velocity stack designs

[Varastorm]

Full of cold & can't sleep

So I've been doing some reading about velocity stack designs.

I spotted these threads & thought it was an interesting read.

Found this on the 'Busa site:

The length of the stack- Positive and negative pressure waves travel up and down the intake track caused by the inertia of the air being accelerated and decelerated. The objective of adjusting the length of the intake track is to have a positive pressure wave arrive at the intake valve just as it opens or just before it closes so to push more air into the cylinder. The length of the intake track is important because the pressure wave moves at the speed of sound and the timing of the wave’s arrival at the intake valve is controlled by the length of the intake which is easily adjusted by changing the length of the velocity stack.

There are three major causes of these pressure waves. The first is the lowering of the piston on the intake stroke causes a negative pressure wave. The second is the scavenging of the exhaust which is seen in the intake track because of the valve overlap period. This creates a negative pressure wave in the intake track. The third is the closing of the intake valve. This causes a positive pressure wave due to the sudden deceleration of the air mass behind the closed intake valve.

The pressure wave(s) travels up the intake track and is inverted (positive to negative or negative to positive) at the opening of the velocity stack and is reflected back down to the valve. If the valve is closed when it arrives, it will bounce off the intake valve travel back to the velocity stack opening where it is inverted again and reflected back down to the valve. If the pressure wave(s) is tuned to arrive at the intake valve at the correct time, the engine can see a momentary supercharging effect of up to 7 psi. This explains why a normal aspirated engine can have over 100% volumetric efficiency.

There are a number of ball park equations for determining the ideal intake track length, but they are very rough estimates. Understanding the above concepts you can see that intake tuning (or velocity stack length) is affected by the type of exhaust system (4:1 or 4:2:1) and the primary and secondary lengths, exhaust balance tubes, cam timing and cam overlap. In addition, muffler design, compression ratio and airbox resonance can play a part in intake tuning.

Sales comment- With the large variety of modified Hayabusas out there, no one length stack is ideal for all bikes. 3 different stack lengths are offered so that the correct stack can be purchased for your bike. General guidelines are provided to assist with selection. A tuner’s kit is offered to assist with the serious racer in optimizing his set up.

Btw, what I've read is the Intake Length is from the back of the valve face to the opening of the plenum.

Metric Induction Wave Tuning Calculator


Welcome to the Velocity-of-Sound.com metric (cm) induction wave tuning calculator. This calculator will determine the most efficient overall length of the intake runners to optimise the intake system. This updated calculator allows the user to decide how many induction waves they want to calculate for. The optimum is the first wave, however, it should be recognised that both the first and second waves are usually too long for most practical applications - NASCAR engines use the third wave. We recommend you start at the third wave and go from there. Just enter the number of waves in the second box. Do as many calculations as needed to get a workable compromise between performance and the practical space constraints on your vehicle. Once you have found the most practical dimensions, all you do is measure the current intake runner length and subtract this from the length advised in the calculator - the bit left over is the length of the velocity stack. This will tell you how long the velocity stacks should be, but remember to allow about a half inch so the stack can slide over or inside the end of the runner (usually the carbie or throttle body).

Please Note: Thanks to a very helpful customer, it now works with major browsers.

Also Note: The Number of Induction Waves in the calculator refers to the number of waves to which the intake runner is tuned. NASCAR engineers use the third wave, you can now to use any number you like for optimum tuning.

http://www.velocity-of-sound.com/veloci ... lator2.htm

The Std VTR cam duration is 245°.

How The Hell Would You Measure Number Of Induction Waves ?

There are actually 3. Here's an interesting article on the subject.

"The intake system (induction system) can be tuned to provide maximum power. Just like the exhaust, you can time the pressure waves in the induction to actually force a lager air/fuel charge into the cylinder.

This theory is derived from the helmholtz theory. You can find many articles on helmholtz theory on the internet. First you need to understand how a wave acts. Take a pail of water where the water is still. Now drop a small pebble into the center of the bucket and you can see that it makes a ripple (a wave) in the water. If you watch the ripple you will see that it travels to the side of the bucket, but it doesn’t stop there. It travels back to the center after bouncing off the sides of the bucket. Then it travels back to the wall of the bucket. It keeps traveling back and forth, but the ripple gets smaller and looses momentum the more it travels."

The idea is to get your "pail" (intake length and diameter) the correct size for a specific, optimum RPM so that the wave arrives at the intake valve just as it opens, forcing more air into the cylinder.

Ever since NASCAR competitors started looking for every single available spec of horsepower, especially for the mandated restrictor plate races on the super speedway tracks, the intake tuning separated the men from the boys. The difference is that they run the engine at or close to the optimum RPM a majority of the time so the results are much more evident.

The HD Destroyer equal length stacks are "tuned" to perform at or near the 8,000-10,000 RPM range as that is where they operate for all of 10 seconds or so.... They start at 7000 at the line and go up from there. Tuning the velocity stacks for a street-ridable bike now leads me to believe that the offset pair (one short and one long) would take advantage of a wider range of RPMs.

Looking at other performance bike sites ('Busa, GSXR, etc.) an interesting fact has surfaced. The velocity stack length is one of the parameters in a long equation of cam duration, valve size, head porting, throttle body size, bore, stroke and exhaust tuning.

It's not a case of one size fits all.

HH

Last one.

The following is a condensed version of some good info I found in a Superflow 1020 Flowbench Manual:
"When the intake valve closes , a pressure pulse bounces back out the intake tract, and then in again toward the valve. By making the intake tract the proper length, the returning pulse can be timed to arrive just after bottom dead center of the next intake cycle, shoving extra air in and keeping exhaust gasses out of the intake port. To use this pulse the intake port must be the correct length. The pulse will help only through a narrow range of rpm. Above or below a certain range the pulse will actually decrease power so proper synchronization is essential. There are actually several pulses which can be used corresponding to the 2nd, 3rd and 4th time the pulse arrives at the valve. The 2nd pulse is the best the others are weaker and shorter.



2nd Harmonic 132,000 / RPM = Length in inches
Tuning Range is: 89% -108% of target RPM. The pulse strength is + or - 10%
3rd Harmonic 97,000 / RPM = Length in inches
Tuning Range is : 91% - 104% of target RPM. The pulse strength is + or - 7%
4th Harmonic 74,000 / RPM = Length in inches
Tuning Range is : 93% -104% of target RPM. The pulse strength is + or - 4%

Pulse strength varies with intake valve opening and flow

The chart shows the pulses which can be used to obtain the inlet system length, divide the number shown by the RPM for peak power.
For example, at 8,000 RPM for the second harmonic the formula would be: length = 132,000/8,000 = 16.5"

This is the desired length from the intake valve to the air inlet entrance For engines with a plenum chamber type intake, the length is from the valve to the plenum chamber. The pulse in the example will benefit from 89% up to 108% of 8,000 RPM, or from 7,120 RPM to 8,640 RPM. The greatest benefit will occur at about 3% below 8,000 RPM. Below 7,120 RPM or above 8,640 RPM the pulse will actually work to decrease engine power".
__________________

The above quotes were pulled from this forum http://www.1130cc.com/forums/showthread.php?t=118537

I've only just finished reading this & only came across it whilst looking to see what the pro's & con's are when running different length velocity stacks. So have done no working out at all yet

[bigtwinthing]

if you like twins, want to be comfy, buy a Multistrada? it handles great, looks great and sounds great. and you can just get on and ride it.

[alanfjones1411]

It's all beyond me.I was lost in the first few lines.

[Pete.L]

Interesting.
So what you really want is a variable stack which will change length and diameter to match any given rpm.
sounds easy

[Varastorm]

if you like twins, want to be comfy, buy a Multistrada? it handles great, looks great and sounds great. and you can just get on and ride it.

Your right Btt, a Multistrada is my dream bike but there's a problem, I can't afford one

I did persuade a mate to sample one from Woods of Abergele once. He signed his life away & took it down the Conwy valley for a ride with me following. He was really impressed with the whole package of effortless power & upright seating position. But it was too expensive for him also he's rich

Another issue, I don't fit on one. At 6'2" & over 20 stone with a 60" chest, it feels like a 125cc

[Stephan]

nice topic you have. Just for your interest, I am still not finished with my setup, but I am close. The biggest improvement was change from Dr Honda stacks (HRC/HP Power design) to oem plastic short/long. From this change, I feel midrange is much better, and top end maybe as well.

BTw, I am just now struggling low end issues with DJ needles at position G2 + 1mm washers ... I need to solve it but I am not going to file them. Strange is that with approx 1.3 mm washers it was ok but midrange punch is not so strong, you was there ...

I don´t say Dr Honda stacks don´t work, but just in my application, together with my Arrow full exhaust, Pipercross filter and stg1 engine, definitely not.

[VTRDark]

I am not going to file them

Interesting read that vara. Makes up for my lack of explanation in the carb balance thread.
http://www.vtr1000.org/phpBB3/viewtopic ... 46#p430627

I suspect that may have got you investigating things.

So what you really want is a variable stack which will change length and diameter to match any given rpm.
sounds easy

And this was one of Tony's ideas for the Storm. The biggest issue is the electronic communication between the TPS and relay to activate the motor to move the stack up and down.

[Varastorm]

Hi Stephan, concerning the filing of needles.

I had an issue with a low speed misfire a few weeks ago which turned out to be plugs. Whilst tracking down the culprit I swapped in some excellent condition practically new carbs.

I've come to the conclusion that the carbs that I had were knackered, worn/oval needle jets. Me lowering & filing the needles was compensating for the knackered carbs. Ah well it only took a knackered plug & 4 years to find out

It now runs brilliantly totally Std not too sure of the mpg I'm going to get, but we'll have to see

What you've discovered with the stacks is why I am looking into it again. the midrange pull is glorious atm, but it seems to run out of puff top end. That's why I was looking into how to work out what the correct height of stack is needed for 9000 rpm.

Tbh, I enjoy the midrange & find myself there most of the time on the road, but it would be great to have another set of stacks to swap in for track days.

I'm full of cold atm & can't be bothered doing anything in the shed

If anyone has the measurement of the inlet tract (Valve to top of the trumpet) that would be great. Obviously I would need to know which trumpet was used

[Stephan]

Plugs, jets and needles are all new, this is ok. But I cannot exclude wear of other parts like carb manifolds ...
I will try bigger main jets, and hope characteristic will be the same and bigger mains let more fuel in around the needles, now it is on the lean side. And I really hope there is no problem with worn parts.

Relating stack lenght, with two long dr honda stacks I feel like afr was not constant from 4-9. When top end was great, mid range was too rich. With oem stacks it seems like difference is not so dramatic. Still talking about application on my bike, far from standard.
I plan to do last dyno this year measuring afr on each cylinder, if I get any usable data I will post.

[VTRDark]

What you've discovered with the stacks is why I am looking into it again. the midrange pull is glorious atm, but it seems to run out of puff top end. That's why I was looking into how to work out what the correct height of stack is needed for 9000 rpm.

Here lays the conundrum because they are not adjustable we cant have perfect all the way through the range. Two long better low down, two short better up top. Honda went for one of each which is like trying to get the best of both worlds but does not quite do it and both ends of the spectrum are left a little flat along with middle being better. The more pro tapered bellmouths like the HRC/HPpower ones are supposed to be good all the way through but I am still not sure if they are as as good as either two long or short standard ones whether you want top or bottom end performance as there is still the height issue there. I believe they they just smooth things out more. Interestingly the Mori ones are more like standard and are one short and one long but more extreme and tapered bellmouths so smoother. Probably the best option for Stephen considering his other engine mods.

[Varastorm]

I looked into making a short Ducati trumpet last year.

I cut a 5mm slot in the prepared trumpet & placed it in a vice with the plan to squeeze it & make it a smaller diameter.

This would of enabled it to sit lower in the plastic oem trumpet.

The bloody trumpet exploded when I put pressure on it, no warning. Just bang. Total failure

[Stephan]

Carl: I know about mori ones, would be nice, but hard to get. I will just work with current setup as I feel progress - like many times before ...

[sirch345]

A very interesting topic, so many variations available. Not something that will be around as it is now in years to come I feel. Enjoy tinkering while you can

Chris.

[Stephan]

I will try bigger main jets, and hope characteristic will be the same and bigger mains let more fuel in around the needles, now it is on the lean side. And I really hope there is no problem with worn parts.

pretty off-topic, but I have to follow this.

After some searching what is going on in carbs, I found my idea of bigger jets dumb. So I put needles back to G2 and 1.3mm + 50 pilots with 1.75, and now it is running with no issues, I carefully say it is running good enough ...

I´ve found this picture quite informative

[VTRDark]

You did say low end issue so bigger mains would not have been right. You have now gone with the bigger pilots and things are good. Two stroke carb pic that one as it has an air screw. I'm so tempted to slice some VTR carbs straight through the middle. Shame I haven't got a band saw. It would have to be an angle grinder or dremmel.