Why Vboost

[Marauder03]

Hi my question is why not have the vboost open all the time? I know the butterflies start to open up at 6k and I just read they don’t open fully until about 8k so why doesn’t everyone unplug the vboost and just run them open all the time? I’m sure there is a good reason for it I just don’t know it, is it all performance based or is fuel economy a factor as well? I’m sure people have experimented with this and I’m curious about the results. I had a 1997 with a stage 7 jet kit and now have a 89 with a header and unsure of the jets that are in this one but they get similar gas mileage, which I didn’t buy the bike for good mpg obviously lol I’m thinking of unplugging mine just to see what happens any real reason not to? Just curious, I’m new to this forum and have spent hours reading and learning here what a great source for information!

 

[veebooster]

Some do exactly that. Rather than using the stage 7 tubes they just unplug vboost while open. That way they can still sync carbs. Try it see if you like it. I think you'll find though that Yamaha had this design down pat. Your bike will likely run better with vboost stock config.

 

[MaxMidnight]

There isn't any reason why you shouldn't, at least you will be able to experience the difference.

Why V Boost?

Engines are full of compromises; they have to be able to produce useable power from just above tick-over to some 8 1/2K in our case.
They also need to be able to be able to get the best fuel mileage for the state of tune whilst also meeting emission regulations.

One factor that will determine how well the above parameters are met is how well the fuel is atomised i.e. the smaller the droplets the more complete the burn, the better the 'bang' the bigger the force exerted on the top of the piston which will produce more torque. Torque x revs = power.

One aspect that determine how well the fuel atomises is how fast the air goes over the main jet. At low engine speeds you need a small diameter venturi to maximise the air speed.
As the engine revs rise there comes a point where the venturi can't pass enough air to supply the cylinder. The engine runs out of puff.
This is where V Boost comes in. By taking the mixture from two carbs you have have potentially doubled the intake area available to flow the air.
The V Boost butterflies open progressively as the demands of the engine increase.

By having the V Boost butterflies open all of the time the engine will perform its best at peak air demand but as the revs drop so will the air speed over the main jet which means poorer atomisation of the fuel which means smaller bang....you can figure the rest?

 

[02GF74]

The V Boost butterflies open progressively as the demands of the engine increase.

I've been researching vboost and that from what I've found that isn't strictly accurate.

Vboost had 3 wires, 2 for power and third is from ignition, essentially the rpm.

The valve opening is based purely on rpm so independent of any load.

 

[MaxMidnight]

Where have I said it was? I was referring to the volume of air required to fill the cylinder.

 

[02GF74]

I was picking you up on thev use of "demand". To me demand is the amount of combustible material = fuel + air that an engine needs, or demands to perform a task.

The vboost valve opens depending on rpm. So it will open the same amount at the same revs whether the bike is being run stationary out of gear or going up a steep hill - the load or demand is not the same in each case.

 

[MaxMidnight]

The original question was 'Why V Boost' and I was explaining that to maximise atomisation of the fuel a high air velocity through the venturi is desirable.

To recap, at low revs smaller diameter venturi work best but can't flow sufficient air demanded (or required if you prefer) at higher revs.

You say 'demand is the amount of combustible material = fuel + air that an engine needs, or demands to perform a task.'
Without V Boost the carbs would not be able to do this.

My original post made no reference as to how the butterflies were operated; I just tried to answer the o.p's original question.

 

[Buster Hymen]

Hi my question is why not have the vboost open all the time? I know the butterflies start to open up at 6k and I just read they don’t open fully until about 8k so why doesn’t everyone unplug the vboost and just run them open all the time? I’m sure there is a good reason for it I just don’t know it, is it all performance based or is fuel economy a factor as well? I’m sure people have experimented with this and I’m curious about the results. I had a 1997 with a stage 7 jet kit and now have a 89 with a header and unsure of the jets that are in this one but they get similar gas mileage, which I didn’t buy the bike for good mpg obviously lol I’m thinking of unplugging mine just to see what happens any real reason not to? Just curious, I’m new to this forum and have spent hours reading and learning here what a great source for information!

This is the best description of the theory behind vboost https://www.bennetts.co.uk/bikesocial/news-and-views/features/bikes/sayonara-yamaha-vmax

 

[Zeus36]

Here are some comments on running V-Boost fully open from the web as well as this site:

Sean says:

"With the V-Boost unhooked into the open position you can't accurately sync the carbs since the vacuum draw is pulling off of both. With it closed the draw is strictly each separate carb."


Another site says:


Having vboost open at lower RPMs will normally make the bike run rich because of double carbureation (intake pulses from adjacent cylinders force incoming air back through the venturi and this double pass picks up more mixture). Usually, if the vboost is locked "on" all the time it is necessary to install a larger PAJ2 (secondary pilot air jet) in order to lean out the pilot mixture.


Posts here on this forum say: ..Dyno tests have also shown that you have less torque / power between 3 & 6k.


Comments on this forum indicate: Full on V-Boost has dropped folks MPG and therefore the fuel range.


Another site says:

The engine isn't throttled up enough to benefit from extra fuel at 3000 rpm's, it's too early to bring the second carb online. It just results in a rich condition. I've seen it on the dyno many times, it actually costs you HP.

Description and operation of the V-Boost is on page 7-61 and 7-62 of this manual:
https://www.yumpu.com/en/document/read/19061326/vmx12-service-manual-v-max-outlaw

 

[9SecVmax]

What a topic. It is still coming up 30 years later. You would think by now, you would have accurate information on how the Engineers came up with V-Boost. Its been so long for me that I may miss something. If you dig hard enough, you will find GK dynamics theory of operation. So much to convey, just not enough time to post all of it. Zues36 is the one that essentially described the "hidden" design that few actually get. Yes, two carbs will feed one cylinder. Length of the runner is tuned to RPM like any other motor. However, using the adjacent cylinder at the proper time will induce a negative intake pulse into the cylinder that acts like a "mini supercharger". GK dynamics never changed there thinking for the Gen II. It's all about accelerative fun. Period. Its the "hit" that makes you smile. There are bikes that are faster. But are they as fun?

Now, does anyone know were the 120HP club went? You'll see me there at 12 or 13 at 130.1 Uncorrected HP on a DJ150. NO stage 7. Stage 7 kit was VERY difficult to tune. I HATED DJ jets. Taking V-Boost off LOST the fun and drivability. You had a nasty hole from 3k to 6.5k then you make 3HP more peak. Was it worth it? Hell NO. My '98 spent more time on the dyno than riding it. I was hell bent on getting the tune absolutely perfect. Took a few months and hundreds of dollars of dyno time and experimentation. ALL Dyno jet kits were always a struggle to get them perfect. Period. I stopped using them with there proprietary main jets and went with Marc's at Factory Pro. Way Way better needles. Then, a friend in Canada turned me on to a very cool idea. Needles from a 1984 Venture that were adjustable. Cant buy in the US. Non US models only. Thank you Tyler P in Ontario.

OK, the year I spent playing, I was right back to V-Boost on. Y off. I even made an adjustable controller using a N555 timer. I could open early at 2k and as late as 8k. It felt quicker opening at 4k. But the dyno showed otherwise. Took controller off. Made consistent 128rwhp SAE every time I ran it. It would WHEELIE in first gear rolling into it to hard. I rode with a friend with UFO Vgas. I'd beat him every time doing roll ons on the I20 in SC. Then he built a PCW 1500. I got whacked.

OK, another tidbit. I had multiple 2004 Kawasaki 700 V-Force ATV's. Someone came up with cross plane intakes to mimic V-Boost. Very impressive intake. I had to flash back and try them. So, I had to retune the 32mm carbs( i think) with way smaller pilots, I used ZRX1200 Factory Pro Needles from Marc. I tuned it while racing it with a wide band LM2 A/F Meter. ABSOLUTELY NO GUESSING. OMG was it a scary ATV. My friend Mike had a BRAND new CanAm 1000 sport. This was more than 10 years ago. He would pull me to about 100 ft then I would leave him for dead. He was so pissed that I beat his 10,000 dollar atv with a beat up used one that he rode off and wouldn't talk or buy me the beer he owed me. Um, it had Stage 1 Hot cams in it to...to be fair.

So what am I saying? Leave boost alone. Just make sure its clean and functioning. Call Factory Pro in Caleefornia and see if they still have the kits.

 

[robdean45662]

Hi fellow makers I just bought a 1998 max and it has a toggle switch someone must have added. The seller says it works to turn on the vboost?????? Does anyone know about these?

 

[MaxMidnight]

There are various gizmos that trick V Boost to opening earlier. Having a switch to turn it off is not commonly talked about as V Boost is one of the reasons folk buy a Max.
You would best be advised to follow the wires and see where they lead (if you'll pardon the pun).

 

[DirtieGirtie]

Rob - I'm in the same boat. My new to me 2002 has a toggle switch below the left side of the gauge cluster. I'll get a pic next weekend when I get home. Once I get my other bike sorted, I'll figure out what this switch does. I feel no difference up or down.

 

[robdean45662]

Same here mine is on the frame just ahead of the faux tank however I was told that when the switch is in middle position its off, pulling it towards the rider adds a certain percentage of boost and if pushed forward adds the total boost. It seems to run a little smoother in either position.
I've only ridden around town so far and what short blasts I have taken hasn't really gotten the full amount of power yet.
I will say I'm not sure I even needs boost this thin is crazy fast with the switch turned off.
Thanks for your help.

 

[NixToy]

The switch was wired to open Vboost at 3k. It was widely used in my time. I never dyno'd it, but Seat-of-the-pants feel is quicker acceleration when WOT from stop lights. Yes, mileage goes down.

 

[Jack Hammer]

Vboost is best kept in stock operating mode. Least amount of secondary trouble. Think about your riding habits.

 

[putin]

The switch was wired to open Vboost at 3k. It was widely used in my time. I never dyno'd it, but Seat-of-the-pants feel is quicker acceleration when WOT from stop lights. Yes, mileage goes down.

Mine has 3 positions: always on, off and stock. When always on - sound like 2 cylinders and vibrates. Also hard to start even with choke open. I realize it when switch by mistake.

 

[Fire-medic]

I prefer VBoost stock, and always-on to idle by the Harley-Davidson contingent.

 

[robdean45662]

I was just afraid of starving a cylinder or flooding it.
Also probably fir a different thread but what fuel do you guys use?
I read where it need Premium and it seems to not like it. Spits and sputers

 

[MaxMidnight]

I suspect that Yamaha chose to open V Boost when they did because that's when you get the benefit.
You won't starve or flood the cylinders by altering the opening point...and IMO you wont get any benefit other than a different power delivery which may well be less than is normally available.
Unless you have increased compression the only benefit of Premium fuel is to the profit margin of the retailer and refinery.