I'm not sure what you mean (CanadianContender) about loosing velocity and gaining volume. And for anyone else, I am going to try to explain this to the best of my ability so that anyone can understand since this topic comes up all of the time!
Here's one for the archives (I hope):
If the speed (velocity) of the air goes down, then you have less air/second going into the cylinder and you have less volume since area x speed = volume/time.
e.g.
2 mm^2 X 2 mm/sec = 4 mm^3/sec.
So you see, if you take the air flowing through a cross section of the pipe (area) and multiply by its speed you get the volume going into the cylinder at that instant. If you drop velocity then you loose volume.
As far as cracking the secondaries and getting more torque, I don't see it happening either.
The intake manifold has basically two pipes going to each cylinder. One pipe is blocked off with the secondaries until the cylinder is capable of using all that air. Since the RPM's are low, the amount of air moving into the cylinder is less than it would be at high RPM.
e.g. (this example shows a hypothetical engine that moves 1 ft^3 of air in one turn of the engine i.e. one revolution)
2000 revolutions/minute * (1 ft^3) air/revolutions = 2000 ft^3 air/minute
4000 rev/min * 1 ft^3 air/rev = 4000 ft^3 air/min
If the math makes sense, then you see that the engine moves more air at high rpm than it does at low rpm. Lets use the same hypothetical engine and say that if the first pipe going into the cylinder can handle up to 2000 ft^3/min easily with no pressure build-up due to resistance in the pipe, then anything above 2000 ft^3/min will cause a pressure build-up in the pipe. Consequently the engine will starve for air at rpms well above 2000 rpm. Therefore when the engine gets above 2000 rpm you would open the second pipe so that the cylinders can continue to get the full amount of air that they need as rpms go up.
Almost Done!
Now lets say that if the pipe feeding air into the cylinder is too big, the air will move slower to fill the required amount.
e.g. our theoretical engine and primary pipe moves 2000 ft^3/min and we are at only 1000 rpm. We only need 1000 ft^3/min or air at this rpm. This means that with this specific pipe, the air has to flow at only half the speed at 1000 rpm to supply the needed air that it does at 2000 rpm.
Now why does this drop the torque?
Air like all matter has mass, and when it is moving it has momentum. When something with momentum (like a car) hits a block (like a wall), there is a crash as the momentum is converted to energy and the wall or car breaks/crushes.
Well, when air is moving and it hits a block it tends to compress (like being in a tire pump). When the air compresses it gets denser. You see where this is going? If we can keep the air flowing faster, then it has more momentum, and if it has more momentum it will compress more when it goes into a closed cylinder.
Therefore at the "Best" rpm for a particular pipe (the manifold) the air will have a high velocity and relatively low resistance to flow. This will mean that at that particular rpm (the "Best" rpm) the air will have the maximum momentum and volume; hence resulting in the greatest amount of power. This is when we measure an increase in torque.
This whole process can be seen in the graph of the torque curve from a standard v6 ford contour/svt/cougar (anything with a dual runner intake). You see the torque curve start to to peak around 2000 rpm and then begin to drop off. That is where the velocity and volume of air going into the engine are at a maximum...the "tuning rpm" of that portion of the manifold.
Why does the torque begin to drop off? Well, the velocity of the air is still high since the pipe is a constant diameter and more air is being pulled (by the pistons) into the engine. The problem is that resistance to flow as the velocity goes up too high. The engine rpms are going faster, meaning it is moving more air. The primary pipe can't flow enough air to provide all the cylinders with a full charge now that they are filling and emptying so much faster. What we really need now is a bigger pipe. Well the engineers provide that by keeping a blocked-off second pipe ready and waiting. When the rpms get to the right pre-determined spot, the second pipe opens and draws in air. The velocity of air through the primary pipe goes down now as the secondary pipe is now providing more air. There is also a little turbulence as the blades open before the airflow smooths out into two columns. This momentary drop in velocity and the little bit of turbulence are the cause of the 'dip' in the torque curve. At this point you see the torque begin to climb again to a new maximum. This new maximum happens when the airflow through both pipes combined is at the highest velocity without restriction and the cylinders are able to completely fill and the air compresses, yielding the densest charge and most power. As the rpms go higher, the need for more air increases and now these two pipes are no longer large enough; higher volumes of air are no longer possible due to the restriction of the pipes and then the torque begins to drop off once again. But of course at this point you are shifting the gears so it doen't matter.
This explains why opening the secondaries too soon is a bad idea, it also explains why when we widen out the intake and exhaust we flow more air but the velocity is down at lower rpm resulting in less torque on the take-off, but it is much stronger at a higher rpm.
I know this is long and some people may not be able to read it to the end, or even understand all of it. However, I know that some people will learn a lot from it.
Hope this helps,
Tom
warmonger
Will cracking the secondaries improve low end without other mods? Don't know, not going to experiment. Mathematics are nice and all, but only in theory. In practice they are many times proven incorrect. It was once 'mathematically proven' that you couldn't exceed 200 mph in the quarter mile...Stick with practice, leave the theories on the bench.