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Discussion Starter · #1 · (Edited)
Help me understand A/F ratio

So how exactly does a full system exhaust and a free flowing air filter change the AF ratio? During the first stroke in a 4 stroke engine, air and fuel is added through the intake port, which is controlled by the ECU. But adding an exhaust and/or filter doesn't actually change the amount of air that goes into each stroke does it? It just makes it more free flowing, but doesn't increase the amount of being used. The ECU controls all of that.

Unless I am wrong and that adding a free flowing system will somehow increase the amount of air that is inputted into each stroke which means that the air within the piston is more dense which in turn makes the bike more lean?

Any help is appreciated, thanks guys
 

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It does change the amount of air going in.

An engine has a fixed volume. As the piston goes down, there is room for a certain amount of air. Unfortunately most of the time it doesn't get filled with 100% fresh air with each cycle of the engine. Some products of combustion don't get displaced by the incoming fresh air and hang around for the next stroke. This oxygen depleted air does nothing for generating horsepower and just takes up valuable space. Increasing flow with different cam profiles, better flowing heads, and better exhausts can increase the amount of fresh air entering and combustion gasses leaving. Therefore we have a higher percentage of available usable air on each stroke. More air + more fuel = more horsepower.

There is way more to it than that and you can even get numbers over 100% but that's a whole other story.
 

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Discussion Starter · #3 ·
It does change the amount of air going in.

An engine has a fixed volume. As the piston goes down, there is room for a certain amount of air. Unfortunately most of the time it doesn't get filled with 100% fresh air with each cycle of the engine. Some products of combustion don't get displaced by the incoming fresh air and hang around for the next stroke. This oxygen depleted air does nothing for generating horsepower and just takes up valuable space. Increasing flow with different cam profiles, better flowing heads, and better exhausts can increase this percentage of available usable air. More air + more fuel = more horsepower.

There is way more to it than that.
But having that oxygen depleted air will still be there whether or not the air filter is there or not right? Having a more freeflowing air filter wouldn't get rid of that air. Which comes back to this, which is quoted from K&N:


7. Can a K&N filter give my engine too much air flow?

No. An engine can only draw in a certain volume of air depending on the engine's size (measured by such things as bore, stroke and number of cylinders). Vehicles are designed to accommodate large changes in air pressure so they can operate at sea level or at an altitude of 14,000 feet. Engine computers adjust the amount of fuel required as a result of changes in air pressure (density). Air filter restriction when the filter is new and especially as the filter loads with dust will result in lower air pressure and availability similar to being at a high elevation. High-flow air filters that were invented by K&N were designed to reduce the work necessary to pull air through the filter and to increase air pressure. Increased air pressure is one of the key elements in producing more power.

And I don't think that the K&N directly allows more air in each compression, but more like provides each compression with more available air which then lets the piston do less work to bring air into the system. Doing less work to pull air in at the higher rpms is way noticeable, that's why the given hp gain.
 

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But having that oxygen depleted air will still be there whether or not the air filter is there or not right? Having a more freeflowing air filter wouldn't get rid of that air.
It might. If the air filter is the restriction it would have an effect. You have to change your filter when it gets dirty don't you? Why...because it becomes a restriction to flow. You need to be able to move air in to move air out. Now if a clean OEM filter is actually a restriction is a whole different arguement. :)

Doing less work to pull air in at the higher rpms is way noticeable, that's why the given hp gain.
That's not really how it works. We aren't really that concerned with how much work it takes to pull the air in since air is pushed by atmospheric pressure, not pulled. (although at some miniscule level there probably is a pumping loss there). We are more concerned with how much air actually moves in.

Here's the wiki on volumetric efficiency. It should help you get your head around what is going on. There are whole books on the subject. Make a pot of coffee and give google a whirl. Your head will spin with how much goes into an engine design...and how brilliant the truely good professionial engine builders are.

http://en.wikipedia.org/wiki/Volumetric_efficiency

I'm going to work. I predict 4 pages by time I get home. :)
 

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Discussion Starter · #5 ·
Thanks for the link, I'll give it a read and I'll let you know if I have anymore questions =] So air isn't actually injected into the system, but forced in through pressure? Is that what you're saying?

Haha, I'll read the link when I get home. I'm at work also ;)
 

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Discussion Starter · #6 ·
Ok so I read it and it makes sense. Now I understand how the filter affects teh A/F ratio but what about the exhaust? Slip on changes the af ratio but not that much correct? How does adding headers to it change it that dramatically? I know the stock headers change in diameter and the aftermarket ones usually are consistent in size throughout the whole piping. That allows for more air flow but how does that affect the amount of air being used in each compression?
 

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The camshaft event timing as set by the manufacturer takes into account the exhaust flow and restrictions to fill the cylinders to an acceptable level. Notice that both intake and exhaust are both open for a time in what is called 'overlap'. This is the most critical phase of cam timing regarding the exhaust system.

If you reduce the exhaust restriction and thereby increase the amount of flow, it can and will cause an imbalance in the combustion chamber with factory timing events. More intake charge will flow into the chamber and can actually flow right across and out the exhaust if the timing is not correct for the flow/pressures being encountered. It is all a delicate balancing act...and changing anything in the system can upset this balance.

Sometimes people purposely upset this balance one way or the other to try to move the powerband to another rpm range or increase the intake flow by reducing the exhaust restriction. ANY large change will require tuning the A/F ratio to produce the same optimum results that were had before the change. Simple!:)
 

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Discussion Starter · #9 ·
Re: Help me understand A/F ratio

The camshaft event timing as set by the manufacturer takes into account the exhaust flow and restrictions to fill the cylinders to an acceptable level. Notice that both intake and exhaust are both open for a time in what is called 'overlap'. This is the most critical phase of cam timing regarding the exhaust system.

If you reduce the exhaust restriction and thereby increase the amount of flow, it can and will cause an imbalance in the combustion chamber with factory timing events. More intake charge will flow into the chamber and can actually flow right across and out the exhaust if the timing is not correct for the flow/pressures being encountered. It is all a delicate balancing act...and changing anything in the system can upset this balance.

Sometimes people purposely upset this balance one way or the other to try to move the powerband to another rpm range or increase the intake flow by reducing the exhaust restriction. ANY large change will require tuning the A/F ratio to produce the same optimum results that were had before the change. Simple!:)
Thank you for the background information! Looks like I have more reading to do =] But that makes sense now. I always wondered because the exhaust is placed AFTER combustion occurs haha. That always got me confused, but thanks. Time to google!

Quick question however. When making it more lean, does that cause more overlap or less overlap? If changing the air flow to be less restrictive, does that change the amount of time the overlap happens? Or just the amount of air coming into and out of the cylinder?

Added to dictionary. :D
Oh man haha, I didn't even know I wrote that! That word was from my middle school days, guess it's still lingering around :)
 

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Quick question however. When making it more lean, does that cause more overlap or less overlap? If changing the air flow to be less restrictive, does that change the amount of time the overlap happens? Or just the amount of air coming into and out of the cylinder?
The overlap is determined by the cam profile. Like you said, leaning it out will just reduce the amount of air being added to the mixture.

If you haven't already, look into exhaust scavenging/pulsing. It's relevant to this discussion and pretty damn interesting.
 

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Discussion Starter · #11 ·
The overlap is determined by the cam profile. Like you said, leaning it out will just reduce the amount of air being added to the mixture.

If you haven't already, look into exhaust scavenging/pulsing. It's relevant to this discussion and pretty damn interesting.
Hi onecartel! Do you mean that leaning out is the excess of oxygen in the mixture? And are you saying that overlap and a/f are independent of each other? Overlap is basically the time when the intake and exhaust valves are open correct? Just to make sure haha. So by adding a free flowing exhaust, the pressure within the cylinder moves out faster when the exhaust valve is open. Would this in turn allow the air coming in from the intake valve to escape out of the exhaust also? How does this change the amount of air that is used in each compression? That's what I don't get.

Thanks for replying!
 

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Discussion Starter · #12 ·
For those of you interested, onecartel mentioned exhaust scavenging, which I just started reading. This is an interesting article about it:

http://honda-tech.com/showthread.php?t=2614040

It ain't just pipes, it's science! Part 1

I’d like to try to explain some basic exhaust theory and clear up some
issues that may not be completely clear.

Everyone knows the purpose of an exhaust system is to provide a means for
the exhaust gases to be removed from the cylinder. You might wonder why
have an exhaust system at all. Other than the frequent need to muffle the
noise of rapid combustion, why not simply open the exhaust port to the
atmosphere, thereby saving both weight and expense?

Some time back in early internal combustion engine history, it was
discovered that attaching a length of pipe to the exhaust port (probably
to direct the noxious exhaust fumes away from a passenger compartment or
out of a room where a stationary engine was housed) often had an effect on
the performance of that engine. Depending on parameters such as pipe
diameter and length, the performance could be adversely or positively
impacted.

I expect it was clear from the very beginning that exhaust gases have
momentum. What may not have been known at the outset is that they also
exhibit wave properties, specifically those of sound. Both those
properties can be utilized to evacuate the exhaust gases more quickly and
completely. The usual term for this removal process is “scavenging.”

There are two types of scavenging: inertial and wave. Inertial
scavenging works like an aspirator whereby some of the kinetic energy of a
moving fluid stream (air, water, etc, generally in a pipe) is transferred
to the fluid in an adjacent pipe. You may remember from high school
chemistry lab class where you used water traveling through the top of a
“T” fixture to draw a quite powerful vacuum in an attached vessel.

The “T” can be likened to a merge collector as used in virtually all
successful racing cars (although often not in dragsters). The most
effective merge collectors minimize the volume increase at the juncture of
the pipes. If this volume is too large, gas speed is diminished and less
kinetic energy is transferred to the gases in an adjacent pipe. Thus, the
scavenging is less complete. Well, so what if there is a little gas left
in the pipes? Consider the engine cylinder as an extension of the exhaust
pipe. A cylinder with residual exhaust gases has less room available to
accommodate the incoming charge of gas and oxygen. Obviously, the more
gas and air you can get into a cylinder, the more power is developed; that
is why superchargers are so effective.

Not only can scavenging be utilized to empty the cylinders, it also can
help to draw in the new charge, by producing a negative pressure in the
cylinder. This gets tricky because there has to be adequate time in which
both the intake and exhaust valves are open, and there is the potential
problem of the new charge passing right through the cylinder into the
exhaust pipe! Gas is wasted and power is lost. Maybe you can design your
cam such that it closes at just the right time to prevent this from
occurring. Or maybe you can make the exhaust pipes just the right length
so that the reflected sound waves (at a particular engine speed) prevent
the incoming fuel and air from spilling out of the cylinder. More on this
later.

A stock S4 engine has very little valve overlap (some at small valve
openings) and therefore there is only a short time during which scavenging
of the cylinder can be accomplished. Even still, there is opportunity for
significant performance gains with effective scavenging of the primary
exhaust pipes (the first pipes that emanate from the ports) where it’s
possible to produce a negative pressure so that when the exhaust valve
opens, exit speed is increased. The result is increased momentum and
possibly improved cylinder evacuation.

On to wave scavenging. An analogy would be tuned organ pipes in which
their length is adjusted such that a standing wave of a particular desired
length (and frequency) is established. This means that some whole number
of waves will fit exactly within the length of the particular pipe. When
the point of maximum amplitude of a wave comes to the end of the pipe or a
change in diameter, the wave is reflected back up the pipe, but as its
mirror image. Thus a positive pressure wave is reflected as a negative
pressure, or rarefaction, wave which, in turn, helps to draw spent gases
from the pipe/cylinder. Wave scavenging is most effective over a narrow
speed range that can be adjusted by changing the primary pipe length.
Thus a torque or power peak can be designed to occur at a particular
engine speed to suit the application whether it is racing or everyday
driving.

What are crossover headers? There are numerous types of headers, tri-Y,
equal length, stepped, unequal length, crossover, etc. Unequal length
headers are by definition not tuned at a specific rpm; rather each pipe is
tuned for a different speed. They tend to perform better than the stock
manifold and may increase performance over a broad speed range. Because
of their unequal length, each pipe will utilize wave scavenging at a
different speed, thus reducing the effect at any single or narrow band of
speeds. They often have sub-optimal merge collectors and so, do not make
the best use of inertial scavenging. Equal length headers can be
excellent wave scavengers, but often have inferior collectors, so inertial
scavenging is not optimized. The tri-Y design is especially good on
4-cylinder engines and is now being used almost exclusively on NASCAR
engines with 8 cylinders. Stepped headers gradually increase the pipe
diameter going away from the port. I believe at least one of the purposes
is to inexpensively approximate a megaphone which is the most efficient
device for returning the pressurized gases back to the surrounding
atmosphere.
 

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So by adding a free flowing exhaust, the pressure within the cylinder moves out faster when the exhaust valve is open. Would this in turn allow the air coming in from the intake valve to escape out of the exhaust also? How does this change the amount of air that is used in each compression? That's what I don't get.
If the cam timing was set for a restrictive exhaust system, the substitution of a low restriction one can and often will cause fresh intake charge to exit out the exhaust on overlap. As the intake charge is now being pulled out the exhaust more total charge is made to flow through the carbs/FI.

A less drastic situation happens when the exhaust just causes a low pressure to happen inside the combustion chamber which pulls on the intake as the intake valve opens on overlap. This negative energy helps the intake charge get started into the chamber and so long as the exhaust valve closes before the charge begins to exit nothing is lost and additional intake charge is made to flow. Pretty neat.

Another situation can happen where intake charge is allowed to pass through the chamber and out the exhaust but is then pushed back in just as the exhaust valve is closing. This prevents the charge from going to waste and is the desired result of the reflection tuning mentioned in your link above.

What will happen on your or my bike is firmly in the 'it depends' category when you change the exhaust or intake. Just remember that the better way to cure a mismatch from an intake or exhaust change is to alter the cam timing.
 

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Discussion Starter · #14 ·
If the cam timing was set for a restrictive exhaust system, the substitution of a low restriction one can and often will cause fresh intake charge to exit out the exhaust on overlap. As the intake charge is now being pulled out the exhaust more total charge is made to flow through the carbs/FI.
I was no whizz in physics so let me get this straight. Low restrictive exhaust means more pressure within the cylinder. This will force out the exhaust faster right? And while it's exiting, during overlap, some intake will exit also. What is the significance of the total charge?

Also, another question, when injecting, does the intake valve release a certain amount into the cylinder each cycle or does it just keep putting more air/fuel in until the valves close?

A less drastic situation happens when the exhaust just causes a low pressure to happen inside the combustion chamber which pulls on the intake as the intake valve opens on overlap. This negative energy helps the intake charge get started into the chamber and so long as the exhaust valve closes before the charge begins to exit nothing is lost and additional intake charge is made to flow. Pretty neat.
How exactly does it cause lower pressure within the cylinder? I understand the simplicity behind it but I don't know how it gets there. Is this with the less restrictive exhaust?

Another situation can happen where intake charge is allowed to pass through the chamber and out the exhaust but is then pushed back in just as the exhaust valve is closing. This prevents the charge from going to waste and is the desired result of the reflection tuning mentioned in your link above.
I got this :)



What will happen on your or my bike is firmly in the 'it depends' category when you change the exhaust or intake. Just remember that the better way to cure a mismatch from an intake or exhaust change is to alter the cam timing.
Alter cam timing? So for example, using a power commander to make it lean or rich, you're basically altering the cam timing? Or changing the amount of air/fuel injected each cycle?
 

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Also, another question, when injecting, does the intake valve release a certain amount into the cylinder each cycle or does it just keep putting more air/fuel in until the valves close?

Alter cam timing? So for example, using a power commander to make it lean or rich, you're basically altering the cam timing? Or changing the amount of air/fuel injected each cycle?
Air isn't injected.

I don't mean to sound like a jerk, but you are getting way ahead of yourself. Try to find some cutaways of a 4 stroke engine in action on youtube or something. You really need to get a handle on how air moves thru an engine before the rest will make any sence.
 

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Discussion Starter · #16 ·
Air isn't injected.

I don't mean to sound like a jerk, but you are getting way ahead of yourself. Try to find some cutaways of a 4 stroke engine in action on youtube or something. You really need to get a handle on how air moves thru an engine before the rest will make any sence.
Hahah, gotcha, not hard feelings. I kinda feel the same way. Might step back and look at this from another perspective.
 

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So for example, using a power commander to make it lean or rich, you're basically altering the cam timing? Or changing the amount of air/fuel injected each cycle?
PC changes the injection duration (and some the ignition timing) while the cam opening and closing points are controlled by their physical positioning. To change the cam timing you must change the relationship of the timing chain sprocket to the camshaft, or substitute another cam with the lobes ground in different places. There are slotted gears available that allow you to move the cam in relation to the chain which makes it open sooner or later as you wish.

How exactly does it cause lower pressure within the cylinder? I understand the simplicity behind it but I don't know how it gets there. Is this with the less restrictive exhaust?
The exhaust 'slug' has mass and velocity and thereby momentum. If the system is properly designed the void behind this exhaust pulse traveling down the pipe will be below atmospheric, and if the intake valve is opened into the chamber (which is below atmospheric as it is responding to the still open exhaust valve) the negative pressure will help the intake charge flow into the chamber.

Also, another question, when injecting, does the intake valve release a certain amount into the cylinder each cycle or does it just keep putting more air/fuel in until the valves close?
The cams open the valves when the lobes press down the buckets and allow the springs to press them back closed as the lobe moves to the base circle. This is fixed by the machining of the cam. (many cars now use variable valve timing but it hasn't spread to the bikes...yet. I'm sure it is coming:)) When a valve opens, if there is a pressure differential across the ports flow will happen...be it exhaust or intake. Of course, the exhaust has a much easier time getting out of the engine as it is pressurized to many times atmospheric while the intake only has that atmospheric pressure to push it into the chamber. The intake system can be tuned to be resonant at certain helpful frequencies which can also help cylinder filling.

I was hesitant to start trying to explain things about this subject because while fascinating....it is very, very complicated and chock full of ifs, buts, thens and therefores.:) MUCH too difficult to do much more than scrape the surface here....and that is taking into account that I know only bits and pieces of this to try to help explain things. Better to indulge in some textbooks if you really want to understand what is going on inside the engine.

The nicest thing going here is that the Suzuki Engineers have already done the work for us....and did it pretty well so staying very close to stock will work well for most things. The 1K's respond well to moving the cam timings around and you can alter the amount of torque produced and where in the rev range this happens. Playing with this on your own engine would be more instructive than any books you might find. Good luck in your research!:)
 
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