Headers can help you make more power, but they can also bring it down. That twisting, snarling mess of tubing is the last function in a complicated equation that makes an engine work. If you want to get the most out of it, you need the right pipes.

“Most of the modern header technology has come from years of collecting empirical data from trial-and-error experience,” Over time, these designs have been refined with the aid of computer software. The big header companies have more resources when it comes to designing headers and exhaust components in the virtual world.

This means less cut and weld development, which takes considerably longer. This doesn’t mean that the traditional trial-and-error development is eliminated; it is however compacted, reducing product development time. This speed enhances the market for mass-production designs, but don’t count out the niche manufacturers. While the big boys have bigger budgets, they also tend to focus on the bigger markets, leaving more on the table for the smaller production shops.

How They Work

Headers can provide more than just sending the exhaust to the back. A properly designed header will actually increase the efficiency of the engine, helping to draw more air and fuel into the combustion chamber through a process called scavenging. This process is very similar to a stove pipe or chimney.

Under the right circumstances, the chimney draws air from the room and releases it out the top, this is called drafting. As wind blows across the top of the chimney, a low-pressure area is created inside, drawing air from the room below, which feeds the fire, generating more heat production.

If the chimney is too small, the effect will be reduced and smoke fills the room. If the chimney is too large, smoke fills the chimney, but the lack of drafting reduces the amount of air feeding the fire.

Engine exhaust works on a similar principle, with mechanical pressure coming from the engine side. As the combustion process ends, the exhaust valve opens, releasing the pressure into the exhaust system.

The air moves from the high pressure chamber to the low pressure exhaust. The rate at which the exhaust gasses move is directly related to the initial pressure inside the header (and exhaust system as a whole).

This is called “blowdown” and is the difference between cylinder pressure and exhaust system pressure. Excessive blowdown pressure means less gas moving into the header on its own, requiring mechanical expulsion (pump action, i.e. piston movement). Reducing blowdown increases the rate of gas movement in the initial stage of the process.

Once past the initial blowdown expulsion, when the pressure between the combustion chamber and exhaust system have equalized, the piston takes over, providing the pump action needed to expel the rest of the exhaust and push it along its way through the exhaust until the piston reaches TDC (top dead center) and the pump action ceases.

The exhaust gasses do not stop when the pump action is finished. The inertia of the hot gasses keeps it moving through the pipe. The exhaust pulse is a not a single action, one blast and it’s over, the repeated pulsing functions like a waveform, just like sound waves. If it were blasted into open space (as in no header or manifold) it would have no positive effect on the engine.

Instead, that waveform pulses through the tubing, pushing the gasses in front of it, while pulling the gasses behind it. This creates a vacuum behind each pulse. Overlap between the intake and exhaust lobes of the camshaft means that as the intake valve is opening, the exhaust valve is still open, allowing that newly-created exhaust vacuum to draw fresh air and fuel into the combustion chamber, beyond what is possible through normal atmospheric pressure in the top side of the engine. This process, called scavenging, is one of the biggest benefits of headers over manifolds.

The basic principles of exhaust flow are fairly simple; getting the maximum exhaust from point A to point B is where things get interesting. There are many factors at work in the scavenging and blowdown process. Primary tube length and diameter are the main components in the fight for horsepower and torque. From here, we will discuss the mechanics of headers in terms of short, mid-length and long primary tubes.

Effective header primary tube and component geometry attempts to take advantage of two distinct forces occurring inside the header to increase performance. One is the kinetic energy of the gas stream and the resultant low pressure area behind it, and the other is the considerably greater energy of sonic finite amplitude waves that originate upon the opening of the exhaust valve.In terms of primary length, amplitude wave tuning is limited to long tube headers. Short and mid-length headers are simply too short to take advantage of the length of the waveform. The most common use for shorty and mid-length headers are cost and clearance.

These two factors will override the desire for a long-tube header, as those two factors can be deal-breakers. If it doesn’t fit, it doesn’t fit. That doesn’t mean they are useless. Shorty headers are designed to be as effective as possible in terms of getting the flow moving by removing restrictions.

Velocity is the key to the shorty and mid-length header design. The faster the pulse moves through the tubes, the less pressure builds up inside. This is a function of blowdown.

Reducing the blowdown pressure increases the amount of gasses that are expelled without mechanical assistance, which in turn reduces the amount of effort required to pump the remaining gasses out of the chamber. So you would think that a large-tube shorty header would be optimum in this instance, well, not exactly.

Smaller tubes acts like longer tubes and bigger tubes act like shorter tubes. There is a fine line between too big, just right, and too small.

The key is matching the header to the engine. There is such a thing as over scavenging, particularly with merged collectors. This leans out the intake track, resulting in a loss of horsepower. If it is not sized correctly, a negative loss of mid-range and top end horsepower will be the result.

These two factors will override the desire for a long-tube header, as those two factors can be deal-breakers. If it doesn’t fit, it doesn’t fit. That doesn’t mean they are useless. Shorty headers are designed to be as effective as possible in terms of getting the flow moving by removing restrictions

Building Better Build Quality

While they don’t necessarily provide a finite performance boost, what the headers are made of does matter. Stainless in the same gauge steel is approximately 1.5% heavier due to the dense nickel-chromium content. Stainless has a lower thermal conductivity coefficient, so it helps retain more heat, which helps propagate the exhaust pulse. Stainless steel often doubles the price of a header, but it will also last longer too.

If mild steel is required to meet the budget, adding a ceramic coating to the inside of the header helps. Heat retention does make horsepower according to the dyno guys.

Adding an additional coating on the inside helps control rust and carbon build up inside the tube which would disturb the gas flow. Coating will also help these expensive headers last a long time.

Gaskets & Hardware

Little tricks like making the header flange a touch larger than the exhaust port to increase scavenging requires the right gaskets. The collector gaskets we use are SCE copper. Since the copper can’t crush like a fiber gasket we use a thin layer of copper silicone on each side of the port opening.

As for our flange gaskets at the head, again we use the SCE flat annealed copper. Any gasket that is flat will seal at the header flange. I never use the embossed copper type. They’re made for the flange to match the port opening and won’t work with our headers.

The gasket must match the header opening more than the exhaust port. Any overhang covering the exhaust will cause turbulence, reducing flow and efficiency. Another importance aspect is locking header bolts. Over time, flanges swell and shrink with the heat, particularly with different metals, such as aluminum heads and steel headers. It is also a good idea to re-tighten the bolts after the engine has been warmed up.

There are a few installations tips that you should keep in mind to get the most out of your install. It is highly recommended that you use a flat flange gasket, either the fiber type or an annealed copper gasket. The embossed copper gaskets will fail.

I suggests using Permatex Ultra Black high-temp silicone on both sides of the gasket. Heat and differentiating metals are the enemy of threads and bolts, breaking off a header bolt is a massive pain.

You can avoid that by applying a generous amount of anti-seize to each header flange bolt. You can also use it with slip-on collectors. Sometime you run into an interference point with the header tubes.

I hope this read gives you a better insite on how headers function, their plus and minus columns relative to design, fitment and projected intent of your motorcycle.

“If you want to ride a rocket…………..you’ll need science ! ”

“Stay Tuned” Jon Cornell

Please use caution if you plan on dealing with or have dealt with this company.

Check out our UFO Air Frame or MaxDaddy Wheels.

In keeping with performance information and enhancements for the Vmax, I thought it only right to explain methods of measuring our performance gains ……or losses relative to delta on Mr. Max.

I will explain the various types of dyno’s and the means of measuring the power that they record. There are three basic types :

1.Engine
2.Rear wheel
3.Rear wheel roller

#1 Engine dyno’s. These are the very large eddy current electric dyno’s that read the power of the engine directly straight off of the crankshaft. This method gives the highest readings. As the engine is not turning an alternator, engine sprocket, or driveshaft in our application, rear belt, sprocket or differential, all of the transmission gears. The unit that measures the power output is in effect a very large electric motor ,with an armature that weights in excess of 200 #.

A load resistance is applied to this electric motor, which in turn is measured in foot pounds of torque. Since this power unit is turning such a large mass and weight, when it is spinning at say 3000 rpm, it stays very stable and steady. The rpm is controlled electronically, which gives extremely accurate rpm and torque readings. Since the engine is having to turn such a heavy load, it cannot accelerate rapidly or stop quickly. Very accurate, however.

Dyno Type # 2 or rear wheel type, measures the power of the engine at the rear drive, be it a rear wheel sprocket or differential. With this type of dyno all the power losses from alternator, drive sprocket or universal joint, clutch, transmission, rear sprocket or diff are all being recorded. Usually the rear wheel of the motorcycle is removed and the motorcycle and rear sprocket or in our case the differential are attached to the dyno fixture. The only thing missing is the rear wheel when the engine is running and the rear diff or sprocket is being turned. It is driving a unit called a “water brake”, which is measuring foot pounds of torque. To measure rpm, there is a small light beam which shines on a small gear with 150 teeth. The light is part of the computer control and measures the top and bottom of each tooth, every revolution or 300 readings each turn. When the computer reads 300 of these signals, it then says that is one revolution. EXTREMELY accurate but not a “quick and easy” for the daily tuner shop.

Dyno Type # 3 or rear wheel roller type (eg. Dynojet), measures the power of the engine when the rear wheel is spinning and turning a roller mass that is part of the dyno. Generally these are called “accelerometer” and do not actually measure foot pounds of torque in the true sense. The dyno has a given load all the time and the computer figures the torque and horsepower, say from 2000 rpm to 9000 rpm in a period of time. If you run in 3th gear, you will get one HP reading. If you run in 4th you get another HP reading and running in 5th gear, another reading. Various tires and tire pressures have quite an effect on the readings also. This type of dyno, due to the ease of mating the bike to the measuring device and not having to remove an engine or rear wheel assembly, does very well for tuning engines. Make a change and it shows good or bad. Very nice tool …..in that respect.

At UFO, I have the best of both worlds. I have an in house Dynojet chassis dyno (# 3) and unlimited access to one of my fellow automotive gear-head friends Superflow dyno (#2) the rear sprocket type, however when I use it, normally I will have an engine fixture in place instead of the complete motorcycle, since I use it mainly for engine development programs. This way I can change heads, cams, pipes and fuel systems very easily. The dyno room itself is properly appointed as well. VERY important for accuracy. A 30 by 30 room. Most of the “dyno rooms” you see are condensed into that previously un-used space in the workshop. At this facility we have a separate control room, which allows us to perform tests and we are protected from the noise and wind that is in the room when the engine is running. We use 3 large 5 foot, 10 hp high volume fans to put fresh air into the room and also to remove it. Blowing directly onto the engine we have another 10 hp high velocity fan with air speed of about 110 mph. The air in the room is changed every 2 SECONDS .

This Superflow dyno will record about 139 different things when the engine is running but we usually record only about 10-15 measurements at one time. These items usually tell us exactly what we need to know about the engine’s level of performance and why it did or did not like a certain camshaft grind or exhaust configuration. Most of the time we do not run our air sensor because we have to remove the air cleaner and replace it with this unit to record the cubic feet per minute (cfm) that the engine is breathing. As we begin to make more important changes it gets installed, so that we can see how many cfm it takes to make whatever horsepower the engine is producing. It also lets us know how close our flowbench numbers are to the numbers that the dyno says the engine is really breathing.

One of the things that we rely on very heavily area the temperatures at various parts of the engine and related parts. The exhaust temps give us a great deal of information about how the engine is running at that moment and usually are our first and fastest signal if something is going wrong. Things like oil temp in the crankcase, cylinder head temps front and rear, left and right, all contribute to what is going on with the engine. Other instruments tell us exactly how much fuel the engine is using, how much fuel is being used per horsepower and what the fuel-air ratio or mixture is.

We have a number of other very sensitive gages that tell us all kind of things, like the pressure or vacuum in the crankcase, valve covers, intake manifolds, air cleaners, etc. Crankcase pressure tells us how well the rings are seating and how much cylinder pressure blow-by we have. All of these gages and instruments give us a little more insight into what is happening with the engine when it is running various speeds and loads. With some of the instruments we can ride the motorcycle at different speeds on the road, record the numbers, return back to the shop, hook the motorcycle up to the dyno and duplicate everything that was happening on the road. Horsepower, rolling resistance, wind resistance, torque, fuel consumption, exhaust temp. How much HP does it take to run 65 or 70 mph? Is my mixture correct at this speed. Is my timing correct for this rpm and load? Going up a hill or down a hill at 60 mph, the engine requirements are entirely different! With the proper instrumentation, you can take out most of the guess work as to what the engine wants under those conditions.

So in closing, much of the time people run on the rear roller dynos (dynojet) and the engine is down 10 hp from what they feel that it should be and they have ABSOLUTELY no idea as to what they should check. All that these dyno’s have told them is HP and torque, and these are calculated. Take “chassis dyno” numbers with a grain of salt……….when they are the only measuring device being used to measure your motorcycle’s performance level. They do not tell you what components or factors need to be addressed. Just a HP barometer. Be aware also that many manufacturers will use this type of dyno more as a method to set the hook on a sale of product than look out for your best interest in your bike’s level of performance.

Thanks again for taking the time to read this post. Hope you have a better understanding of putting your motorcycle through a polygraph test.

Jon Cornell

OK, guys this week I am going to write about the illusive bastard us gear-heads spend our lifetimes chasing……..because that IS our life. The Horsepower God. Exactly who is this man? Tech Tips is going to cover a lot of facets of interest in building performance motorcycles so we might as well build our information base with definition of our goals in mind. Too much horsepower is just enough. Every time I build a bike, I build it with the next level of performance in mind. I want my bikes to be able to “outride” me. When I have ridden a bike to the point it no longer intimidates me and it is “under my thumb” then it is time to “turn up the wick” ……….again.

HORSEPOWER:

Horsepower = torque x rpm ÷ 5252

The constant 5252 comes from our old buddy James Watt’s 17^th Century definition of horsepower. He must have been one of the original gear-heads. He just had to have a way to measure his toys enhancements. So, if 33,000 pound-feet per minute is divided by 2 times pi (6.2832), the result is 5252.1008 rounded down to 5252.

For example :  An all stock Gen 2 Max makes a maximum torque of 112 ft/lbs at 6500 rpm when measured at the rear wheel by a dynamometer.

What is it’s hp at this rpm? 112×6500 ÷ 5252 = 138.61 hp.

Horsepower is a measurement of how much work (force over distance) an engine can do in a given amount of time. It is a function of a given amount of torque (force) at a given rpm (distance). At a given rpm, horsepower is directly proportional to torque. The only practical method for measuring horsepower is by measuring engine torque and rpm with a dynamometer.

By increasing torque at a specific rpm, horsepower increases at a corresponding amount. Also, if the torque remains constant but the rpm increases, then horsepower increases in direct proportion to rpm. Even when torque starts to drop off (beyond the engine’s torque peak) , as long as rpm increases faster than torque drops, horsepower will still increase. Horsepower can also be described as how much and how often a cylinder fills, and how often if fires in a specified time period.

TORQUE  = HORSEPOWER X 5252 ÷ RPM

Example: Gen 2 Max makes    172 hp x 5252 ÷ 8000 rpm

This = 112.91 ft/lbs torque. I have backed up these numbers spot on with the chassis dyno.

Torque is a twisting or turning force that engineers measure in “pound-foot” (equal to a 1 lb. force applied at the end of a 1 foot lever). An engine’s power is actually established by first measuring torque at a given rpm and then calculating horsepower.

******NOTE Since horsepower is equal to torque multiplied by rpm, any torque increase results in a power increase at a given rpm level. This is why you are better off concentrating on improving your torque instead of horsepower for the best performing engine. You make the torque……..you will have the horsepower!  Most of the established racers and qualified engine builders concentrate on improving torque within the rpm range their engine needs to operate.

An engine’s torque is determined by the percentage a cylinder is filled at a given rpm. The greater the cylinder fill is, the greater the torque. If power is going to be increased, than you are going to find that it is crucial to improve an engine’s ability to breathe.

Peak torque is reached when an engine runs out of air OR loses its ability to breathe better. This is the point of maximum cylinder fill. As mentioned earlier, an engine can continue to make more horsepower even when torque is falling as long as rpm is increasing faster than torque is falling. So, if maximum torque is the point of maximum cylinder fill, then maximum horsepower is the point where torque is falling off faster than rpm is increasing.

For a given engine, the intake and exhaust tracts, cylinder head(s), cam(s), displacement, stroke and rod length are among the factors that control the amount of torque and the torque peak. Also, the mean airflow velocity can be quite a factor on peak torque rpm.  Mean flow velocity is dependant on runner or pipe cross-sectional area, not volume. Rest assured that the peak torque rpm can be controlled by selecting parts with specific cross-sectional areas.

The most successful race engines are normally designed to turn a higher rpm and have greater cylinder fill and torque at the higher rpm levels. This type of engine design however will narrow the use of that power plants application, for instance a drag race oriented engine where concerns of torque and horsepower below shift power values are not a part of the program. Typical high performance engine modifications generally shift an engine’s maximum torque to a higher rpm range, and this can result in the loss of low end power. The key to building a “happy” engine is to maximize torque in the rpm range most important for meeting your objectives, goals and riding styles.

Although all engines produce torque, an engine that produces peak torque at low rpm if often referred to as a torque engine, while one that produces torque at high rpm is typically referred to as a horsepower engine. Since torque and horsepower curves always cross at 5252 rpm, and engine will always produce more pound-foot of torque than horsepower below 5252 rpm, and more horsepower than pound-foot of torque above 5252 rpm. This explains how a four cylinder in-line four Japanese engine that revs to 13,000 rpm can produce higher horsepower than a V-twin, but make less torque. Long stroke, small bore (under square) engines like the V-twin boys typically are lower revving with high torque below 5252 rpm (like our friends with the Yamaha Warriors). On the other side of the street, big bore, short stroke (over square) engines normally are higher revving and produce high horsepower at high rpm from a relatively low torque reading.

The relationship between torque and horsepower is crucial to understanding the different characteristics of under and over square engine designs and being able to maximize each design’s power output.  So, to sum up in general, increasing the engine’s stroke shifts the torque peak to a lower rpm. Decreasing the stroke (formula one car stuff, very cool) will bring the torque into the higher rpm range.

I hope this week’s Tech Tip gives you a better understanding of the definition of horsepower, torque and how engine configuration comes into play on the particular engine design’s ability to produce them.

See you next week and remember………”Stay Tuned”    Jon Cornell

The Vmax engine likes a BSFC number of .490 to .495 to make maximum power. Ideally I would like to see this number at each RPM, 2500 to 9500 RPM but this just doesn’t happen. Generally the BSFC is high at low RPM, good in the mid range and then high on the top RPM. The BSFC mixture on a lot of carburetors is up and down like a prom dress at each RPM and there is very little you can do about it. When the carb varies widely this way, it is called a carb that does not track properly. This is due in part to carb design, fuel height in the bowl, passage diameters and lengths within the carbs and so on. The BSFC number is a means of judging how efficient an engine is and some of the engines I have worked with are able to get down to .430 to .450 and make peak power. These are very efficient engines and they are making 2 to 2 ½ horsepower per cubic inch. Your generation 1 Vmax is 1200cc or 73.22 cubic inches. The formula for cubic centimeters to cubic inches is : c.c. x .0610239 = c.i. To go back to c.c. multiply c.i. by 16.387. So if your gen 1 Max was down to the .430 BSFC you would be making as high as 182 hp. Most of these BSFC gains are found in the cylinder head. Good head……is good head. Enough said.

There are three different types of carburetors used over the years in the motorcycle industry. Between these three they differ greatly in the means that they meter gas and air. A carburetor is an instrument that operates on air speed and not volume. When the engine is not running the carb has lots of volume but nothing is happening. As air begins to move through the bore or venturi and the air speed picks up, it starts functioning as a metering instrument. The higher the air speed, the better it works. The three types of carburetors are listed below and I will try to explain their operation.

1. BUTTERFLY TYPE
2. CV TYPE : SLIDE AND BUTTERFLY COMBINATION (STOCK VMAX)
3. SLIDE TYPE (AS IN MIKUNI AND KEIHIN FLATSLIDES USED IN OUR VGAS )

First is the butterfly type, such as the S+S and Harley Davidson Screaming Eagle units, basically have two metering or jet systems, which make it rather hard to correct the mixture from just off idle to full throttle. Not very linear fuel delivery /engine demand. It’s like having a two speed transmission, 1st and 5th gear. As the throttle is opened from the idle position, the engine is being fueled from the low speed or pilot jet. As the butterfly opens further, we start loosing the signal at the low speed jet and start relying on the main jet. Problem is that the air speed is so slow at this point, that the signal at the nozzle for the main jet that the fuel does not want to be pulled from the float bowl. The jump from one jet to the other is called the transition period. It is very hard to move from one jet to another without having a hesitation problem. Usually to correct this, we have to richen up one jet or the other, which makes the mixture richer than we want it in other areas. If we make the bore of the carburetor larger ,the air speed is slower and the signal at the nozzle is weaker and the low an mid range rpms suffer more. Today there are a number of companies that manufacturer add-on jets to be used on the butterfly carbs as a third jet system and several help the problem slightly. With a butterfly carb with a venturi of say 42 mm, it is 42mm at idle and 42 mm at full throttle. The air speed over the nozzle in the venture at 60 mph is 19 feet per second, which is very slow for throttle response.

Second is the CV or Constant velocity carburetor (Stock Vmax, gen 1). This unit uses both a butterfly and a slide to control the air flow and meter a mixture over a wide rpm range. The butterfly shaft is controlled by the throttle cable and the slide is controlled by the manifold vacuum. As the manifold vacuum goes down, the difference in pressure above the slide causes it to rise and open up the carburetor bore. There is a very light return spring to push it closed as the manifold vacuum goes back up. The advantage to this type of carburetor and why it is so widely used by motorcycle manufacturers is that this type of carb can be leaned down a great deal (EPA emissions compliance) and also very rider friendly as it will not hesitate when opened quickly. We can open the throttle in 1/100th of a second and yet it may take two seconds for the slide to rise to the full open position. The slide is controlling the air to the engine and not the butterfly. The butterfly is controlling manifold vacuum. Yamaha uses this type of carburetor so that it can meet the government emissions standards. They can lean it way down and not have hesitation when the throttle is quickly opened. With the other type of carburetors, if they are leaned down to meet this test, they will cough and hesitate badly when you try to ride them.

It’s not a tough job to lean most carbs to meet this emission standard and the tests are run in a stationary position. If you take them out on the street with the jetting arrangement that is used for the tests, they will run terrible. Quick throttle movements cause coughing and hesitation. This is the area when the CV type of carburetor shines. There are so many variables in metering one of these carburetors properly, that it would take two days just to list them. When the factory installs a new CV carb and starts to meter it for that specific engine, a carburetor engineer form the carburetor factory will spend 8 hours a day, and a good 30 days changing, adjusting, and fiddling with everything getting it dialed in. Most people only think of changing a jet or so. In a job of this type, the engineer will probably get the main jet in the first 10 minutes of work, which means he will spend the next fricken 29 days, 7 hours and 50 minutes on the balance of the metering.

Last but certainly not least, is the regular slide carburetor (round slide, flatslide) Mikuni and Keihin, Delorto, Bing, and Quicksilver. Most manufacturers of performance motorcycles in the world today use this type of unit. Most likely the reason is that this type is very easily adjusted for each engine and that engine’s requirement. You can put one on a 125cc engine and adjust it pretty good and you can install the same carb on a 1500 cc engine and adjust the metering with good results as well. No way with the other two types. Even though there are about 5 jets in these carbs, each one has a totally different function and corrects only that area. If you are having a problem just off idle, correct it with a low speed jet and it doesn’t effect the rest of the range. If you are having a problem at 60 mph you can correct that and not effect 60 or 80 mph. From idle to full throttle, the air speed over the fuel nozzle is always faster on this type of carburetor than on the butterfly type. At 60 mph on the same size carburetor as the butterfly type, the air speed is 119 ft per second rather than 31 ft per second!>

Most of the slide carburetors are what we call a smooth bore unit, which means when they are wide open the bore is very clean and uncluttered. About all that is in the bore is a small 1/8” diameter needle in the air stream. As the throttle is opened, the slide lifts slightly and all of the air flowing through the bore is passing by the fuel nozzle at very high speeds at an area of only about 10 mm. As the slide moves higher the area becomes 15 mm and the air velocity stays about the same. This continues right up to full throttle or 39mm in our case, with the air speed always very fast. This is the reason the slide carburetor is so responsive. It is what we call a variable venture carburetor. We find that this type of carburetor is not as prone to reversions and fuel stand-off and the regular venturi type carbs. The back side of the slide works as a reversion dam when we have strong pulses in the system.

I have found that a great deal of the mechanics and individual people are afraid of these carburetors due to the fact that they have a number of jets. Because they have a number of them doesn’t mean you have to change all of them. You are 10 times more apt to have to change a main jet on a butterfly carb when you change from one exhaust system to another, than you will with the slide type carb. What all the jets amount to is, that the manufacture can get the things so fricken close that you don’t have to fool with them once set up properly.

I hope this article helps bring more clarity to the thought process behind carburetor design without getting too technical and yet provide you with the reasoning behind why certain types of carburetors have different applications based on intent. Factory carbs……..emissions and “rider friendly”. Flatslides, performance first closely followed by adjustability of individual circuits with target rpm areas.

Thanks for your time. See you next week.

STAY TUNED Jon Cornell

With the ignition off remove the left side chromed plastic “H” shaped cover which is held in place with (2) large phillips head screws that covers your intake manifolds. While looking under the carbs near the middle of the rack of carbs turn the ignition on and you will be able to see a cable pulling a little match box size black box forward. This box is a cable adjustment pod controlling the amount of movement cycle of the v-boost butterfly valve assembly. When the little black box goes to the end of it’s travel forward, the v-boost butterfly valves, which open and close a passage between the front and rear carbs on each side of the bike, will be fully open.

The goal of this exercise it to have the vboost in its fully opened position at all times. We find performance value when applying performance enhancements such as an aftermarket exhaust system. This procedure will decrease the performance of your Vmax if you have stock exhaust or slip ons.

You will find three electrical boxes under the left faux air scoop. The one with Yamaha embossed on it is the servo control module. With the ignition off locate the round connector with five pins (there’s only one round one). Unplug the connector (it’s quite tight) and then plug it back together so it JUST makes contact. Turn the ignition on while watching the cable controlled black box mentioned earlier. Unplug the connector when the box is all the way forward. This is when the v-boost butterfly valves are fully open. It might take a few attempts pulling the plug at just the right time.

Now each of your cylinders is “seeing” two carburetors at all times. I can not count the number of phone call conversations I have had where I have asked
the person on other end of the phone,”Do you understand how V-boost works?“ Nine times out of ten, the answer I get is “It gives the engine more fuel at
6000 rpm. Yes, and no. The v-boost unit is merely an rpm regulated assembly which opens and closes a passage between the front and rear cylinders located in a tube which connects the front and rear cylinder intake manifolds on each side. THE V-BOOST DOESN’T NOT CHANGE THE METERING OF ANY FUEL. The opening and closing of the v-boost has no effect on your fuel economy, only where the metered fuel can travel. Closed, only the fuel and
air mixture from one carb is visible to only one cylinder. Open, and the fuel and air mixture from TWO carbs is visible to each cylinder.

When the cylinder head was put into production for the Vmax in 1985, the engineers quickly found out that at about 6000 rpm the volumetric efficiency of their four valve DOHC cylinder head would out flow the surface area of a single 35mm venturi opening of Mikuni’s largest CV carb available for their application. Whenever we dyno a stock VMax and it is down about 10 hp from the 110 rwhp that we typically see, the first thing check is to see if the v-boost is opening at all, or perhaps not opening all The way. I have run across many that were out of adjustment as well. Our dyno charts show very clearly the difference when a v-boost stays closed vs. when opening @ 6000 rpm or just plain open all the time when we have a stock exhaust in place. A single 35mm carb starts to become restrictive at this rpm. Allowing each cylinder to now have 70 mm surface area of air available deletes this restriction and the engine continues to build horsepower and torque. If you open the v-boost at 3000 rpm, WITH an all stock Vmax, with stock pipes, I guarantee you will make 6-8 less horsepower and about the same decrease in torque as well. We have the dyno charts to back it up. Anything you may read anywhere on the internet is just plain inaccurate, no foundation. There is just not enough piston speed at this rpm to consume The additional dense fuel being metered
from two carbs at this rpm. Don’t you think if there was an advantage to opening it below 6000 rpm the factory engineers would have applied the v-boost earlier?

Now with an aftermarket exhaust and less restrictive intake, like a K+N Filter or four separate filters, we have found the Max likes the more dense fuel charge available from two carbs per cylinder and we meter our jetting accordingly. The typical stock dyno chart on an all stock Max has two “humps” in the
horsepower and torque curve. The lower one is the v-boost operation the higher one is the air box design. When we put our exhaust systems on and remove the stock airbox, re-meter the fuel management with a stage 7 Dynojet kit and run four separate air filters, we are able to get a nice clean linear dyno curve, with peak torque building nicely through the mid-range.

I hope this article helps you understand the theory behind the v-boost as well as it’s operation, function and performance modification there of.

Thanks for taking the time to read this and remember, STAY TUNED!
Jon Cornell