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turbo8u | Jan-22-04 01:11 PM |
Member since Jun 15th 2003
10552 posts,
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#67197, "RE: 4-1 or 4-2-1??? That is the question!!!"
In response to Reply # 8
Jan-22-04 01:18 PM by turbo8u
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Originally posted by ajna_star
the only way to give our cars good low end power is the bottle
uh, marc? lol thats complete and utter bullshit.
read away, and be enlightened.
the (longggg) summary of header design theory. The gist is this. The headers are supposed to route the spent exhaust gas out from the combustion chamber to the atmosphere. But with good design, a header can -expediate- this process by a process called "scavenging". What happens is this.
1. When the combustion cycle has completed, the exhaust valve opens to let the spent exhaust gas go -OUT- into the headers. The upward stroke of the pistons helps to push the exhaust gas out. The over-simplified theory is that once the piston has reached the top of the rotation cycle (where it will start to go down), the exhaust valves closes and the intake valves opens. Thus the combustion chamber has now been emptied of spent exhaust gases and when the exhaust valve opens, fresh air (mixed with fuel vapour from the injectors) will now flow in. The downward stroke of the piston helps to create a low pressure area in the combustion chamber, encouraging more air to flow in.
2. Those who have gone through physics would know of the fact that air flows from an area of high pressure to an area of low pressure. The bigger the pressure differential the more air will flow (and faster). This is a physical law. The ideal situation would be that the low pressure is at vacumn (i.e. no air at all or totally 0 bar pressure) but that is a physical impossibility. Even outer space itself contains very minute amount of particles so except for artificial lab conditions, we cannot even find a nearly perfect vacumn. What this also means is that when the exhaust gases goes out of the combustion chamber, they will NEVER empty completely. There will always be some residual gas left. This of course prevents perfect filling of the combustion chamber during the intake cycle (and why for normal non-race tuned engines such as the lower tech'ed non-VTEC engines, you can never completely fill the cylinders so each cylinder of a 1.6l 4-cyliner 4-stroke engine will never fill up to 400cc, i.e. the volumetric efficiency is below 100%. They often go as low as 75% or less for the bigger c.c. engines found on the luxury cars like BMWs, Mercs, Audis, etc).
3. How do headers come into the picture ? The combustion sequence of 420A is I recall 1-3-4-2 (I think this is correct but it doesn't really matter in the theory). What we want to happen is that the earlier cylinder helps to fill the later cylinder. How ? Take cylinder 4 for e.g. When the combustion completes, the exhaust valve opens. Let's ignore what has happened earlier. What happens now is that the exhaust gas goes out into the "leg" of the header that connects to cylinder 4. Actually it goes out in a pulse (which contributes to the unique exhaust sound). So when the exhaust valve opens, a huge amount of exhaust gas shoots out into the leg of the header, followed by the rest of the gas. The "tail" of the gas or pulse will never completely finish though so there will always be some residual exhaust gas left. Now, the next cylinder to fire is cylinder-2. The same thing will happen when its turn comes to purge the exhaust gas.
BUT, we can do some 'trick' here.
Recall that header designs comes in 4-2-1 (tri-Y) or 4-1 or a few other fancy designs (such as cross-flow) etc. Why so complicated for such a simple thing which only needs to route exhaust gas to the exhaust pipes ?
Well, the process is like this. WHEN cylinder-4's exhaust gas is expelled, it travels from the cylinder head area down towards the collector (i.e. where two more more pipes of the header merges into one). What happens at the header is that when the exhaust pulse flows through it creates a complementary effect - it helps to "suck" any residual gases that may be remaining in the other pipe/leg that connects to the same connectors, helping to create a very low-pressure condition in that pipe/leg. Visualize it this way.
In a 4-2-1 header, pipes for say 4&2 are joined in one collector (3&1 in another), then they join in one final main collector. What happens is that when the exhaust pulse from cylinder 4 goes through the collector, it pulls any residual gases in the pipe/leg connected to cylinder 2 out as well. It helps to promote a very low pressure condition not only in its own pipe but that of cylinder-2 as well. So what happens is that when this very low pressure condition exists, IF the combustion cycle of cylinder-2 SO HAPPENS to have just been completed, when the exhaust valves opens, the purging of the spent exhaust gases will be more complete (since the leg of the header connecting to cylinder-2 will have extra low pressure). This helps to more completely empty cylinder 2 so that when the intake valves opens, more air can flow in. (actually the theory goes a bit deeper than that. By simultaneously opening the intake and exhaust valves, we can also make use of the exiting exhaust gas to help pull more air in. This is what allows VTEC engines to "overfill" the cylinders, acheiving upwards of 105% volumetric efficiency). This process is called scavenging.
Now the extra low pressure condition in the header leg for cylinder 2 will not exist forever, once the cylinder-4 exhaust pulse have passed, pressure equalization will occur again and the pressure will start to rise/equalize with the other areas of the header. So the critical thing in the design of the header is to make sure that each exiting exhaust pulse helps to scavenge another cylinder so that more air can flow into the engine. With more air, more fuel can be injected and thus more power. This means we need to TUNE the lengths of each leg of the header as well as the location of the collectors as well as the size of the pipes AND the collector, etc. Too large a pipe will lower the pressure of the pulse and slow it down as well so it will interfere with the scavenging process. This is why some 4-2-1 headers connects cylinder 4 & 2 together but others will connect 4 & 3 instead. Also the rate at which the exhaust pulses flows is constant (near speed of sound) BUT the interval between the combustion cycles of cylinder 4 & 2 for e.g. SHORTENS as the rpms goes higher. So scavenging that works VERY well at 3000rpm may serve to INTERFERE at 7000rpm. So, if you simply whack a header from what-not engine into your 420A, you may very well interfere with the scavenging process THROUGHOUT the entire rpm band if the header is a mismatch.
_________
96 talon esi-t
san clemente, ca
as needles of ice
are the ill winds' talons
the coldest of shadows
they seep unto the bone
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