Bellmouths, or to you chaps, velocity stacks...

[Stevex]

Does the length of a bellmouth (velocity stack) fitted to PHF36's alter the way a Guzzi 850 V twin produces power?
I've read that shorter 'mouths are better for producing low rpm power whereas longer ones help at high rpm.
Is this limited to a race prepped engine or would one expect to feel a difference on a road bike, for example a LM2?

[Triple Jim]

I've noticed that what's considered correct on the Internet and what really works are not always the same.  You may learn more by experimenting.  You can make extensions to standard stacks with PVC pipe, and try different lengths to see what works for your engine and what doesn't.  A friend of mine used PVC elbows between his carbs and filters because of space limitations, and found quite a bit more bottom end and midrange for his 2-stroke.

[johnr]

It must make a difference to the length of the intake column of air and therefor make a change, just like altering the effective length of an exhaust pipe will. 

Whether that change is an improvement or not is another question as the 'over breathing' that air column tuning provides is affected by the cam.

There is a formula for it some where. I may have seen it in a little book called 'Speed and how to obtain it'

[Rough Edge racing]

 Correct,try it out.... on the chassis dyno ,any length velocity stack reduced the power of my Triumph racer. Another engine  might see totally different results...The length of the intake tract, exhaust and cam are a team...But on a stock bike the effect of a short stack might not be noticed...