Hi all,

I've recently discovered the use of the so-called "velocity stacks", mainly for gas engine conversions and similar sized engines. They all appear to be a tube bolted to the carb intake, with the end cut at 45º facing the airflow, so prop blast is forced into the carb - creating a sort of forced induction effect.

I'd like to know if there's a noticeable perfomance increase with that mod, as well as possible design considerations. I hope I don't say too many stupid things, but:
- The tube seems to have the same internal diameter as the carb, but why not give it a different shape? Why just no use a 90º elbow instead of the 45º cut? Will I gain something if I try what I've seen in bikes - cone or trumpet shaped carb intakes.

I was going to turn a simple velocity stack with my lathe but would like to know if it's worth the time and if other shapes are worth a try.

This is a black art at best. What works wonderfully on one set up will be a dog on another... The only way to know is to try it and see. If you wanted to do a bell mouth shape, I would look at the motorcycle world for ideas, as you have done already. Good luck with it...


Mark

I always thought a velocity stack was a tuned length tube that relied on the pressure wave, moving at the speed of sound, to reinforce/supercharge the intake of air into the cylinder. The specific length of the tube causes an increase in intake volume at a particular RPM only. The issue of ram air from the propeller is a separate issue, not to be discounted however.

Allan, you are correct. Especially in relatively still air. But the problem with model aircraft use is that the air around the carb mouth is anything but still... Which is why things work great on one model and terrible on another. Vehicles use large volume airboxes to provide that still air reservoir, but we have no such thing on our planes.

Also, be careful about trying for pressure increases with prop airflow and ram effects.. If you do not vent the reference side of the regulator to the same pressure, you will get mixture changes with the pressure fluctuations. This will cause no end of problems and make tuning impossible. This effect is the very reason so many people vent the diaphragm into the still air inside the fuse to provide stable running.


Mark

Interesting! So basically a (good designed) velocity stack acts like sort of a "tuned intake pipe" using waves to force air inside?
Tuned pipes are actually better than using a free exhaust on 2-strokes, so this sounds like there could be some potential improvements. In fact, I now remember a small trimmer engine I saw at my university. It had a telescopic tube fitted into the carb's intake. By varying it's lenght, you could hear the rpm change, so it makes sense to think that.

And yes, that is actually a very different thing oposed to a ram air system. Even if you put the intake right behind the prop, I'm sure there would be some fluctuations (apart from those cause by varying rpm). Cowl shape (inside & outside), plane attitude, will probably cause the device to work partially or even decrease performance. So I'm kind of unpassionate about trying to make one. Making a thought design is a nice excuse to learn aerodynamics, but if I want to try something fast I'll just have to experiment. Maybe a tube of varying lenght, and several shapes. That is, for a plane and a certain rpm (more likely top rpm). I guess if I still found a design that helped with top rpm, it could still degrade midrange performance. So unless someone has a radically different point of view, I'll make some experiments when I have the time and lack of projects to work on.
Mark, could you further explain that venting issue?

You don't need them. Many times you must cut a ugly hole in a beautiful cowl if you do use one. I have two gassers with the carb very close to the inside of the cowl and they run fine.
Just my 2 cents

Its not about the air being rammed into the engine. The intake plenums are setup so the air comming in is tuned. Wether it is a trumpet or "velocity stack". The two really have the same effect. If you think about the air being drawn into the plenum on the velocity stack, the air will hit the end of the barrel and get sucked in on the short end of the taper. The reason the taper is there is to deter from a vacuum effect. If air was blowing over the tube end, without that slat, the air would be sucked out of the carb. Fighting the intake charge. Now if you spun the taper on the velocity stack around so the tall part of the stack was forward, a siphon effect would be happening. Low performance. Most are better off leaving the trumpet on there because 360 degrees of comming in is air is better than having 180. If the "velocity stack" was mounted in the cowl one may have better results. But neither one will have a ram air effect. Hope this helps.
Adam

Many velocity stacks are square on the end (no slant). When they are slanted, best results are usually obtained with the high end of the slant forward (you do not want ram air into the stack or you will have a difficult time maintaining proper mixture at all throttle settings). And yes, a properly designed velocity stack will usually give a slight improvement in performance.

Another function of a velocity stack is to straighten up the flow of turbulant air. I used to have a Roadrunner, 440 Wedge with Hilborne injectors, constant tuning nightmare. Leave the stacks for the Quarter Mile Dudes.


Keep The Faith: Johnny

Im thinking of making an exhaust and wanted to make a megaphone shape like a tumpet straight off the port and wanted to try and use the theory of the velocity stack basicly have the speed of sound travel from the port back to the port as the port opens back up but am having trouble getting this worked out sounds crazy I know. But when flying my trainer a couple of years ago with an os la the nut fell off with vibration and the end came off the exhaust during flyte but I thought it sounded pretty grizzly just above tick over and would love to experiment as I hate that screaming noise and those quiet pipes you can barely hear any help on how to maximize the noise and get the calculations for the velocity stack design also was at an airshow and was looking at a spitfire and the exhaust pipes were tiny is there a short distance exhaust pipe tunning formula

speed lover,

2 stroke exhaust is a spooky, black art of a design problem. Formulas can get you in the ballpark, but then you must test if you want to really get there. Does an unmuffled tuned pipe not get the noise where you like it? Anyway, there are no magic formulas for what you are trying for. There are chapters in text books and entire text books written on this subject....Some reading is required if you really plan on understanding what you are doing and getting results that aren't just a function of guesswork. Some books to look at:

2 Stroke Performance Tuning by A. Graham Bell

Design and Simulation of Two Stroke Engines by Gordon P. Blair

The 2 Stroke Tuner's Handbook by Gordon Jennings

Also look at MOTA simulation software, it is pretty reasonably priced and does a decent job on small 2 strokes, glow and gas.


Mark

I "lift" the thread again to ask if anyone here could give me some measurements to make one for the Homelite 30cc, to know where should I aim for if I decided to give it a try (I can always machine a couple of "long" stacks and vary length and shape to experiment).
During this time I've seen more gas engines and they all have in common that many of them, like half or more, seem to have one, and with the high end of the slant backwards, so I start to think that in most cases it's just for aesthetics rather than for real performance. In fact, I believe a poor match even spoils or degrades some of it.