Megaphone length

[dadsolds]

In both Bell and Morrison's formulas there is a term for the number of degrees that the exhaust valve is open before TDC. At what lift is the valve considered open enough to allow the exhaust gases to propagate the initial positive wave?

[stay`tee]

Put the bike on the dyno a couple of weeks ago, didnt have enough time for serious testing, only time for "what if", ,, had trimmed the megaphone back to the soot witness mark, no other changes, straight up five extra hp at the wheel, adjusted timing and fuel, picked up seven in total  ,, put a small reverse cone on and seen no extra in peak, however almost eight percent increase in average ,,, 

  "I guess Mister Watson, all jokes aside, that is the answer to the threads topic".

[wobblywalrus]

The method Stay-Tee mentions in the previous post seems promising.  We tune at elevation 150 feet and this is a lot lower than the salt flats.  Is there an adjustment we can calculate for the cone length determined by low altitude dyno work to make it work better at higher altitude?