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Date: Mon, 20 Jun 2005 18:22:44 -0600
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Fuel Injector Position 
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When you have long manifold runs, the accelerator pump volume becomes HUGE. 
Perhaps I am dating myself, but the old Chrysler 300 had four-barrels 
outboard of the valve covers and the manifold runs crossed the entire 
engine. The accelerator pumps on those carbs were enormous. Blipping the 
throttle was like flushing a toilet.

         The reason this is so is because you must instantly provide the 
fuel for a whole manifold full of air. You go from a pretty good vacuum to 
a full atmosphere in the time it takes for the butterfly to open. As this 
initial slug of air is flying past the butterfly, you must quickly dump 
fuel into it.

         On the quickly closing side of the problem, the fuel wetting the 
walls of the manifold is, indeed, a serious issue. Fuel doesn't evaporate 
that well at room temperature and pressure. Thus, the less volitile 
fraction of the gasoline wets the manifold walls. When you suddenly shut 
the throttle, you get a big dip in pressure. The surface area of the wetted 
walls is quite large and the fuel layer is thin, so when the pressure 
drops, all the fuel flashes into vapor.

         If the manifold is very long, you can get enough fuel to cause a 
few misfires which fill the exhaust with fuel/air mixture. Then as the 
mixture becomes leaner, the engine fires, touching off the mixture in the 
exhaust system. Ka-bang.

         On cars with carburetors and long manifolds (like air-cooled VWs) 
they put a throttle position damper to prevent the throttle from closing 
too quickly.

         Bill Dube'