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 Posted: 01-27-2009 08:42 pm
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subwoofer

 

Joined: 04-01-2008
Location: Sandefjord, Norway
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Finally had some time to cook up a reply to this:

pc wrote:
Fuel delivery in a carburetor is a function of air volume, i.e. Velocity x Area.  In the case of a constant area carb (Weber, Delorto, Holley, Mikuni,…) fuel flow varies with velocity and the venturi cross section is fixed.  In the case of a constant velocity carb (Zenith-Stromberg, SU, Hitachi,…) fuel flow varies with throat cross section and velocity is fixed.

It is actually a function of air mass, since fluid density is a part of the Bernoulli equation (describing the pressure change for a given change in flow velocity), and the reduced pressure above the jet is the driving force of fuel delivery. The need for retuning due to altitude and atmospheric conditions is most likely caused by non-ideal behaviour of a carb and the combustion cycle.

EFI fuel delivery is a function of whatever the system designers builds it to be.
 
basic electronic control schemes commonly used include:
<snip different metering strategies>


I am aware that there is a lot of different strategies used to calculate the pulse width for the injectors, but is does not change the fact that fuel delivery in any EFI system is strictly based on time.

The controller delivers a certain amount of fuel at a specified time window, regardless of the momentaneous flow of air. Because of that, you can run into problems if air flow isn't evenly distributed over the full 720 degree cycle of the engine.

If the air is flowing slowly past the injector at the time if fires, the charge is denser than if the air is moving at a higher speed. Since the two cylinders running off the same TB are spaced 180/540 degrees apart, not 360/360, it would be reasonable to expect that one cylinder will get a richer charge than the other, unless the volume of air in the common part of the manifold is much larger than the volume of the individual intake runners. Since the bifurcation is quite close to the carb/TB, I don't think that is the case for the Stromberg equipped 907.

Just thinking out loud...

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Joachim