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Old Aug 15, 2012, 01:44 AM
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Adelaide, South Australia
Joined Sep 2003
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It's very difficult to say exactly why fuel sometimes spits back because there are so many different things all happening at the same time. Some of the problem could be that an engine is running much slower than it's peak HP revs (almost no one runs them at peak HP revs) so crankcase compression is building up before the port closes causing a flow reversal. However the area of the port that's still open at this time is very small which would limit any reversal. Add to this that crank timings vary wildly between engines designed for the same (and even different) applications.

As a for instance, one of my CL engines has quite a late (55 ATDC) port closing time and it's only running at 7900 revs but spits no fuel at all. The only difference between it and an RC engine is that the venturi area is much smaller giving a higher velocity at the spray bar and the distance between the spray bar to the top of the inlet is longer than for an RC carb. The longer inlet may be just long enough to trap any spit back when the port next opens. A simple way to test this out would be to add a slight extension on a carb with an engine that spits back fuel.

As for terminology, dwell is the wrong term to use for the difference in exhaust/transfer port timings. It's almost universally called blow down time (or period) which effectively is the period when the engine evacuates the exhaust to a level where the remaining pressure is low enough for the incoming compressed charge to start flowing into the cylinder. An engine designed for moderately low revs will have less blow down time (degrees difference) because there's more actual time for the exhaust to escape. A high revving engine conversely needs a longer period (degrees difference) because of less time for the exhaust to escape. On average most engines have the transfer ports opening at ~60 degrees BBDC while the blow down is set by the exhaust timing which, on average, is ~70 degrees BBDC for slow revving engines and ~75 for high revving.

Dwell is really only used for the time a piston remains almost stationary around TDC for combustion to begin. The longer the dwell the more ignition can be retarded and allow for pressure to rise while the rod is increasing its angle to get more leverage on the crankshaft. The larger the rod length/stroke ratio the longer the dwell.
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