Topic: Let's talk velocity stacks: engine death or hot stuff?

[50words]

I love velocity stacks. The precision tuning involved is so cool. However, I recognize that the screen that they come with doesn't filter nearly as well as an air filter or pod filters. But they are seriously cool

I am thinking about bobbing my bike before next summer, and I would rather do velocity stacks than pod filters, but not if my engine will grenade in a thousand miles from the extra grit. Can anyone with stacks tell me about their long-term experiences with them?

Edit: And if I do go with velocity stacks, how much bigger will I need to make the jets on my carbs? Is there a way to find out?

[78 k550]

My 72 500 had stacks on it when got it. the foam is long gone. Looks like to be something there. I would contact UNI Filter and buy some materal and cut to fit. If I get another 500 running I'll give them a try. Jettring is a trail and error thing. Ask in your area or shops and see what they recomend.

Paul

[cd811]

on early 750 carbs, with 4-2 pipes(no baffles), with pods,  I am running 120 main and 40 pilots

[Uncle Ernie]

First of all, they are going to be a lot louder.
Second, they look and may sound cool, but if you want to keep the motor a long time- don't use them. Racing engines aren't expected to last long wout being rebuilt.

[KB02]

This is something that "Two Tired" kinda posted a little while ago, when I asked a simular question, that has stuck in my mind:

Ifyou look at the stock Air Box on on these bikes, the rubber boots that connect the box to the carbs, are, in essecence, Velocity stacks. Albeit, short and rubber and connected to a common volume air box, Velosity stacks are all they are. Hence, with this information, one can only concude that they are going to get better performance than, say a Pod filter. HOWEVER, like evryone else above has said, the lack of filtration may become a problem.

Has anyone seen a POD filter and Velosity stack combined? (is it possible to get the best of both worlds?)

[turnenwheels]

i really like the way pods look, very industrial and old school, stacks, not to sure about the jet ?
But There are some many different suggestions with pods...size up, change this change that,
I'm running 120's, Its been suggested to run 130's...

[ProTeal55]

When I bought my bike it had stacks on it.
I like the look of them on the bike , kinda goes with the overall Bobber feel I am going after.
I went to Home Depot and got some 90 degree copper bends , and staggered the stacks.

[kyre]

that looks interesting! never seen it done before. any performance flaws? or does it run the same without the 90degree bends?

[dusterdude]

yea id like to know that myself,i wonder if there might be any airflow issues with them.

[ProTeal55]

Me and the guys at work argued this the other day over a beer or three 

You would think it would mess with the jetting by adding the bends to the stacks , but it doesn't.
I ran the bike with the stacks directly on the carbs , and then after , with no difference in idle,accleration,de-cell,etc...
We actually ran a series of tests at the shop , for throttle response , and all came back the same, no difference....

I actually think , the bike runs better/smoother with the bends on the carbs.. I have yet to pull the carbs to see what jets are in the bike , but maybe the previous owner had the wrong ones in their , and with the bends now on their , made up that difference , thus making the bike run smoother.

I am actually going to do alittle "surgery" to the bends today , to get alittle more shock factor outta them. Will post pics when complete..

[Vagelis]

I've had stacks on my 400 for 15 years now with no problems. A pain in the rear to rejet and sync. the carbs in the beggining to get everything right. About the filtering I just got some foam filter for A/C's cut to fit and that's it. Every month I just change it with new pieces.
You should hear it redlining (especially with the Supertrapp) music to the ears!!
Keep safe.

[TwoTired]

You can get a pod filter to fit over the velocity stack. It is less efficient, however.

The rubber boots won't do anything to help performance other than make the intake resistance-free (which only improves the performance with a lot of other changes to tune the bike properly). A velocity stack is a precision piece of tuning that gives the bike forced induction for a very narrow portion of the power band. Basically, they prevent the shock bubble from the intake valves from escaping, so that it returns to the valve just as the valve is opening again, forcing extra air inside. It's a sweet bit of precision tuning, if it is done properly.

Performance by what definition?  Maximum power?  At what throttle setting?  For how long?

Velocity stacks were/are quite popular in the drag racing or land speed record arenas.  I can't think of anywhere they were used with success in a duration event. 
The funnel does a small bit of compressing the air, using the mass and velocity of the air at higher speeds.  Also, part of what a velocity stack does is help keep the intake airflow laminar, as in non-turbulent.  This is something most pod filters ignore.   Anyway, turbulent flow is bad because the eddies cause higher and low pressure areas in the intake runner tract.  The positions of these pressure differences vary along the tract with the air speed.  When these pressure differences extend over the fuel exits into the carb bore, it can upset the flow of fuel into the carb bore, making some RPMs richer and other leaner, even though mechanical mixture ratio settings were maintained.  A velocity stack can only do the job properly if it's mouth is completly unobstructed.  A filter or screen at its mouth will certainly turbulate the airflow and negate much of its beneficial effects.

I'm curious as to just how the "shock bubble" is kept from escaping?  In a tuned exhaust pipe for two 2 strokes, there is a reverse cone to reflect a pressure wave back toward the combustion chamber.  A velocity stack has no means of reversing any shock waves.  There is only the atmospheric pressure at the mouth of the stack, assuming the carb slides are wide open.

While I don't think the rubber funnels in the stock air box are tuned for a specific RPM or RPM band, they do help with the laminar flow through the carb bore and in keeping fuel flow predictable throughout each throttle setting.  The turbulence from the air filter is well ahead of these rubber cones.

As to the topic title, I vote for engine death.  Just about anything that makes an engine produce more power will increase wear and shorten it's maximum life.  But, when you add dust and dirt particles blasting onto the cylinder walls during combustion, it's certainly going to errode those walls faster than with clean air.  It's a classic trade off; power/durability/longevity.  But, then, I feel I've already shortened the life of my engine by using the foam Uni filter in place of the stock paper type that blocks smaller abrasive particles.
Don't know how I sleep at night...

[TwoTired]

Here's a short bit on velocity stacks/intake runners: http://auto.howstuffworks.com/question517.htm

By "shock bubble" I meant "pressure wave." They work by being just long enough that the wave does not escape before it heads back towards the intake valve. Properly tuned, they would work at the max power rpm of the engine. Otherwise, they are just for looks and sound.

I understand that the intake valve closure can create a back pressure wave travelling the reverse course of the intake runner.  But, when it reaches the open end of the intake runner, what causes it to reflect back toward the intake valve or combustion chamber?  Why doesn't it just...leave, or escape back into the atmosphere?

I read your reference article.  But, all they say is that it does, with no supportive reason or justification.
Hand waves are only good if you are skilled with "The Force".

[TwoTired]

I did some review and research on pressure wave physics.  During the review, I finally remembered what the physics principles are, and where I encountered them in practice before.  Not to put too fine a point on things.  But, I think the pressure waves have more to do with tuned intake runner lengths than they have to do with velocity stacks.

Try this out:
Fuel air mixture traveling at high velocity smacks into the intake valve as it closes.  This creates a high pressure area and a pressure shock wave alternating with a low pressure wave, that travels back toward the open intake.  If there is nothing in its path, like the carb throttle valve to reflect the wave, it will indeed exit the open mouth of the intake.  But, that is not important.
In the running engine, we have multiple valve closings and the reflected pressure waves have a frequency.  This means there are high an low pressure areas "stacked" in the intake runner, whose entire volume is moving en mass toward the cylinder.  The critical timing is to have a high pressure part of the air stack arrive at the intake valve while it is open.  And the column of stacked fuel air mixture must be longer than the frequency component of the pressure wave, or it will be lost to the atmosphere.  That's the theory, anyway.

The rub.

I've seen tuned intake runner lengths in drag race cars. The carbs sat about a foot and a half above the engine on these long tubes.  These were engines that were turning eight or nine grand so those lengths ought to be comparable to what is required for our SOHC4s to have benefit.  The touchy thing is that carburetors flow fuel through their jets based on the pressure differential between carb bore and ambient atmospheric pressure.  High and low pressure waves in the carb bore wreak havoc with selecting jets which react to these changes with varying flow rates.  When the length is tuned for a specific RPM, are the carb orifice exits in a low or high pressure part of the "stack" of air in the runner?
As I recall, the drag racers with tuned intake runners switched quickly to injection rather than carbs to avoid (or reduce) burning up pistons from detonation on lean going carbs.

[dusterdude]

thats ok 50,you`re not the only one on this forum thats confused.

[TwoTired]

For some reason I was confusing tuned intake runners with velocity stacks.

I HAVE seen the two schemes combined into one device.  I recall seeing a couple of auto race engines had port injection at the manifold near the exhaust port, the long tuned runner stack pointing straight up and the end of the runner was flared into a velocity stack funnel.  Looked like one machined part.   Probably called them velocity stacks just because of the flared end, and an interest in brevity.

Cheers,

[lefty]

for clear info on the subject, and how a negative pressure wave will scew everything up, as well as some interesting stuff on airbox size, read Kevin Camerons sportbike performance handbook.