Velocity Stacks

Hi my name is Adam and I was wondering if you know any formulas to aid in designing velocity stacks.  I am going to use them on a snowmobile but the theory should be the same for all two strokes I want to make them resonate at around 8000 rpm.  Thanks for your help.



    I think what you are referring to is the sonic waves that occur on the intake side of the motor. Velocity stacks change the tuning of the wave to a broader rpm band. Unfortunately, all this is determined by engine design, exhaust pipe design, intake design, bore/stroke ratios, rpm, and just about everything else you can think of in this motor. Probably not what you wanted to hear. Most of the snowmobile engines I have seen, are tuned to work with the airbox the manufacturer designed for it. When you remove the airbox, most of the time performance actually goes down. There is a formula to figure a velocity stack at the rpm you are talking. Whether it would make more power, remains to be seen and it would probably be less than 1 or 2%. The book "Two Stroke Tuners Handbook" by Gordon Jennings lists all of the formulas you are after. They are ungodly lengthy, and complicated, but they are there nonetheless. If you would like to make your velocity stacks work, I would suggest studying this book. You will need to fill in all the blanks, as far as CC's and engine design and such to come up with the right formula. But again, as you will find in the book, the gains are not all that great. A good set of pipes on your snowmobile will reap huge rewards, compared to the pitiful results that the velocity stacks will give you back. Hope this gets you interested in the book, as I think you will enjoy all the possibilities that it gives you. 




Please submit whatever questions you have to  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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