Topic: velocity stack pros and cons?

[Retro Rocket]

Slightly off the velocity stack point, i have been trying to figure out a better way to get more air to my engine and have come up with this. I have gutted the stock airbox and plan on using a final filter inline green foam filter inside the airbox, i am plumbing a couple of aluminum tubes into the top of the box and running tubes over the tappet cover to "ram feed" the airbox. I will post pics in the "project shop" very soon. The only positive i see from using stacks is the "look", apart from that they just reduce the life of the engine..

Mick

[eldar]

Well you DO get more air to the engine with stacks. AS for the ram air you mention, the tubes will have to be a fairly good size to really provide any benefit from what others have said in the past.

Screens do come on some. It will keep out larger items such as rocks and bugs and such. Wont stop dirt at all. I know that someone here tried nylons and they seemed to work ok from what he said. Will filter smaller items than screens but still no really small particles.

I guess you just have to decide where you want your trade off. Better filtering = less air flow but longer engine life   less filtering = more air but shorter life
you have to find the middle ground you feel comfortable with.

[Soos]

You can get better flow with a filter, but would have to mod the airbox to accommodate a larger filter somehow.








A crazy idea....
As the intake pulls on air, each carb flows the volume of air in the cylinder...

As with exhaust pulses, and the scavenging effect....

can you do the opposite on the intake side?
Have the air build up pressure from being sucked at high velocity when have the intake valve shuts, allowing a "pulse" of air to be rammed into the cylinder as the intake valve opens up again?


I understand this would be a REALLY long velocity stack....


But would it work?



I know the scavenging effect of the exhaust is RPM dependent, as well as (basically) works by creating a vacuum at the precise time, unlike what I am thinking where things would be dependant on a positive pressure being built up by the incoming air.


A object in motion tends to stay in motion unless a equal and opposite reaction occurs.
So If the air were being pulled fast enough(high rpm's) in a long enough tube, the air that is already moving at the end of the tubes would pressurize the carb end of the stacks....

The volume of the stacks would need to exceed the cylinder volume I am thinking....
But I am by no means an engineer. Just a machinist with a wild imagination.




Any thoughts?



l8r

[eldar]

If you maintained a constant pressure of air, a "pulse" may be possible however the tuning required would probably be almost impossible with carbs. Hard to say.  I am reasoning that first you need to have that constant pressure as otherwise, it would change depending on wind and speed. Second, you have to figure out the length of each pipe for each carbs based on that carbs state of tune. the other issue is that air is always flowing at least into the carbs but without pressure, would try to flow back out possibly creating a fluttering effect.   All this is theory of course!

[tygrant]

Its funny you say that i have imagined that same thing with intakes. i allways had pictured that happening on my mustang intake manifold that i replaced. check out the volume each intake port has, how each intake tract loops down around underneath.

[bwaller]

Last summer I happened by a friends pit at Mosport and he had the cylinders off his TZ250 exposing a badly scored right piston on the intake side caused by what looked like a piston circlip. Happens though that the circlip wasn't his but must have been picked up by the rear tire and tossed into the right carb.

No question this is bad luck, but you get my drift, there is much more crap on our streets than our racetracks.   

[TwoTired]

I have gutted the stock airbox and plan on using a final filter inline green foam filter inside the airbox, i am plumbing a couple of aluminum tubes into the top of the box and running tubes over the tappet cover to "ram feed" the airbox.

It seems you have ignored how the carbs actually work.  When the carb throat atmosphere is lowered by the engine sucking in air, the higher relative air pressure on the surface of the gas supply in the carb bowls pushes the fuel into the carb throat through the fuel jets, at a rate to provide the proper mixture for combustion.  If you pressurize the carb throat with ram air, this increased pressure pushes back on the fuel flowing through the carb jets and leans the mixture.

Note the carb jets don't flow fuel when the engine doesn't run, and the carb throat pressure is the same as atmospheric.  If you pressurize the carb float, air can flow backwards through the fuel jets into the carb bowl fuel reservior.

Years ago, drag racers learned that if you pressurize the carb throats, you also have to pressurize the entire carb assembly, or at least the ports that supply the force that pushes the fuel through the carb jets.  The cost was several engines burned up as the engine went lean, as soon as air speed forced the ram air system to provide some denser air.

V stacks only have a place on the race track where your engine only has to last to the end of the race.  That is where you appropriately run them open, and expect frequent engine overhauls.  Functionally, anything you put at the stack entrance, filter-wise, upsets smooth airflow and negates any beneficial effect of the stack, aside from an aesthetic visual appeal for some individuals.

Cheers,

[Soos]

It seems you have ignored how the carbs actually work.  When the carb throat atmosphere is lowered by the engine sucking in air, the higher relative air pressure on the surface of the gas supply in the carb bowls pushes the fuel into the carb throat through the fuel jets, at a rate to provide the proper mixture for combustion.  If you pressurize the carb throat with ram air, this increased pressure pushes back on the fuel flowing through the carb jets and leans the mixture.

Ahhh....
TT, many of us owe you a undying thanks for showing the folly of our ideas....Now whether we would admit it, dunno.
(me included... many times over on many subjects!)


Hrmm.... that makes me rethink even trying the long velocity stacks to get the "pulse" air feed......
At the wrong rpm's that could cause the "pulse" to arrive too soon, and pressurize the carb throats, killing the vacuum of fuels.... making it run lean until the "proper" rpm range is hit.


And from what I have found on exhaust gas pulses and the scavenging effect, on a 4-4 system, at certain RPM's it is actually somewhat a power robbing effect. Hit the sweet spot, and you suddenly gain a few hp though.


As TT said.... At the track(or bonneville salt flats ) stacks are worth it.
Depanding on how crazy I get I might play with stacks... but I'm doubting it more and more.
(unless I can con some of the engineers at work to figure a shape, length and all for this, to eliminate some of the trials and errors bound to occour)




l8r

[547]

uuuhhh, der
i can see that there is way too much involved here to not play it safe and go with the filters.
now i just need to find some cool filters, something inconspicuous.

[Retro Rocket]

Maybe ram feed was not the correct term. The set up i am going to use is meant to get a flow of cooler air to the airbox, and that will help it run better. If ram air systems don't work then why the hell is there heaps of "rev heads" using " bug catchers" on their cars???

Mick

[Soos]

speaking of cooler air intake temps, is it feasible to hook up a intercooler?




l8r

[TwoTired]

Ram air systems DO work if they are engineered/implemented correctly.

Fuel injection, ox sensors and closed loop fuel metering computers would certainly help.  You just can't slap on ram air to a carb throat that relies on differential pressure to pull fuel through the metering jets.  Besides, just because there is a scoop on a car hood, doesn't mean it is ram air or behaves as one.  Some help cool the entire engine compartment.  Some are there just to wow onlookers...like velocity stacks.

Anyway, I'm trying to get you to understand how the carbs work on the SOHC4 so you'll not wonder why your engine self destructed when you hit the ton even though it ran great around town.  There are lots of parts/systems on cars that won't work on the SOHC4.  You going to put Recaro seats and a 5 point harness on your bike?

If your air induction scheme pressurizes the air box feeding the carb throats, the engine will lean at higher speeds and this will vary with the head winds.  It makes jetting the carbs a nightmare.  I'd rather navigate Escher's Staircases.

Cheers,

[scunny]

"You going to put Recaro seats and a 5 point harness on your bike?"
very nice TT

[Soos]

Ram air systems DO work if they are engineered/implemented correctly.

Fuel injection, ox sensors and closed loop fuel metering computers would certainly help. 

that gives me ideas as well.......

l8r

[lordmoonpie]

Two tired is right all round - ram air works well if the system is designed to do it. My Moriwaki VTR has a fully pressurised airbox and ram air tubes to feed it. To keep the fuel coming though it has a very high flow fuel pump to keep the carbs fed. That might be an option for the older bikes too - I'm sure you could hide a CBR600 fuel pump under the seat to feed the carbs, but setting the whole lot up could be a nightmare!! 

[eldar]

Easlier option is just getting CRs! Sure they cost 600 or so but once you figure in the amount of materials and the time you would spend trying to tune, hell you would probably have the 600 PLUS change!

Another option is to turbo it but you would want to drop your compression if you do that and that has its own host of issues.

[Retro Rocket]

I think Two Hands tends to blow things out of proportion and assume everyone is stupid or uneducated, there is no need to patronize with your responses. I am well aware after owning several highly tuned muscle cars and owning over 30 motorcycles, most i have built myself, so i am very familiar with all types of induction. I have used the method i referred to earlier and have never had a bike "blow up". I gut the airbox and run pods on the carbs inside the gutted airbox, i then fix a couple of 40 to 50mm pipes to the airbox, run them over the tappet cover which gives me a supply of cooler air to the carbs. Because of the diameter of the pipes running into the airbox i doubt very much whether it will force too much air into the carbs as the filters generally determine how much air will go in anyway, as the pods are restrictive by nature, so all i get is a good flow of cooler air. I have used this method several times{ i have owned 18 750 honda's} and never had a problem yet. I will be running CR 29's on this bike and they flow way better than stock so i can't foresee any problems.

Mick 

[TwoTired]

I think Two Hands tends to blow things out of proportion and assume everyone is stupid or uneducated, there is no need to patronize with your responses. I am well aware after owning several highly tuned muscle cars and owning over 30 motorcycles, most i have built myself, so i am very familiar with all types of induction. I have used the method i referred to earlier and have never had a bike "blow up". I gut the airbox and run pods on the carbs inside the gutted airbox, i then fix a couple of 40 to 50mm pipes to the airbox, run them over the tappet cover which gives me a supply of cooler air to the carbs. Because of the diameter of the pipes running into the airbox i doubt very much whether it will force too much air into the carbs as the filters generally determine how much air will go in anyway, as the pods are restrictive by nature, so all i get is a good flow of cooler air. I have used this method several times{ i have owned 18 750 honda's} and never had a problem yet. I will be running CR 29's on this bike and they flow way better than stock so i can't foresee any problems.

Mick 

Well then, good luck to you sir.  I'll stop trying to help, as you've made it clear you have far more experience and knowledge than I do.  Whoever wishes to follow your self proven successful designs and amply demonstrated power gains is free to do so, of course.

Happy trails!

[crazypj]

If the airbox leaks, you'll just have a cold air induction which is exactly what the induction systems on sport bikes operates as at 'low speeds' (up to about 85~110, depending on engine size)
 They don't really do much else or 'ram charge' until 120~130 and up.
 Kawasaki measured 2psi 'boost' at 190+ mph on 1200cc motor
 If you work out air requirements of engine at max rpm you'll find its going to be difficult to ge any meaningful pressure difference at lower speeds.
PJ

[eldar]

While I do understand why honda put the box behind the engine, I wonder why they did not cold air intake. I suppose BACK IN THE DAY when engines were not as understood and engineering was not as good, they probably figured it was ok. Or they figured it was too much work. After all, old carb equipped vehicles used cold air intakes with a tube running up from the manifold for cold weather.

I would think that after experiencing the sudden lack of air from strong winds, I am betting that the tubes probably help to reduce that.

[voxonda]

Hi Twotired,

PM sent.

Regards, Rob

[Retro Rocket]

Another clever answer  Let me see, last time i looked a nice supply of cooler air to the induction side of an engine { especially a worked one} usually did result in small horsepower gains, not huge by any means, but definitely cooler air does help. Instead of putting words into my mouth maybe you should think about what i'm doing not what your talking about.

Mick

[cezarious]

hi guys!i'm kinda new here,so pardon me for no introductions wutsoever

i know this topic been long dead but as i've just completed a short course on manifold tuning,i couldt help but give my 0.02 cents

mind u i'm not that great a student but this is what i've gathered from the course

velocity stacks basically works like a funnel,the same way how you would pinch the end of your gardening hose to make the water shoot out faster,it accelerates the air from the inlet and increase the pressure at the outlet. it called the venturi effect

all carbs works on the venturi effect.it needs the acceleration to determine how much fuel it needs to release to the intake.if air was constant flow or no air moving, the pressure at the float and pressure at the jet nozzle would be the same and no fuel will b released from the carbs.so basically all carbs has built in velocity stacks

by adding the velocity stacks you increase the pressure even more at the inlet of the carbs so that you have more air coming in,and when theres more air you need more fuel.this is where the jets come in.your jets are like injectors,they control how much fuel is released,for example if you are using 30cc injectors they would only release 30cc every time,i'm not sure exactly how much a 120 jet releases coz i slept during the carb tuning class but main line is the bigger the jet the more fuel is released.too small the jet and the airfuel mix would run lean and the engine would overheat,too big the jet you would waste a lot of fuel and ruin the environment with alot of unburnt gases.but it does have an upside the fuel actually helps cooldown the engine.so to heck with the green house gases and those bloody tree huggers.if your running with more air find the biggest jets you can fit....and tell them WE WANT MORE POWER!!!bwahahahahhaha

now for the pulse wave effect it has to do alot more with sound waves than anything else.lets take a look at what happens in the engine a little bit more closely.when the intake valves open air is moving from the velocity stacks to the carbs to the runners through the intake ports the suddenly the valves close all the air flowing hit the valve face,some would come too a halt some would even bounce backwards,what ever it is the congestion would create a positive pressure and this positive pressure has no where to travel but back through the ports to the runner to the carbs to the velocity stacks but wait as this positive pressure travels the valve opens again now a negative pressure(aka vacuum) is produced at the at the valves and this negative pressure also travels through the ports to the runners to the carbs to the velocity stacks.and thus a pulse wave is formed.this positive and negative pressure going backwards is similar to a sound wave of high and low amplitudes you see in those oscilloscopes.

so whats up with with this waves man...is it good?is it bad?now bear with me a little on this,we're gonna do a small science experiment...everybody pick up a piece of metal pipe layin'round your workshop hold it right in the middle n give it a nice knock on sumthing solid.not too hard boys you'll dent your pipes.do you hear it?the sound man.its traveling from one end to the other.its the resonance effect(eat that bill nye the science guy lol).you see the vibrations in the pipe is resonating in the pipe shaking the air molecules n making a sound that travels back and forth.every object has a resonating factor even air molecule.and the air pulse waves created by the opening and closing of the valves also resonates back and forth within the total intake tract

the thing is you want to time the resonance effect so that when the waves hit back at the valves at exactly during the time when the valves open.engine displacement is fixed,when i say fixed i mean the stroke and bore of the engine is determined by the piston conrods and crank,but air is compressible.if u manage to get the resonance wave to reach the valves at exactly wen it opens and not a milisec late,you'll manage to put more airfuel mix into the combustion chamber.more air more fuel = MORE POWER!!!it does require some maths n again i slept during that part but i do remember all we were calculating was how long the intake tract should be.the main deciding factor was the length of the total intake tract from the valve seat to the port to the manifold gasket to the runners to the carbs n the end of the velocity stacks.and it doesn't have to be as long as my exhaust and point towards the sky.the resonance bounces back and forth right?so i don't need to make a super long v.stack that reaches the valve at the first interval,i can make a shorter one that bounces ...ummm lets say 4 times n reaches the valve at the same exact time as the long one.

extra note here,its also a good idea if you know which rpm range you want to tune your vehicle.for example a dragster would want peak torque at the low range and peak hp at high rpm while a tracker would be opposite high power at low range and peak torque at high rpm with all those corners and stuff.since engine revs also determine the opening and closing of the valves it would also determine the pulse wave produced and thus you would tune/design the manifold to the target rpm.but since i'm such a bad/lazy student.i go by the old adage "if it ain't broke don't fix it".i mean c'mon the bike was already design to work perfectly at all conditions.who am i to challenge the thousands of dedicated engineers hired to design the bike.so for my cb500 im just gonna measure the length from the carbs to the airbox coz im sure those dedicated engineers already had a target rpm in mind n did their homework.once measured make the v.stack n install bigger jets.theres gonna b a significant increase i can bet you         ................on stock

the thing is the engine is like your bowel system and if anybody tells you its a breathing machine like your lungs inhaling and exhaling,you put their head between your crotch and giv'em the ol' triple H pedigree ahaha.exhaust is just as important as intake.you cant just keep on eating without pooping thats called constipation,and you cant just poop without eating thats diarrhea...it works with the same resonance principle as the intakes and if you timed it right you get the scavenging effect,where exhaust gas actually pulls the airfuel mix as it exits the chamber....pulling even more airfuel mix creating,you guessed it man,even MORE POWER!!thats why forced induction are for pussys n a N/A will beat the crap out of them everytime.you don't need no exhaust driven turbos or belt driven superchargers.a healthy fit tuned racer will always beat sumone running on those lifesupport bypass machines heheh...

again applying the old adage im gonna use the stock as a template n make two 2 to 1 exhaust,ive got a feeling it compliment the  v.stacks...if not i'll just blame those dedicated engineers at honda lol

anyways,i hope you all dont take offence wit my dry sense of humour...i can be quite the dork,cheers!!

[dave500]

ZZZZZZ,ZZZZZZ,ZZZZZZ.