Velocity Stacks

[Terry Syd]

First post

First post, might as well be this thread. Years ago I was at a tuner who used a flow bench. He showed me a beautifully shaped airhorn that attached to a motorcycle carb. He ran up the vacumn on the bench, I forget the reading, say 300 cubic feet a minute. Then he rolled up a small piece of clay about the size of a pea and put it on the edge of the air horn. The flow jumped up something like 325 cfm. He added another, then another, he ended up with 6 of these ugly pieces of clay on the edge of the air horn and the flow was up about 400 cfm.

I laughed and said it looks like the way the air horn was designed it restricted the air flow. Naw, he said, the carb it was designed for only flowed 240 cfm. So I said it was worthless, no he replied - it does make the owner feel better about his bike.

Having said that, I do believe there is an argument for resonance tuning. However, I would be more interested in something in the intake tract to increase volume to change the resonance than something stuck on the end of the carb. Has anybody experimented with changing the intake volume with an interference pipe, perhaps a hose to a small enclosed chamber (something like the old Yamaha 'Boost Bottles')?

I've done work on bikes using lengths of tubing (cut the volume to size), but I was wondering if anyone had experimented with chainsaws.

 

[Bill G]

Nope, I think Marky still has it .

Seth,

Did you sell it complete with the bar or do you still have it.

Bill

 

[arlen]

Dolmar optional filter

Dolmar is calling there new optional filter system a velocity stack filter.
http://www.arboristsite.com/attachment.php?attachmentid=84283&d=1229567846

David

 

[timberwolf]

I laughed and said it looks like the way the air horn was designed it restricted the air flow. Naw, he said, the carb it was designed for only flowed 240 cfm. So I said it was worthless, no he replied - it does make the owner feel better about his bike.

Welcome, I'm not jumping on you as a new guy here, but this I don't think is quite true. It does not matter where the restriction is everything in the path of flow is a restriction to flow. It does not matter if one part of the intake flows 300 cfm and another 250 cfm, both parts will restrict flow and cause a pressure drop across them decreasing air density in the process. It does not really matter which order the ristrictions occur if the greater restriction is first the air density is dropped more sooner, but it is still dropped further by the second restriction.

This cfm flow thing I think is greatly missunderstood, just because a part flows 250 cfm does not mean it won't flow 300 cfm or 500 cfm. Just that the more it flows the greater the pressure drop at 250 it might drop 10 inches of water at 300 maybe 20 inches and at 500 cfm maybe more like 60 inches. It matters too if the test is conducted under pressure or vacuum, under pressure air density is increased, under vacuum air density is lowered though the test piece, what this does to the results depends on the design of the flow bench and how the calculations are done.

That said, a part that flows 300 cfm makes for a drop of say 10 in wc yet if that same part flowed only 250 cfm the pressure drop may be only 7 in wc. 0 flow and you always get 0 pressure drop, the graph will max out at the other end under perfect vacuum over 400 inches of water.

The cfm term is a reference and without knowing the pressure drop used to determine the measurment it is all but a worthless sales pitch. Many flow bench tests are done at 6,8,10,25 or 28 inches of water often the determining factor in which is used is the capacity of the flow bench, it takes some serious power to pull 28 inches of water at 1000 plus cfm.

 

[timberwolf]

As for the velocity stack, it can work, moving air has inertia and the longer the air column the greter the energy stored in the intake to aid with packing charge into the base. But with more energy in the air column it has more ability to do bad just as well as good. At design RPM if it's righ power will be gained, but off powerband power will be lost, it good if it's tuned right, but if not it's worse than not having a stack at all.

The effects of intake tuning can be much greater than simple inertia gains with a longer intake, lengthening the intake has the effect of lowering the resonant frequency of the intake as the sonic pulse or wave requires more time to travel back and forth the length of the intake. So just adding a velocity stack to a stock saw may well hurt performance unless you got lucky with the added length just right to drop the resonance in the intake a full harmonic.

Base volume, RPM, intake durration, intake diamiter, length, charge density and temperature all factor into the problem.

 

[Terry Syd]

Flow vs resonance

No offence taken. If I understand your post, then the airhorn will also create a restriction and therefore a carb without an airhorn will be able to flow more air on a flow bench. If so, then an airhorn is not an item to increase flow.

My post was directed at the two different concepts of flow versus resonance. On a flow bench there is a constant flow of air, however on an induction system we have a 'gulp' of air that causes a pulse in the system. I am more interested in tuning the nodes of resonance from the 'gulp'.

On a work saw we don't have the room to make a pipe to work with the exhaust resonance, however we may have enough room to work with the intake resonance.

I found that by using a long section of clear plastic reinforced hose attached to a homemade fixture on a rubber carburetor boot that I could shift the nodes around to different RPMs. This creates some difficulties in a broad powerband engine such as an enduro bike as many times two nodes would appear. However, sometimes even that could be an advantage by setting one node up for the lower part of the powerband to torque over obstacles and the other node either before or after the peak torque of the pipe in order to stretch the strong part of the powerband.

Using a long hose on a bike is not a problem. I could have more than a meter of hose strung under the rear fender. The tuning of the length would create the volume of the intake. I would just keep trimming off the hose, replace the cork, and see what effect it had on the powerband. If needed, the volume of the hose (filled with water) could give me the volume of a boost bottle if I wanted to use a shorter unit.

I did have one system that was controlled by a valve. The valve could be operated by a lever on the handlebars. One position gave me a stronger low end for tight woods work, the other postition was tuned for maximum horsepower.

I note the 'velocity stack' on the Dolmar. It may be that the stack is simply tuning the length of the intake tract. Changing the resonance node could also be accomplished by changing the volume of the intake tract prior to the atmospheric opening. Instead of increasing the length, which could cause handling problems and make the saw unwieldly, we might be able to do the same thing by putting a small section of hose on an intake elbow.

A mod like that would allow the saw to retain the same overall configuration, including the stock intake filter. The node could be adjusted for maximum RPM or it could be adjusted lower to give the saw stronger torque - or maybe a valve put in so that the operator could choose the powerband in the field.

 

[timberwolf]

If I understand your post, then the airhorn will also create a restriction and therefore a carb without an airhorn will be able to flow more air on a flow bench.

Not what I was trying to say, a flat faced carb opening will have a significant veina contracta with large eddies just inside the opening into the carb bore restricting flow. Such an intake would reduce the effective intake to 80% of the bore area. A simple radius entry can improve it to more like 95% and an eliptical taper about 97%.

The air horn and carb will each have their own flows but together will flow another value again.

 

[Terry Syd]

Volume

Here's another idea for the perpetual 'tweakers' of engines on the forum. I noticed that the downtube on the motorcycle frame contained an enclosed volume. So I piped the induction system into that volume to see the effect. Unfortunately the volume was too large and moved the node way too low in the powerband. However, it did verify the possibility of using an existing structure on the bike.

For a saw, perhaps after testing a volume would be arrived at that would create quite a large 'boost bottle' to be attached to the saw. In that case, using an existing hollow structure, such as a tubular handle on the saw, could substitue for the additional volume.

 

[Mr. Firewood]

Thats because in 2001 there were only 10 members total. The sad thing is I remember this thread from when it was new

I searched and that was my 5th post i ever made here

 

[Terry Syd]

Old Guy?

Gee, I'm just an FNG, but already I can claim that this ancient thread contains my fourth post. I wonder if that helps my rep?

 

[Sethro]

Seth,

Did you sell it complete with the bar or do you still have it.

Bill

The bar went to the west coast (Berry Creek) and the powerhead went to the east coast to Marky Mark .

 

[Bill G]

The bar went to the west coast (Berry Creek) and the powerhead went to the east coast to Marky Mark .

I figured it was gone.

Bill

 

[Taxmantoo]

Using a long hose on a bike is not a problem. I could have more than a meter of hose strung under the rear fender. The tuning of the length would create the volume of the intake. I would just keep trimming off the hose, replace the cork, and see what effect it had on the powerband. If needed, the volume of the hose (filled with water) could give me the volume of a boost bottle if I wanted to use a shorter unit.

I thought that the resonance was determined by the length of the tube, not the volume, and a 15,000 rpm 2 stroke saw would be running a tube roughly 1/4 the length of a 7500 rpm four stroke car engine.

 

[Terry Syd]

Resonance

As Timberwolf mentioned in one of his posts, there are a lot of variables that enter into the resonance equation, length being just one of them.

The section of hose off the intake is not an open hose, it is closed off with a plug of some sort. The purpose of the hose is to increase the volume of the intake tract so that the pulse thinks that the length is longer. Depending upon the volume added, you can adjust the return wave to the cylinder to hit the piston just as it is closing the intake port.

The wave, upon reaching an open end of a pipe, reverses in form. Whereas it starts out as a negative wave when the piston opens the port (the 'gulp'), when it hits the open end of the intake it reverses to a postive wave and heads back down the intake tract towards the clyinder. That positive wave can then be used to pack extra mixture into the crankcase, provided it is timed right. Using a long plastic hose makes it easy to change the volume, you just keep cutting off sections and trying it. When you get that wave (or one of its harmonic cousins) hitting at the right time there is a noticeable increase in torque.

 

[CR888]

Interesting stuff here, l wonder how many builders are looking at the resonant effects of the intake tract and testing different lengths and sizes. I imagine the OEM's do that work for them and changing things can result in lost hp as much as increased hp. ln any case l find intake tract tuning a more interesting subject in performance 2T than drilling a hole in a muffler can. When tuned well the resonant effects of an intake can make a very noticable difference.

 

[gunnusmc03]

Most builders have a mad scientist laboratory in their basement. They test this crap all the time.

 

[Terry Syd]

When tuned well the resonant effects of an intake can make a very noticable difference.

Ain't that the truth, even some guy without a clue about tuning will notice the jump in power.

If I had a dyno I'd be tempted to use the 'hose' method on a saw's intake to see if it could be tuned. The dual intakes of a strato complicate things, but maybe they could be used to an even greater advantage. Since the jetting on a saw carb goes rich without an air bleed jet, maybe staging the strato port to charge more efficiently at higher RPMs could add the extra air to use that rich mixture.

Wouldn't that be a hoot, just like coming on boost with a turbo!

 

[CR888]

l also find it interesting when using a backpack blower and a handheld at the same time (yes l do it often, can't help myself) how they harmonically balance and kinda feed off each other. l imagine this is why twin outboard two strokes sound and run the way they do. l just love it when things come together in 2T tuning, and intake resonance plays its roll. The Japs have made formulas for this however trial and error is probably just as easy considering whats involved and the low cost of tube or whatever. l like ya thinkin Terry!!

 

[Knot_Smart]

Ain't that the truth, even some guy without a clue about tuning will notice the jump in power.

If I had a dyno I'd be tempted to use the 'hose' method on a saw's intake to see if it could be tuned. ........

I'm one of the guys without a clue but your idea of using hose reminded me of the Ramcharger's 'High-N-Mighty' test cars where they used hose and various boxes to tune the plenum, same deal on the exhaust with cones. Hokey looking for sure and using the dragstrip (mostly?) for a dyno, I believe some of their learning with these cars went right into limited production stuff (early 60's Max Wedge crossram etc). I know, not chainsaw related at all but I find this stuff interesting anyhow...

 

[CR888]

I think modern high output 2strokes can no longer dismiss the benefits of the ram air effect that intake resonance can achieve. l know for saws this is a bit of a problem as far as space is concerned (but still possible). l can see low range torque increasing and a higher effective compression ratio under operation increasing. l think the industry term is inertia super charging or something similar. Nobody denies the benefit of a tuned expansion chamber, intakes can benefit too. Why not use what is naturally available to help the induction process...who says forced induction can't be used on a saw!lol