia behind the column of air. This also means that short runners can actually remove some low end torque from the combo because there is very little velocity in the incoming air.
When the intake valve closes, it sends a pressure wave back up the intake tract much like when you slap your hand over a tube. If you tune things carefully, that sound wave will be headed back to the intake valve at the same time its ready to re-open. Long intake runners (like Tubas) develop those harmonics lower in the rpm range. Short intake runners (like flutes) develop those harmonics higher in the rpm range.so being able to have bothh can be benefitial if done correctly.
Basically its about playing with the "harmonics" of the air and the mass and velocity of airflow in the runners. Every engine parameter has an RPM where it works best which is why its important to match them together. As rpm increase, the velocity of airflow will increase in the runners. Once it reaches a certain velocity (actually around 300 mph) it will start becoming restrictive. So, as the velocity increases, the more inertia it carries with it. Once the piston reaches the bottom, the column of air will use its inertia to cram a little more air in there before the valve closes.
A long skinny runner will hit that peak velocity early in the powerband and use its inertia to cram extra air in the cylinder down low in the rpm where torque is favored. Above those rpm it starts choking off flow and restricting high rpm power. A big short runner will reach that peak velocity later in the rpm band, meaning you have to rev the engine higher to get it to get any inertia behind the column of air. This also means that short runners can actually remove some low end torque from the combo because there is very little velocity in the incoming air.
When the intake valve closes, it sends a pressure wave back up the intake tract much like when you slap your hand over a tube. If you tune things carefully, that sound wave will be headed back to the intake valve at the same time its ready to re-open. Long intake runners (like Tubas) develop those harmonics lower in the rpm range. Short intake runners (like flutes) develop those harmonics higher in the rpm range.so being able to have bothh can be benefitial if done correctly.
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