I notice that sometimes people have confussion on what restriction, air flow velocity and resonating effects is.
I wanted to do a write up that would help people to understand how everything works together and explain things a little better.
Restriction is a matter of how much air flow (volume) should be ingested (or forced) or exhaust flow should be sent out of the exhaust system at one point and it is not causing a "bottleneck" in flow which will coincidentally make the engine work harder to either get air or send out exhaust waste.
There are a lot of people who are very quick to change their intake manifolds without understanding how everything plays into effect of each other.
Bigger isn't always better and there are reasons behind it...
Lets look at it from the point of view of intake manifolds:
When your intake valve opens, the piston is moving downwards in the cylinder which is creating a strong pressure wave drawing air into the combustion chamber to achieve maximum cylinder filling. Maximum cylinder filling depends upon the capacity of air (the amount of air able to enter at one point) and the velocity of air entering (the speed that the air is traveling towards the combustion chamber).
There are key parts that determine the capacity:
1. Cylinder Head Intake Ports diameter
2. Intake manifold runner diameter
3. Intake manifold plenum diameter
4. Throttle body size diameter
5. Air box / Intake tubing diameter
For most vehicles, the intake ports will technically be the most restrictive part of the intake system but this does not mean this is a bad thing necessarily. Now of course, the larger the diameter, the higher capacity of air that could be present at one point. Example: Think of a cup that is 2" diameter compared to a cup that is 3" diameter that are the same height, the 3" diameter cup would be able to consume a higher volume of fluid at one point than the 2".
Here is the kicker though of why bigger isn't always better...
Remember, the engine is a big air pump, as the piston is moving downwards, it's drawing in it's own pressure of air (vacuum) into the combustion chamber but the larger the key parts are, the lower the pressure will be and the slower the speed of air will travel into the combustion chamber.
The bad side of slowing down the velocity of air...loss of low end power. Why? Imagine a straw that is 1" in diameter and you are using your mouth to suck in air and you do it a certain strength of drawing air in. Now change the straw size to 2" and once again use your mouth to suck in air at the same strength. Theoretically, air will travel slower (velocity) with the 2" straw than the 1" straw. The straw is your intake system, your mouth sucking in is the same effect as your piston moving downwards to BDC (Bottom Dead Center) during the intake stroke and the intake valve being open.
Focusing on the intake manifold, the longer and narrower the runners are, the higher the intake velocity will be. These types of runners are optimal for low end torque cause like said, air is capable of traveling in faster with less intake strokes occuring (less engine revolutions occuring) but is not necessarily optimal for high RPM performance (above 5-6k RPMs). Shorter and Wider intake runners are optimal for higher RPM performance because you have a higher air capacity which is necessarily for as many engine revolutions (think: more engine revolutions, the more intake strokes that are occuring, the more air that needs to be present in order to spray more fuel and keep your powerband climbing) but is not optimal for lower RPM or low end power because of the slowed down air velocity with the wider runners. Also note, the more revolutions that occur, the stronger the velocity build up.
Now you might think to yourself "How can I have best of both worlds?"
A couple ways to think of it but it's more of a expensive route of experimenting and designing a intake manifold. There are calculations out there to help you develope the best possible manifold design according to the powerband you're looking for and the amount of airflow that you require to achieve the power you're requesting.
Valve Overlap plays a large effect into this as well.
First let me explain that when it comes to exhaust systems and picking out the most ideal exhaust system, velocity of exhaust flow should be the most important part. With that said, every vehicle/engine with an exhaust system will HAVE backpressure.
When your exhaust valve opens, the piston moving towards TDC (Top Dead Center) is pushing exhaust waste outwards to the exhaust system but at the same time it's creating a exhaust pulse (positive pulse) that resonates into the exhaust system. When this wave reaches the end of the exhaust system, the action of your intake valve opening creates a negative pulse to travel back through your piping, into your combustion chamber and back outward to your intake manifold. This moving of pulse waves is called a resonating effect and it all happends in about the speed of sound.
How does valve overlap help you? When the wave travels all the way through your exhaust system, back into your combustion chamber and into your intake manifold, as your exhaust valve closes and the piston moves towards BDC during the intake stroke, the pulse waves help increase the velocity of the sluggish air traveling from the atmosphere. If you have a short and wider intake manifold runner while having a properly sized exhaust system and camshafts that create a sufficient amount of valve overlap (example: On a DOHC vehicle), this could be a very optimal setup while having a great midrange to high end powerband.
Going back to what I said earlier on how your intake manifold being smaller and more "restrictive" is not always a bad thing because the smaller diameter will increase the velocity of the air charge before it enters the combustion chamber assuming the valve is open.
Another note about resonating effects is that when the piston does reach BDC and the valve closes, air that was being drawn is now going to hit the valve and cause a resonating effect and send a pressure wave back outwards to the intake manifold plenum and out the throttle body opening and as the intake valve opens again, the pressure wave now helps draw in more air and increase the velocity of the air charge once again into the combustion chamber.
To explain more about camshafts and valve overlap if you don't understand
Valve Overlap is the occurance right in between the end of the Exhaust Stroke and the beginning of the Intake stroke because both the Intake Valves and Exhaust Valves are open which as I explained the cause of this can improve resonating effects which can help you improve cylinder filling.
Camshaft duration is talking about how long the valves are open for.
Camshaft lift is talking about how far the valves are being pushed downwards into the combustion chamber.
Camshafts do nothing but move around your powerband and can also effect the velocity of air depending on the RPM. Granted I mentioned that valve overlap can increase resonating effects but it's all to a degree and it all depends on the rest of the intake system (including the ports) and exhaust system (including ports).
The valves are another thing that can effect the velocity of air for the same reasons. Increasing the size (diameter) of the end of the valves and the valve seats will of course increase the possibility of having a larger capacity of air entering at one point but once again...can effect air velocity.
Now to some wanting to ask "How does this all play into effect with forced induction?"
Forced Induction changes the game a bit because now instead of relying on vacuum pressure creating by the natural occurance of your piston moving downward, you're relying on a compressor to now compress the air and force it down the engines intake system. This is also going to increase the engines VE (Volumetric Efficiency) even to the point of going pass 100%. Basically, a engine's displacement is talking about the capacity of air that could physically be able to be drawn into the engines cylinder at bottom dead center (stroke x bore) naturally. If now you're having a compressor send in MORE air than it could naturally draw in, you're now reaching the possibility of it reaching more than 100% Volume Capacity during it's natural capabilities.
Depending on the type of compressor you're using, you might or might not have a intake manifold and your compressor might be pre or post throttle body (Pre Examples: Turbocharger, Centrigual Supercharger - Post Example: Roots Type Supercharger).
If your compressor is pre throttle body, increasing the size of the runners, plenum and throttle body (talking about diameter size) it could be able to support a higher CFM of airflow that your compressor is capable of flowing through, which means, you could still keep up a great velocity of air traveling inwards throughout your intake system and into your combustion chamber.
If your compressor is post throttle body, you are almost following the same laws as before BUT the turning of your roots supercharger will make a stronger draw in for airflow but the air traveling from the intake box or tubing before it reaches into the supercharger housing will NOT be above atmospheric pressure, it will be just at a higher vacuum pressure traveling inward. So basically, increasing your throttle body or intake tubing can and will slow down the air traveling towards the throttle body.
Going back to valve overlap, it is kind of a touchy subject when it comes to forced induction applications. Typically people say that you want less valve overlap with some setups, some say it doesn't matter.
If you have compressed air traveling inwards during the intake stroke, for optimum power, you would think that with less overlap, you would have less wasted pressurized air traveling out of the exhaust system towards the end of the exhaust stroke which means MORE cylinder filling, hence, more air molecules and more of a possibility for you to spray more fuel and create a stronger ignition on the top of the piston to create more power ultimately. At the same time, with some valve overlap occuring this would help resonating effects.
The touchy part typically comes up with turbo setups. They say you should have less overlap for this reason:
Since the turbine on the turbocharger is of course reliant on the exhaust energy from your engine, the amount of exhaust exiting will effect the response of the turbine which ultimately effects how fast your compressor wheel will spin to create X pressure that you're aiming for (Note: Compressor wheel has to spin a certain RPM to create X pressure in the compressor housing to shoot towards the throttle body). If you're increasing valve overlap, you're lessening cylinder filling and also you're shortening the strength of the exhaust being pushed outwards to the turbine. This also exposes hot exhaust gases to cooler temperatured air entering the combustion chamber which may lower the EGTs (Exhaust Gas Temperatures) which will alter the energy of the exhaust leaving the exhaust valves. This may effect the response negatively.
How much negatively can it effect you? Depends on your setup BUT from my knowledge (not to sound cocky at all) of turbocharger systems, there are MANY ways to improve the response of your turbocharger like....
1. Increasing the size of the exhaust ports
2. Looking into a non-journal bearing turbocharger
3. Properly sizing the turbocharger to your engine
Those are just some simple ways. In my opinion, it for the most part shouldn't make or break your performance unless you have a very high strung engine. Also note that valve overlap isn't necessarily needed for resonating effects of the negative pulse "pulling" in a intake air charge because of the fact that there is a turbocharger on your exhaust system with a turbine. Post turbocharger as well, exhaust velocity isn't important and backpressure is your enemy for the fact that it can disrupt the response of the turbine spinning in one direction and pressure waves fighting against it's operation. The most ideal exhaust system for a turbo setup is to be as short (lenght) and wide (diameter) as possible so it has the littlest backpressure being created especially for the fact that velocity "laws" don't matter after the turbocharger.
Another note about Valve Overlap, increasing Valve Overlap may cause issues with fuel enrichment just from a fact that it will change the consumption of air at wide open throttle (WOT) but also during idle and low RPM conditions (this is of course saying that if you DON'T have Variable Valve Timing system that doesn't involve Cam Phasing), fuel enrichment requirements may change now in order to have a balanced idle quality and better driveablility. Remember, increased overlap is not only happening at wide open throttle but in ALL engine conditions. If you have a MAP based system, this is especially true because now you're having a constant fluctuation in manifold pressure...which...is how your MAP sensor is only able to meter air (this is of course talking about a stock computer & calibration). If you have a MAF based system, depending on the aggressiveness of the camshaft profile, a fuel enrichment calibration might not be so critical for the fact of how your MAF is able to meter air. This of course depends on your application.
I hope everyone has learned a little bit or a lot from this write up. I just see a lot of people who might not have caught onto how things effect each other and it's not all about 5 + 5 = 10 HP because that's not at all how it works. Everything effects each other and there are much more details that I left out that I would highly suggest everyone look into. Resonating effects are important and effect a engines operation and how efficient the engine runs. If you did indeed read all of this, you will see how repeatative it is but it just goes to show how everything plays into each other.
Hopefully this all makes sense. I wrote it at 5 am without any sleep lol
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