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| Induction System
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I just added this to help show and prove the induction wave theory. Above is a dyno sheet of a bike in my shop. The bike dynoed at 90 TQ / 80 HP using a SE air cleaner. The graph above has two runs. The run with the lower TQ was the same bike with the air filter removed. Notice that it had an increase of 3 TQ and dropped 1 in HP. The second run shows a velocity stack that I kept cutting the length to acquire the highest peak TQ. This is a graph of the final cut. The graph actually read 100 TQ before the final cut was made. The final cut is now too short shown by the drop in TQ from 100 to 97. That is an increase of 10 TQ just by playing with the the induction length (changing the induction wave timing). The picture on the right side of the graph is of the velocity stack that was being used. If the camshaft had a different intake valve closing timing, the length of the velocity stack would be different. Notice the extra drop in TQ at 2700 RPM. Also notice the A/F ratio became richer at 2500 RPM and leaner at 4000 RPM. This shows that the VE (volumetric efficiency) dropped at 2500 RPM and the VE increased at 4000 RPM making the AFR leaner. If the AFR was corrected, it would also raise the power even more.
The induction theory is very similar and coincides with the exhaust pressure wave theory. Be sure to read both articles. The intake system (induction system) can be tuned to provide maximum power. Just like the exhaust, you can time the pressure waves in the induction to actually force a lager air/fuel charge into the cylinder.
This theory is derived from the helmholtz theory. You can find many articles on helmholtz theory on the internet. First you need to understand how a wave acts. Take a pale of water where the water is still. Now drop a small pebble into the center of the bucket and you can see that it makes a ripple (a wave) in the water. If you watch the ripple you will see that it travels to the side of the bucket, but it doesn’t stop there. It travels back to the center after bouncing off the sides of the bucket. Then it travels back to the wall of the bucket. It keeps traveling back and forth, but the ripple gets smaller and looses momentum the more it travels.
Check out this video. Now imagine if you did the same experiment on a large pond. The wave would still travel the same as in the bucket, but there are no bucket walls to deflect the ripple. The shore will deflect the ripple, but the ripple will diminish before it reaches the shore. Keep this in mind as we discuss it later.
This also occurs inside the induction system. Think of it this way. If you had a dam blocking a huge amount of water. There is a gorge on the other side of the dam followed by another dam located a mile down the gorge. If the first dam ruptures then all of the water travels down the gorge, then hits the second dam. What happens when the wall of water (the wave) hits the second dam? The wave bounces off of the dam and travels back up the gorge. The “gorge” is the induction system and the “dam” is the intake valve. Of course the “water” is the air/fuel charge.
The reason that I keep saying “induction system” is because the induction system includes any components located from the head of the intake valve to outside air or where the air begins to enter the induction system. When the throttle plate is open and the intake valve opens, the air/fuel charge starts to travel down the induction system and enters the cylinder. Then the intake valve closes, but the air/fuel charge still has momentum and slams into the closed intake valve (kind of like a train wreck), then the wave bounces off the valve and travels back up the induction system. Remember that this happens every time that the valve opens and closes. What happens to the air/fuel charge (the wave) when it is traveling up the induction system (after it bounces off the intake valve) and while the intake valve opens again? You have A/F charge trying to enter the cylinder while the wave is trying to force it in the opposite direction. The result is less A/F charge being able to enter the cylinder. This will decrease the V/E (Volumetric Efficiency) of the engine. The wave traveling in the opposite direction will eventually exit into outside air. Have you ever wondered after building an engine, that it just doesn't seem to have the power that it should? This is one of the reasons why. It's not the biggest parts that you can install in the engine but, it's the combination and the tuning of the induction and exhaust that makes power. Just look at the modern Crotch rocket. If you slap on a set of drag pipes, it will loose most of it's power. Slap a straight pipe on your 2 cycle dirt bike and see what happens to you power band.
If you added a velocity stack to the outside end of the induction system, the wave would travel up the stack then exits the stack. When it exits the stack it will cause a negative pressure wave (reversion wave) to travel back toward the intake valve. This is very similar to the exhaust wave tuning theory. The waves are traveling at approximately the speed of sound. Taking that into affect, it is possible to time the wave to arrive at the intake valve when it is opening. This will actually force A/F charge into the cylinder almost like a forced air induction system, causing a higher V/E.
The way to tune the timing of this wave is to first figure the intake valve duration, open and closed. After figuring out the degrees that the intake valve is open, this gives you the target that is needed to shoot for. Calculate the amount of time it takes after the valve closes until the valve opens again. This amount of time is how long you need for the wave to travel to the end of the velocity stack and the reversion wave to travel to the intake valve. There are many online calculators to help calculate intake length. The shorter the velocity stack, the quicker the wave returns to the intake valve. Lengthen the stack to increase the travel time. Don’t forget that the diameter of the stack and the diameter of the induction system also effects the wave's travel time. This is an induction system on a Range Rover V8. Notice the different length velocity stacks. A plenum fits over the stacks 
If you’re not confused yet, let me add the “Ram Air” theory. This theory goes a step future. Instead of letting the wave escape into the atmosphere or outside air, it is basically trapped inside a plenum to help force more A/F charge into the cylinder. Team Integra has a nice article on this theory
Click on this link Team Integra  This theory is that if you add a plenum at the end of the intake runner or velocity stack, the plenum would act as an accumulator and stores the wave until it is to be sent back down to the intake valve (if timed properly). This is like the pebble dropped in a bucket of water that was discussed earlier. The “bucket” would be the plenum. Just as the wall of the bucket reflects the ripple back to the center, the plenum reflects the wave back to the intake valve. If you increase the diameter of the bucket, it takes the ripple longer to reach back to the center. This is the same with increasing the volume of the plenum. Not only does the length of the induction system affect the timing, but the diameter also affects the timing. There are several article on the internet that can help calculate and fabricate induction plenum.
The automotive industry has been using this theory for many years. It is possible that it first started being used in racing in the 1960’s. Here is a link that helps explain the number of bounces that occur and how they are timed.
Click on this link It would be impractical to capture the first wave due to the length of the intake runner needed. NASCAR is using the third wave to enhance power. This theory is now commonly used in the automotive industry on most automobiles. As we all know, the automotive industry would not spend the extra money to add more cost to the production of an automobile if it wasn’t well worth it. By adding a plenum, the wave will not escape into the atmosphere, but will travel back and forth between the plenum wall and the intake valve. The wave gets weaker (less momentum) each time it travels.
Now go to the next step.
We can time the induction waves to enter the cylinder at a precise time. So, if you had a cam that produced maximum torque at 4000 rpm, and the exhaust was tuned for maximum torque at 4000 rpm, you can now tune the induction system for maximum torque at 4000 rpm. So in theory if these are timed properly, you would have the exhaust pulling on the charge while the induction system is pushing the charge, acting as a forced air induction system. Now all of your power will occur at 4000 rpm, but now you have to wait for the engine to wind up to get to the maximum torque band. You can lower the torque band and re tune the intake and exhaust to occur earlier. One way to do this is to advance the cam timing (see "Camshafts" article). 2,000 – 3,500 rpm is the area that is used more frequently during normal riding. Wouldn’t it be neat to be able time your induction system to peak at different rpms. Guess what, you can! The automotive industry has incorporated the variable induction system. There are many different ways to incorporate these systems. The more common way is to have two different length runners timed for two different induction timings. They use a computer controlled actuator valves to separate the two runners. The longer runner is used for lower end torque, then the valve switches to the shorter runner at a predetermined rpm to increase upper end torque. This results a longer torque band. Incorporate this with VVT (Variable Valve Timing) and you could literally create torque band through any rpm band (depending on how many intake runners you can add). Here are a few pictures of these systems. This is an intake off of a Jaguar V6. Notice the hole in the intake at the bottom side of the picture. A electrical operated valve fits inside the hole, which can open or block flow and route it through the other valve located on the left side of the picture.
Basically there are two separate intake runners (one longer that the other) made in one unit. The valves choose which one flows. 
Jaguar X Type This is a variable exhaust. The vac. pod opens and closes the exhaust pipe which will cause it to reroute the exhaust travel. This lengthens or shortens the exhaust travel depending on which way the ECM wants it to travel. | |
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