By: Brian "Tapper" davis
I'm going to cross post this from my sticky on VTXOA. It tens to answer a lot of questions. Hope you enjoy it.
This is long, so be prepared. Hopefully, its worth the effort. The object here, is to give our readers a good baseline of information about how their VTX's work, and address some of the most common questions produced on this forum in a thoughtful and complete way.
Having read hundreds of posts on these topics, I decided to take a stab at writing something to clarify the many misconceptions on this subject. Whether it’s interminable questions about backfiring/popping, or arguments about “load”, or discussion about whether aftermarket fuel computers are necessary and which is best, we see a lot of bad information and mistaken ideas being tossed around as fact. So lets take a look at the basics, and then talk about a few specifics.
Pressure is the thingThe single most important thing to understand about an internal combustion engine is that at its very root, it’s nothing but an air pump. If you can get your mind around that one simple concept, understanding everything else is a lot simpler and more straightforward.
Ever consider your own lungs? As it happens, your body is also a pretty efficient air pump. When you take a breath, your diaphragm muscle contracts and pulls your lungs downwards in your chest, and this increases their size by stretching them. This increase in volume causes a decrease in pressure (applies vacuum) to your windpipe, and this causes air to rush into your chest. Air always flows from an area of high pressure to an area of low pressure. How fast it flows, depends on the difference in pressure. The bigger the difference, the faster air moves. The bigger the volume, the more air needs to move to equalize the pressure. When you exhale, the reverse happens – your diaphragm relaxes, causing the volume in your lungs to decrease, which increases the pressure inside your lungs, which makes air flow out of your windpipe – to the outside air, which is now at a lower pressure than your lungs.
The pressure is the thing. Differences in pressure make the air move. All around you, at this very minute, is a bunch of air that stretches up above you for several miles. The weight of all that air, presses down on the air below it, and causes the air that surrounds you to be under pressure. This is called “atmospheric pressure”. It is the baseline around which we talk about the pumping of air. You should understand something else here, the concept of “vacuum”. Vacuum, at least with respect to motors, just means “lower pressure than atmospheric”. If you create a vacuum, what you are really doing, is creating an area where the air pressure is lower than the surrounding atmospheric pressure. If you give it a way to do so, air is going to try real hard to kill that vacuum by sending air into it. Nature abhors a vacuum. Air always moves from an area of high pressure to an area of low pressure, and the amount of force it does so with is proportional to the difference between the two pressures. Sounds simple, no? It really is, but it also confuses folks.
Remember I said a motor was just a big air pump? It is, and it works exactly the same way your lungs do. On the intake stroke, as a piston travels down its cylinder, the area in the cylinder increases rapidly – and causes a vacuum. When the intake valve opens, air rushes into the cylinder, and causes a drop in pressure in the intake manifold that is proportional to the amount air moving into the cylinder. On the exhaust stroke, the burning fuel and air has created very high pressure in the cylinder, and when the exhaust valve opens, the pressure in the exhaust header is much lower than the cylinder, so the gas rushes out into the exhaust pipe.
If this sounds simple or obvious to you, then great. But make sure you really do understand these concepts, because understanding the way a motor works means understanding the sequence of pressure changes. If you don’t understand, then none of the rest of this is going to gel for you.
Different Strokes Move the World, Willis.Ok, now its time to see what the air pump really does, and what drives it, and that means understanding strokes. For the purposes of this article, we’ll focus on a 4-stroke motor, since our bikes use them, and since the vast majority of motors are 4-strokes.
Lets simplify, and look at each stroke.
Intake stroke – This is the first stroke in our series of four. In this one, the piston is moving downward in the cylinder with the intake valve open, causing a vacuum to be created in the cylinder and more importantly, in the intake manifold where the carburetor or fuel injectors live. The vacuum (area of low pressure) created in the intake manifold is the power that drives the fuel system, and gets the fuel into the cylinder where we want it. The larger the vacuum (lower the pressure), the more fuel is pumped into the cylinder. The faster the piston moves downward in the cylinder, the bigger and faster the vacuum is created, and the more fuel gets pumped. Therefore, it also follows, that the higher the engine’s RPM, and the harder an engine is working, the higher the vacuum (or lower the pressure) exists in the intake manifold.
At this moment, I hope your brain is saying to you “whoa – that vacuum measurement is sure an important number to know if I’m going to know what my motor is doing”. Damn skippy it is – so lets define something important right now:
The amount of vacuum produced by a motor is a direct measurement of the load placed on the motor. Therefore, when discussing motors, Load means the amount of air being demanded by a motor, which is equivalent to the amount of vacuum developed in the intake manifold, which is the same as the pressure difference inside the manifold and the outside air.
Woo! Now we understand the mysterious load. It’s a great number to know, and is used by fuel injection computers to determine how much fuel to squirt into the intake manifold. It’s also the energy that makes a carb shoot a given amount of fuel out of its jets. We’ll talk about carbs and fuel injection a little later, so lets move on with the strokes.
Compression Stroke – Ok, the piston has moved as far down in the cylinder as it possibly can on the intake stroke (called bottom dead center or BDC), and starts its way back upward. The intake valve closes and so we have a volume of air and fuel trapped in the cylinder, and suddenly the space it occupies is gonna get squished real hard (compressed), as the piston begins moving upward in the cylinder. This causes the pressure in the cylinder to increase very rapidly, since the charge has nowhere to go. One thing to remember here – fuel burns much faster under pressure.
Power Stroke – Ok, all the valves are closed, we’ve got our air/fuel charge compressed really tight, and put it under a lot of pressure as the piston gets to the very tippity top of it’s path through the cylinder (called Top Dead Center). At this point we set the charge on fire, and as it burns it produces a whole bunch of gas, which rapidly increases the pressure in the small space, and pushes the cylinder downward very hard. This is where all the power in your motor comes from, this rapidly expanding gas pressure. A couple of things to note here.
First, we ignite the mix using the spark plug. If the plug were to fire at exactly top dead center (TDC), we could arbitrarily call that the zero position of the crankshaft, and indeed, that’s exactly what we do. Remember, the crankshaft is rotating in a circle throughout this process. One rotation of the crankshaft (360 degrees) means one complete up and down motion of the piston. When we talk about “timing” in a motor, what we’re really talking about is the relationship of the spark to the moment of TDC or the zero position of the crankshaft. If we fire the spark plug before TDC (BDTC, or during the end of the compression stroke), we call that advancing the spark. If we fire the plug after (ATDC, or during the power stroke), we call that retarding the spark.
Exhaust Stroke – Ok, the piston is now hauling Cranberries downwards, and the cylinder is full of hot and probably still burning gas and residual fuel/air. As the piston crosses bottom dead center (BDC) again and starts upward, we enter the exhaust stroke. During the stroke, the exhaust valve opens, and since the cylinder is now at a much higher pressure than the exhaust pipe, all that hot gas and stuff goes flying into the exhaust header, and out the pipe. That’s the end of the 4 strokes, and when the piston crosses TDC again, the whole process starts anew, and away we go.
A couple of things to note here. You’ll notice, that during all four strokes, the crankshaft rotates two full rotations, and travels through 720 full degrees. The camshafts, which actuate the valves, have traveled through one full rotation, or 360 degrees. You often see guys tossing around degree measurements when discussing things like spark timing, cam timing, cam profiles, and so forth. So it’s important to be able to relate the concept of degrees, to the actual rotation of the parts. If you were really good at math in school, you’ll immediately notice that any given instant in the cycling of a motor can be described using simple trigonometry, which should give you some pretty important insights into the ways fuel injection computers and electronics work on your motors. But that’s a bit lofty a subject for this article, so lets move along, shall we?
It Gives me GasWell, now that we’ve got the strokes, let talk about gas, and the fuel system in general. I’m going to focus on describing a fuel injection system, because once you understand that, understanding a carbureted system is vastly easier.
So. We know that load (or vacuum demand) is the way fuel is delivered to the cylinder, but how does the motor know how much fuel? And how much fuel is actually needed?
Lets start, by talking about the air/fuel ratio.
A ratio, just means “amount of one thing compared to amount of another”. In our case, it means amount of air compared to the amount of fuel. When you see someone talk about air/fuel ratios, they usually put the air part first, followed (or divided) by the fuel part. So an A/F ratio of 13 to 1, or 13-1, or 13/1 just means “thirteen parts air to one part fuel”.
Now, lets talk a little simple chemistry. The burning of air and fuel is just a chemical reaction called combustion. Chemical reactions are described by chemists in chemical equations, and the science of balancing these equations, or determining how much of each chemical reacts, is called “stoichiometry”. Lets look at a simple one:
1O2 + 2H2 --> 2H2O
It’s a little hard to read that, but it just means “One molecule (consisting of two atoms) of Oxygen plus two molecules (consisting of two atoms each) of Hydrogen will react and form 2 molecules of water”. You’ll notice, that in this reaction, everything gets used up, and no atoms are left over, and so the reaction is said to be “stoichiometric” (i.e. nothing is left over), and now you know where that big word comes from that gets tossed around when people talk about A/F ratios. It just means “completely burnt with nothing left over”. Glad we got that out of the way.
Now that we understand stoichiometric A/F ratios, lets define a couple of things.
An A/F ratio is said to be “Lean”, when there is air left over after burning.
An A/F ratio is said to be “Rich”, when there is gasoline left over after burning.
Now, the perfect stoichiometric A/F ratio varies based on a lot of factors, like the formulation of the gasoline, or the density of the air, but in general, we can say it’s in the neighborhood of 14.7 parts of air to one part of gas, or 14.7/1 (the VTX really likes 13/1 to make best power).
So now we understand ratios, and what ratio we are aiming at, but how does the motor get the right amount of gas in to make that ratio? Vacuum, that’s how.
A carbureted motor does it directly, by varying the size of its jets. The carb sits in-line between the outside air pressure (air filter) and the manifold pressure (load or vacuum). The action of the air flowing through it into the manifold causes suction in the jets, which pull gas out of the bowl, and spit it into the intake manifold. The bigger the jet, the more gas gets spit out for a given vacuum. But it’s a little different in a fuel-injected motor, since the fuel injectors are turned off and on by the computer (ECU). So how does the computer measure the load to know how long to turn the injectors on? By using sensors.
There are several involved in this process, with the most important being the “MAP” (manifold absolute pressure) sensor, which directly measures the pressure in the intake manifold. The computer then compares this pressure to the outside air pressure, or barometric pressure (obtained by reading the BAR sensor), and now knows just how much vacuum the motor is pulling. In order to get load though, the computer needs to know how much air is being pulled into the cylinders, so it needs to know the air density as well. It does this by looking at the intake air temperature measure by the “IAT” sensor. The ECU has a table of density values in it that compares density at various temperatures and pressures. It just takes the values it read from its IAT and (adjusted) MAP sensor, looks up the density value for these readings in its table, and “hey presto”, your ECU now has a direct measurement of the load on your motor. Now that the ECU knows the load, it just has to figure out how long to turn the injectors on to get the right A/F ratio (which it does by looking it up in a fuel table by comparing load to RPM), and boom – we got gas. The same table is also used to look up the right time to fire the spark plugs (timing). Honda calls this “three dimensional fuel programming”, and I guess that’s sort of accurate.
There are other sensors that play less important roles in the process, like O2 sensors on California bikes, WTS (water temperature sensors), and so forth. But the MAP senor is the big one.
The pleasure of the SensorsBy now, you've probably heard someone say that "every bike runs a little different", and maybe you've wondered why. Didn't all these bikes come off the same assembly line? Didn't the same machine mill all the heads? Isn't it the same configuration, and the same parts? Why do they run different?
It's a fair question. The answer, is that your tune is derived from the plasure of your sensors.
Thing is, no two sensors will report the exact same reading from the exact same thing they are measuring. The reasons for this are many, but the biggest is that Honda doesn't require absolute accuracy from the companies it buys sensors from. They allow a certain amount of variance and error right from the get go. So that MAP sensor from Denso might have a stated accuracy of +- .1v at 90 degrees or whatever. This guarantees right from the start, that you have a certain spread in the tune achieved by the ecu's of all bikes, because there's a spread in what their map sensors are reporting as the manifold pressure. Beyond that, other things can skew the readings reported by the sensors as well - things like outside temperature, dampness in the wires, oxidation in the wire harness, etc. The age and condition of your wire harness greatly effects the resistance of your wiring, and this in turn directly effects the voltages reported by your sensors. Remember Ohm's Law?
Voltage = Amperage x Resistance
So, if you vary the resistance, and the amperage is the same, then the voltage varies. And the ECU reads voltage from the sensors to determine what the sensor is measuring.
So since everyone has a wire harness in different shape, and all the sensors are varying a little in the first place, we end up with bikes tuned all over the map. Thus, we need to find a way to bring our fuel curves back where we want them, and compensate for the error introduced by our wambly sensors.
After Market Fuel ComputersThis is a good time to talk about aftermarket fuel controllers, so lets hit on what they do real quick. The 1300 guys can manipulate their A/F ratios by changing jets, but it’s a bit more complex (and expensive) for the 1800 riders. For the VTX, there are basically three types of controller available. All three do exactly the same thing – they manipulate the amount of time the injectors are turned on during the intake stroke, by intercepting the voltage the ECU sends to the injectors. How the decide how much longer or shorter to turn the injector on, varies a bit though, and that variety is important thing to know when selecting which controller to use on your bike (if in fact you choose to use on at all – none of them are ever really necessary, regardless of what pipes or airboxes you add on to the bike.)
It’s also important to understand this: None of these controllers is ever a necessity, regardless of whether you change pipes or airboxes on your bike. The stock ECU will, in almost every case, adjust to the changes in airflow you’ve caused, and give you a reasonably good A/F ratio. But you should understand, that the ECU is not programmed to give you an A/F ratio that is optimized for horsepower from the factory – instead, Honda worries about things like pollution, engine temperature, and rider perception, and so the ECU can be said to be “de-tuned” in order to address these other concerns. Likewise, you have a certain amount of error introduced by your wambly sensors. So the real function of these add-on controllers is to correct the error (or eliminate the de-tuning) that Honda induced in your fuel curve on purpose, and to compensate for the error in your fuel system sensors, in order to reclaim the lost horsepower and improve engine efficiency (possibly at the cost of making more pollution, hearing more deceleration backfiring, etc). Adding on aftermarket pipes or airboxes can sometimes exaggerate this de-tuning as well, so we need to be able to modify our fuel curves to match the configuration of our bikes. Got that? These boxes aren’t really necessary, but if you’re hunting more horses, they can sure find them. Likewise, it's almost impossible to get your bike in perfect tune without one.
So lets talk about the specific boxes:
TFI – This unit is marketed under several brand names by Techlusion, Cobra, DFO, and others. It does one thing, and one thing only – it extends the fuel pulses produced by the ECU, and therefore, it can only richen the fuel mix – and never ever lean it. Typically, these units are designed to act like variable fuel jets – by adjusting two or three settings, you can add more or less fuel to the mix for a certain RPM range – usually low, middle, and high. It determines RPM by looking at the signal frequency produced by the ECU. Because these units are extremely simplistic, they won’t really optimize your fuel curve, although they can reduce or eliminate decel popping, or sometimes correct a really lean condition, But in all cases, your fuel curve will vary from lean to rich along it’s total fuel curve, so you’re only going to help the lean spots, and hurt the rich spots. These units get sold with a lot of pooey about bikes always being lean, or “knowing load”. It’s crap, since we’ve seen hundreds of fuel curves off the dyno now that amply demonstrate that most VTXs run a little lean at low rpms, and rich in higher rpms (as a rule, but not always). This load nonsense is just that, since all of the available fuel controllers modify the signal produced by the stock ECU. Therefore all of them “know load”. Don’t get suckered by marketing malarkey. Frankly, while these units can reduce decel pop, I can’t see that as a goal worth achieving, and so I never recommend these units. I think you can do better, for slightly more money.
Power Commander III – This device also modifies the fuel output from the ECU, but does so based on a table of values referencing the throttle position sensor (TPS), and rpms. Since the position of the throttle plates directly affects the amount of air being allowed into the engine, the value of the TPS is a pretty good substitute for the “Load” value derived from the map sensor by the ECU. In effect, the PC3 produces an overlay curve of its own, and this can be manipulated to correct the de-tuning in the stock ECU, and for sensor error. PC3’s can either lean or richen the mix, and so are capable of completely correcting the fuel curve produced by the stock ECU. Because the new curve is applied proportional to load, its effective regardless of the ambient air pressure – since the curves are proportional. Tuning the PC3 correctly requires a session on a dynometer, so these units are somewhat expensive – but set up correctly, they will produce optimum fuel curves under nearly all conditions. Of course, changing equipment on your bike will mean the curve is no longer a perfect match, and another adjustment session may be required in order to get a perfect fuel curve, so the PC3 is not without its gotchas. But for best performance, the PC3 comes out on top. It’s a very flexible and effective unit. Newer PC3’s also have functions that can emulate a tunable accelerator pump, or allow you to map each cylinder independently. Good stuff.
HPP – This unit adjusts the output fuel pulse by manipulating the value of the MAP sensor, based on calculations derived from reading the oxygen content of your exhaust with an O2 sensor. In theory, this results in a “closed loop” system, one that adjusts itself looking for optimal F/A ratio by reading the exhaust in real time. In practice, the O2 sensors are pretty inaccurate at higher RPMs, and so the device does some educated guessing, and therefore the fuel curve isn’t always perfectly optimized. However, experience has shown that properly set up, these units perform very well, and are an excellent alternative for someone who doesn’t want the hassle of fiddling with their fuel tables, or going to dyno sessions. They can also adapt when new equipment is installed on the bike. So this is the unit to use when performance with convenience is your goal, and you are willing to give up a few percent of horsepower to gain that ease of use. Decent stuff, not perfect. Unfortunately, these are no longer being made, so a used one is the only way you'll find one. Note also, no support, no repairs available, etc.
"Alpha-N" fuel controller - This unit has been recently released, and claims to be a fully closed loop system - meaning it uses feedback from an O2 sensor to optimize itself at runtime. Like the PCIII, it uses the throttle position sensor and RPM to determine an approximation of the load currently placed on the bike. It uses narrow band sensors only, so the accuracy of the Fuel/air ratio measurement is rough, and so it will tend to produce more accurate results at cruise, than under acceleration. However, this unit is so new, that we have very little experience with it here on VTXOA. And so we'll have to wait and see how it will perform. In theory, it could be a very good box, but experience has shown that the gap between theory and fact is often wide, and so some skeptical thinking is required here when considering it for purchase. Once I've gotten one of these on a dyno, I'll revise this and report on it's efficacy. As usual, it comes packaged with the usual outlandish marketing claims. It's also fairly pricey, although it could be considered comparable to a PCIII + a custom dyno tune.
Alas, there is currently no unit on the market which can manipulate the timing tables, and that sucks. These big motors cry out for the ability to optimize timing tables to match configuration or changes to the fuel tables, and there’s no doubt that 4 or 5 horsepower lurk in the inability to manipulate the spark. Perhaps someday Dynojet will follow through on their promises to provide a timing module, but until then, the ability to manipulate spark effectively simply doesn’t exist. I yearn for a fully programmable ECU. I also yearn for a 20 year old hottie with great legs and big boobs. I’ll probably get my timing before I get the boobies though – more’s the pity.
Burn Baby BurnThe last thing I want to address today is the subject of deceleration backfire, or “popping”. This topic generates a lot of concern from inexperienced riders, or even from experienced guys who just hate the noise, so lets take a look at what causes it. But first things first, lets define the issue:
Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.
No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:
A motor in perfect tune will exhibit deceleration backfiring.Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:
Getting rid of the noise means de-tuning your motor.Yup. If you’ve jut got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing. So lets talk about what causes the problem.
Ok, so you’re riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.
First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.
1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.
So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Honda has added a device called a “programmed air injection valve” (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it’s also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but it’s there, even when you can’t hear it.
So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.
There are a few ways you can do this.
First, use the stock pipe. It will hide the sound, by absorbing it into mass, and masking it with the larger baffle space. Second, you can add more fuel during deceleration. This has the effect of raising the chamber pressure slightly, which burns a little more before the exhaust valve opens. Lastly, you can remove the Pair valve, which reduces the amount of available oxygen in the pipe to burn the unburnt fuel.
Well, there you go. I hope this long piece has given you some information you can use, or clarified things a bit for you. I may ultimately add to this, and discuss valve timing, the effects of compression on performance, or other topics as they motivate me. In the meantime, feel free to criticize, correct, or just tell me how dumb I am. I’m still learning too, after all.
