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Old 01-12-2005, 09:16 PM   #13
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Originally Posted by vrago
I'm in learning mode now,

What do velocity stacks do exactly?

Tuned for what?
Velocity stacks are used to smooth the flow, and prevent the inlet stream from separating from the inlet tract walls on entry. Which would actually reduce the effective area.

This is a typical piece of crap velocity stack beloved of Harley riders and VW builders. Note the sharp entry. Avoid them.

Now here is the business. Note the fully radiused lip. That's what you want. This allows the full inlet area to be used, and as the diameter gets smaller downstream, the gas flow is gradually accelerated.

Tuned length is a different subject, Sort of, as velocity stacks are often used to vary the tuned length of an inlet system. My own preference is to see the inlet tract either shortened or lengthened downstream of the carb or throttle body, but to each his own. But either way, every inlet should have a full radiused entry, or as close to one as we can get.

An inlet system is tuned as follow: When the inlet valve opens there is (hopefully) a lower pressure in the combustion chamber. When you open a valve and there is a pressure differential in the adjacent chamber, a wave is created. In this case, the higher pressure inlet tract creates a negative, or suction wave that travels back up the inlet tract. At full throttle, as long as the inlet tract is otherwise unobstructed, this negative pressure wave travels right through the carb or throttle body, and to the end of the inlet tract where we have just installed our velocity stack.

So what happens when a negative wave reaches an open end? It also reflects back, but this time as a positive pressure wave. And what we want is to have the length of the inlet tract just long enough so that this reflected wave arrives at the valve just before it closes and shove a little extra fuel/air mixture in, where the closing valve traps it. The math is pretty straight forward, the speed of sound (which is the speed that all our waves travel, which is why it is often called acoustic tuning) is @1100 ft/sec. All you have to do is figure out your cam timing, and what RPM you want this effect to happen.

The waves are pretty much a full throttle only thing. Waves will move quite nicely through smooth cross sectional changes, but a throttle slide sticking halfway through the bore will kill them dead.

And there are two big flies in the ointment. The first is this only works at specific RPM, or multiples thereof.

The second problem is that there is not always room for a single pulse to travel up and back. Inlet tracts tuned for that tend to be pretty long. But you can always cut them in half. A system tuned for 16000 RPM will also work at 8000 (the waves just travel twice up and down, but the pressure phasing is almost the same), and it will fit a lot better.

The effect of all this is great at 8000 RPM if that is what we have tuned for. But it pretty much sucks at 6000 because it is now out of phase (though it may be back on the job at 4000). But there is hope.

Modern airboxes are also frequency tuned (the term is Helmholtz Resonator if you want to do some more reading), and they are tuned to take out the flat spots created by inlet length tuning. Waves resonate in an airbox system just like they do in the inlet tract. And these can be tuned to have the airbox have it's highest pressure at any RPM you want. Modify a modern airbox at your own peril. There is a lot of engineering behind them these days.

Pretty much the same thing happens on the exhaust side, except everything there is tuned to give the lowest pressures (as opposed to our inlet example where we want the highest pressures). But, in general, longer primary pipe lengths work at lower RPM, and shorter ones at Higher RPM. In fact, a two stroke's expansion chamber is nothing but a bunch of tuned lengths with diffusers and convergent tapers designed to amplify and lengthen the effect of these waves bouncing back and forth.

But watch that wave speed. Due to temperature, the speed of sound, and of our waves, in the exhaust system is @1700 ft/sec.

If you space these events out (inlet tuned for 8000 RPM, Airbox tuned for 6000 rpm, exhaust tuned for 7000 RPM) you get a nice wide powerband with few flat spots. If you bring them together (everything tuned for 8000 RPM) you get a rocket with no powerband at all. It's like turning on a light switch. And it will have a flat spot you could shoot pool on.

Lot's of good info in books and on the web if you want to dig into the subject in depth. Cheers.
- Jinx

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Jinx screwed with this post 01-12-2005 at 09:51 PM Reason: removed double quote
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