Velocity stacks pros and cons??
- Thread starter jcw0411
- Start date
Swapmeet Louie
Put some metal in yer eye....
search this is a well covered subject.
DesmoBro
Busted Nut
I personally have never run them (I too think they look Boss) The draw backs i've heard are the obvious sucking in a rock at speed, and that they are only for race applications (if you plan to rebuild the motor again anyways) For daily driving purposes Pods are your next best bet....Crazypj Is basically the go to guy for small cb's from what i've heard you send him the carbs and Voila Pods work great
surffly
Over 1,000 Posts
Amsterdam360 said:
might want to search for the millions of threads with people that cant get them to run right.
Dont be fooled but that "pre jetted" joke that some claim.
even 30mm is overkill on a stock motor. Really I can see zero reason to run mikunis on a street engine. And im sure that most bike would be better served by spending the $350 on other things.
Stacks offer several advantages over other intake types.
Mainly, it's a flow issue. Without filters of any sort, there are less restrictions on getting air through the engine and anyone serious about building for power knows that air is what you're after. That's not all there is to the story, though. Obviously running without any intake will remove all the restriction and so the stacks do something very special and it all has to do with the way they're shaped.
Because of the bellmouth design, stacks smooth the air as is passes through the intake. Not only does this provide a stronger signal to the carbs, it also allows the air to form a uniform pattern while traveling through the intake tract. Air that is smooth and all traveling in the same direction is more dense than air that is flowing every which way (such as it is after passing through filters and such). All things considered, velocity stacks will offer slightly more horsepower than other intake options.
The secondary consideration with stacks is that they usually come in different available lengths and this allows you to "tune" your intake in order to operate best at certain RPM ranges. I wrote this document a while ago and I've learned a bit since then, but this is the basic gist:
[quote author=Sonreir]As many of you may already know, CV carbs don't respond well to changes in the intake system that introduce turbulance. Turbulance, mostly, is due to pod filters and other "high flowing" intake systems. The solutions to this problem are numerous and include construction of air boxes to provide "still" air, introduction of air correction systems within the intake itself, and even sticking with the stock air box and making no intake changes whatsoever. For those whose minds are set on a solution that will provide better airflow while still allowing the use of their stock CV carbs (as opposed to a switch to Mikuni VMs or the like) a velocity stack offers itself as a good option. A velocity stack serves a number of purposes. First and foremost, it smooths air flow entering into the intake. Smooth airflow means MORE airflow. Because the air molecules are all heading in the same direction and not fighting among themselves, you can make more air and more fuel into your cylinders. More air and more fuel means more power!
Additionally, in reference to our CV carbs, smooth air means we can tune them more reliably and not be forced to a switch to VMs. The secondary reason to use a velocity stack is to increase the (you guessed it) velocity of the intake charge. This is a good thing because the intake valve is still open when the piston begins its compression stroke (this is especially true for high duration cams). What this means is that if the intake charge is moving too slowing, it won't have enough interia to overcome the increasing pressure in the cylinder caused by the rising compressing piston and the fuel/air mixture will be forced back out of the intake, costing us power due to poorer cylinder filling. Finally, the tertiatary purpose of a velocity stack is to change the length of your intakes. This allows you to create an intake length that will resonate at a certain frequency (RPM). This resonance creates pulses or waves of higher pressures within the intake and the object is to time one of these pulses to hit the intake valve just before it closes. A well designed velocity stack with full taper will accomplish its primary and secondary purposes with little left over for you to worry about and so we will focus the remainder of this document on the resonance effect (also called pulse tuning or ram induction).
Much like small waves lapping up against the edge of a pool and then heading back toward their origin, these resonance pulses will rebound and travel back and forth along the intake tract. The number of bounces the pulse incurs before being utilized by our tuning determines how we address the pulse and also the amount of pressure the pulse creates. For instance, the third time the wave bounces back and forth is called the (funnily enough) third wave or third pulse. Just like any other wave that bounces between objects, much of its energy is spent during the reflection process and so each each higher numbered wave will have much less power than the lowered number wave before it. "Easy!", you say. "I want the most power and so I will tune for the first wave!" Well, that's exactly what we want, in theory, but this is often quite difficult to do in practice. The speed at which these pulses travel is variable depending on your intake conditions, but is roughly the speed of sound (1125 feet per second) minus a bit for any bends or restrictions.
Now lets examine your intake system a bit more closely so we can figure out how to best capture that rebound. Before we begin with the math, decide which range in the RPM you wish to the effect to take place. As with nearly evertything in engine performance, you have to choose a "best" and in order to make the most of your machine you're better off stacking all of these "best" points at exactly the same spot. For argument's sake I will choose 8000 RPM. For the following calculations, I will also assume the stock duration for a Honda 360 which is 221°.
1.) A 4-stoke engine rotates twice for every opening of the intake valve, and so we use 720° as a starting point.
2.) To determine how long our intake valve remains closed, we take 720° and subtract the duration of our cam at 221°. The answer is that the intake is closed for 499° of the crankshaft's rotation.
3.) Now we need to figure out the number of rotations per second by taking our 8000RPM figure and dividing by 60. We have 133.33 RPS.
4.) Now convert this number to degrees per second by multiplying by 360°. We have 48000°/sec.
5.) Next, take the number of degrees the intake valve is closed and divide that by the number of degrees of rotation per second and we have .010395 seconds.
6.) Finally, we take the number of feet per second which the pulse is travelling and multiply that by the number of seconds we calculated at step 5. The answer is 11.695 feet. Now divide this number by two because the pulse which we are measuring is, first, traveling away from the closed intake port and rebounding back to it. This gives us an ideal intake length of 5.848 feet in order to catch the first pulse.
Obviously a six foot intake is going to be longer than many of our bikes and so capturing the first pulse is rarely, if ever, feasible. Even targeting the second pulse can be difficult and NASCAR builders usually aim for third pulse. Depending on your application you should try to get fourth pulse but can settle for as low as sixth. Knowing the necessary intake length for each pulse you capture is as simple as dividing the number we got from step six by the pulse number. For intance, to capture the fourth pulse, we divide 5.848 by 4 and get 1.462 feet, or 17.5 inches. Doable, but still long. Going down to fifth wave gets us 14.0352 inches, so we'll give that a try.
Because this is the total length of the intake and not just the length of the velocity stack, we need to first subtract the existing intake length before we can determine the length of the velocity stacks. The intake length from intake valve to carb venturi opening for a Honda 360 is 214mm. A quick metric conversion from 14.0352 inches minus 214 mm gives us 142.5 mm. So for our particular application, we're looking for velocity stacks that are 142.55 mm in length. Simple, no?[/quote]
As for cons, there is one big one. Lack of filters means your engine is sucking is dust and one of the major components of dust is silica and aluminum oxide. Pretty much the same stuff the goes into making sand paper. Every five thousand miles or so you'll want to pull the engine and spec it for wear. Pay special attention to piston ring gaps, cylinder walls, and valve seats.
Mainly, it's a flow issue. Without filters of any sort, there are less restrictions on getting air through the engine and anyone serious about building for power knows that air is what you're after. That's not all there is to the story, though. Obviously running without any intake will remove all the restriction and so the stacks do something very special and it all has to do with the way they're shaped.
Because of the bellmouth design, stacks smooth the air as is passes through the intake. Not only does this provide a stronger signal to the carbs, it also allows the air to form a uniform pattern while traveling through the intake tract. Air that is smooth and all traveling in the same direction is more dense than air that is flowing every which way (such as it is after passing through filters and such). All things considered, velocity stacks will offer slightly more horsepower than other intake options.
The secondary consideration with stacks is that they usually come in different available lengths and this allows you to "tune" your intake in order to operate best at certain RPM ranges. I wrote this document a while ago and I've learned a bit since then, but this is the basic gist:
[quote author=Sonreir]As many of you may already know, CV carbs don't respond well to changes in the intake system that introduce turbulance. Turbulance, mostly, is due to pod filters and other "high flowing" intake systems. The solutions to this problem are numerous and include construction of air boxes to provide "still" air, introduction of air correction systems within the intake itself, and even sticking with the stock air box and making no intake changes whatsoever. For those whose minds are set on a solution that will provide better airflow while still allowing the use of their stock CV carbs (as opposed to a switch to Mikuni VMs or the like) a velocity stack offers itself as a good option. A velocity stack serves a number of purposes. First and foremost, it smooths air flow entering into the intake. Smooth airflow means MORE airflow. Because the air molecules are all heading in the same direction and not fighting among themselves, you can make more air and more fuel into your cylinders. More air and more fuel means more power!
Additionally, in reference to our CV carbs, smooth air means we can tune them more reliably and not be forced to a switch to VMs. The secondary reason to use a velocity stack is to increase the (you guessed it) velocity of the intake charge. This is a good thing because the intake valve is still open when the piston begins its compression stroke (this is especially true for high duration cams). What this means is that if the intake charge is moving too slowing, it won't have enough interia to overcome the increasing pressure in the cylinder caused by the rising compressing piston and the fuel/air mixture will be forced back out of the intake, costing us power due to poorer cylinder filling. Finally, the tertiatary purpose of a velocity stack is to change the length of your intakes. This allows you to create an intake length that will resonate at a certain frequency (RPM). This resonance creates pulses or waves of higher pressures within the intake and the object is to time one of these pulses to hit the intake valve just before it closes. A well designed velocity stack with full taper will accomplish its primary and secondary purposes with little left over for you to worry about and so we will focus the remainder of this document on the resonance effect (also called pulse tuning or ram induction).
Much like small waves lapping up against the edge of a pool and then heading back toward their origin, these resonance pulses will rebound and travel back and forth along the intake tract. The number of bounces the pulse incurs before being utilized by our tuning determines how we address the pulse and also the amount of pressure the pulse creates. For instance, the third time the wave bounces back and forth is called the (funnily enough) third wave or third pulse. Just like any other wave that bounces between objects, much of its energy is spent during the reflection process and so each each higher numbered wave will have much less power than the lowered number wave before it. "Easy!", you say. "I want the most power and so I will tune for the first wave!" Well, that's exactly what we want, in theory, but this is often quite difficult to do in practice. The speed at which these pulses travel is variable depending on your intake conditions, but is roughly the speed of sound (1125 feet per second) minus a bit for any bends or restrictions.
Now lets examine your intake system a bit more closely so we can figure out how to best capture that rebound. Before we begin with the math, decide which range in the RPM you wish to the effect to take place. As with nearly evertything in engine performance, you have to choose a "best" and in order to make the most of your machine you're better off stacking all of these "best" points at exactly the same spot. For argument's sake I will choose 8000 RPM. For the following calculations, I will also assume the stock duration for a Honda 360 which is 221°.
1.) A 4-stoke engine rotates twice for every opening of the intake valve, and so we use 720° as a starting point.
2.) To determine how long our intake valve remains closed, we take 720° and subtract the duration of our cam at 221°. The answer is that the intake is closed for 499° of the crankshaft's rotation.
3.) Now we need to figure out the number of rotations per second by taking our 8000RPM figure and dividing by 60. We have 133.33 RPS.
4.) Now convert this number to degrees per second by multiplying by 360°. We have 48000°/sec.
5.) Next, take the number of degrees the intake valve is closed and divide that by the number of degrees of rotation per second and we have .010395 seconds.
6.) Finally, we take the number of feet per second which the pulse is travelling and multiply that by the number of seconds we calculated at step 5. The answer is 11.695 feet. Now divide this number by two because the pulse which we are measuring is, first, traveling away from the closed intake port and rebounding back to it. This gives us an ideal intake length of 5.848 feet in order to catch the first pulse.
Obviously a six foot intake is going to be longer than many of our bikes and so capturing the first pulse is rarely, if ever, feasible. Even targeting the second pulse can be difficult and NASCAR builders usually aim for third pulse. Depending on your application you should try to get fourth pulse but can settle for as low as sixth. Knowing the necessary intake length for each pulse you capture is as simple as dividing the number we got from step six by the pulse number. For intance, to capture the fourth pulse, we divide 5.848 by 4 and get 1.462 feet, or 17.5 inches. Doable, but still long. Going down to fifth wave gets us 14.0352 inches, so we'll give that a try.
Because this is the total length of the intake and not just the length of the velocity stack, we need to first subtract the existing intake length before we can determine the length of the velocity stacks. The intake length from intake valve to carb venturi opening for a Honda 360 is 214mm. A quick metric conversion from 14.0352 inches minus 214 mm gives us 142.5 mm. So for our particular application, we're looking for velocity stacks that are 142.55 mm in length. Simple, no?[/quote]
As for cons, there is one big one. Lack of filters means your engine is sucking is dust and one of the major components of dust is silica and aluminum oxide. Pretty much the same stuff the goes into making sand paper. Every five thousand miles or so you'll want to pull the engine and spec it for wear. Pay special attention to piston ring gaps, cylinder walls, and valve seats.
Racer X
Active Member
Hi My name is Eric and I run open velocity stacks. 
I resisted it for years and would not do it on the street. But I have been through it on the dyno. The stacks make more top end power and that is all I need to see. I can pull my motor down between runs, So wearing out pistons and cylinders is of no concern.
Ram air and an air box has proven to make better throttle response out of a turn at 150 mph . Honda has spent years developing there ram air system, But for a bike that is only going 100 to 125 mph its stacks all the way.
I resisted it for years and would not do it on the street. But I have been through it on the dyno. The stacks make more top end power and that is all I need to see. I can pull my motor down between runs, So wearing out pistons and cylinders is of no concern.
Ram air and an air box has proven to make better throttle response out of a turn at 150 mph . Honda has spent years developing there ram air system, But for a bike that is only going 100 to 125 mph its stacks all the way.
jcw0411
Active Member
surffly said:
Yea man. This is why I am asking now before I make changes. I appreciate all the info from everyone. Thank you Sonreir for the education. In my case cracking the engine every 5k miles doesn't really sound good. Ultimately I just want the most performance with the least drawback. Surrfflly like most things you can search on the internet I have found good and bad things about the mikunis. I slapped some cheap k&n style filters on my keihins and with minimal tuning had it idling and running well. The bike I have when I bought it had no air filters or anything. The other thing that I wanted to investigate was space and that is ultimately what drew me to the velocity stacks. I want to minimize the amount of space my carb setup takes up for battery and whatnot. Like I said I am very new this is my 1st bike and I appreciate all the feedback. This site is tops! I have a lot to think about.
