A forum dedicated to the 1992-1995 Honda Civic.
mike_belben@yahoo.com:
Quote:
Originally Posted by 00Red_SiR
Negative (returning) expansion waves are undesirable in headers and manufacturers often design in things like steps and anti reversion chambers to help reduce this pulse. Scavenging is very desirable but it is a "suction" created behind the departing pressure wave pulse, when the exhaust valve opens. That suction helps create a vacum in the cylinder so it will fill more completely with a fresh intake charge and therefore make more power. A Negative returning pressure wave HURTS the scavenging effect because it actually pushes exhaust gasses back into the cylinder reducing cylinder filling efficiency.
at no point in an otto cycle engine's 720 degree cycle is a negative exhaust manifold pressure a bad thing. when present any time the exhaust valve is open, it is a great thing.
negative pressure, any negative pressure, IS "suction". any pressure lower than the current one will have a suction effect on a fluid as far as i am aware. the more negative it is, the greater the magnitude of "suction."
fluids flow from area of greatest concentration to area of least concentration. the exhaust valve opens prior to bottom dead center when chamber pressure is still quite high. a very high pressure blast is the driving force for most of the exhaust stoke's duration, called "blowdown." the exhaust manifold pressure will rise as the piston slows and the manifold's exhaust concentration increases due to restriction. if the chamber pressure is lower than the exhaust pressure at any time that the valve is obviously open, flow will reverse and fill the cylinder with exhaust gas. if the cylinder pressure is greater than the intake manifold pressure during overlap, the inert exhaust gasses will expand into the intake, then be reinducted after the exhaust valve closes and the pistons downward travel creates the pressure depression that draws the intake charge back in. the mixture now has patches of inert gasses that do not burn. this mix burns in layers, one at a time starting at the plug. if layer 3 is loaded with contaminants, layer 10 is not going to be burned as rapidly as it could have been without inert material. chamber pressure and temperates are lower, torque falls off. intake manifold design, to an extent can be used to compensate for reversion by ensuring a more positive pressure at the inlet valve during the overlap period than what is in the chamber.
i personally like to think of the blowdown (initial high pressure"poof" that comes out immediately when valve opens) as a comet. the head is thick while the tail tapers off. in a book i read, the head of the gas column is likened to a piston in an engine. it makes compression in front of it, and vaccuum behind it. this is the pressure that is used to manipulate the scavenging of adjoining cylinders through merge paired headers.
Quote:
Originally Posted by A20A1
I would rather tune the tube lenghts and diameters to get the most power for my application.
exhaust pulses travel at roughly the speed of sound, which varies somewhat with RPM, gas temp and pressure. (edit: thanks to luke for pointing out that intake is velocity dependant while exhaust speed is more "fixed.") while i am all for planning out a sweetspot, there is no tuned length that works perfectly everywhere. i think good results can be obtained using an exhaust that scavenges particularly well at one rpm and an intake that "supercharges" well at another, achieving a broad powerband.
i am not qualified to say i know which works or doesnt first hand, someday maybe. for now i can only piece together bits and pieces from the tests and experiments of others and form assumptions.
one thing i do know, sharp edges and abruptness make turbulence, and turbulence flows significantly slower than a laminar flow. not to imply that exhaust gas is laminar, i think the reynolds number is too high but am unsure. if the fluid flowing one direction sees a completely smooth path it should have little obstruction in that direction. if travelling in the opposite direction there IS an obstruction that creates turbulence, less mass of this gas should travel as far given the same amount of time than it would without the obstruction. i believe this is the idea behind anti-reversion chambers. i dont like the chambers as much as i like port extension into a larger primary tube at the flange.
so we know that there is no way to maintain a negative pressure at the valve at all RPMs and that flow reversal will occur. by placing this obstruction in the path of reverse flow, perhaps we can shear a percentage of the mass that would otherwise enter the chamber. if successful, the chamber will contain a lower percentage of contaminants. the burn rate will be increased and detonation resistance increased. having more room for fresh fuel is just icing on the cake. might it be a miniscule difference? certainly. so is the difference between going to the next round of eliminations or loading the trailor.
Quote:
Originally Posted by S2 INSIDER
theoretical mumbo jumbo!
next time my little brother's digital camera is here ill try to snap some photos of pressure graphs broken down by crankshaft degree from some books.
when the clutch is disengaged the tire might as well be bolted to the crank. a fast car is one whos crankshaft has the ability to transition from a low RPM to a high RPM in a short amount of time, IE accelerate. i guarantee reversion hurts acceleration, and until i personally build/test several headers and see HP, TQ, BSFC and ET suffer, i am all for anti reversion devices.
hope this helps some of the newer guys.
