Fuel Stand Off
Posted: Tue May 17, 2011 11:54 pm
CAn anyone explain to me what causes fuel stand off... Would be interested to know...
cheers
Mark
cheers
Mark
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elansprint71 wrote:Very good John.
Now can you explain about a Mexican stand-off?
Andy8421 wrote:Bill,
The reason you haven't seen standoff is that it is only visible at full throttle and high revs. You wont see it just blipping the throttle. Search the web for videos of twincam dyno runs and the standoff is clear.
Just to expand on previous explanations;
When the piston decends and inlet valve opens, air doesn't go rushing through the carb and inlet tract - the air column takes time to accelerate. Instead a low pressure zone develops at the valve throat causing a low pressure pulse to travel back along the inlet tract at the speed of sound. This pulse exits the carb mouth and a high pressure pulse fills the gap and travels back toward the valve. In an ideal world this positive pulse reaches the valve at about the time it is closing and pushes a little more mixture into the cylinder giving a 'ram charge' effect.
The positive pulse in turn gets reflected and travels back up the inlet tract, carrying mixture with it that exits the carb mouth. This doesn't happen in isolation, subsequent valve openings cause futher pulses to form. At certain engine speeds and with certain inlet tract lengths the pulses all line up and a standing wave is formed, which is stationary relative to the carb. At this point a 'cloud' of fuel vapour forms and seems to hovver over the carb inlet.
Edit: Just found this. Look at 1:20 onward. Not standing waves, you need to get the revs just right, but clearly shows standoff effect.
http://www.youtube.com/watch?v=RSSYcVTUrT8