Changing injector pulse rate at idle?

[Ed Carryer]

Mike Sargent wrote:
   This results in a consistant pressure across the injector, as well as
   providing higher flow as the throttle opens.

The reason for varying the fuel pressure
with manifold pressure is to mantain a constant delta-P across the injector
and therefore remove that as a variable in determining fuel flow rate
vs injector pulse-width.
It therefore does *not* provide higher flow as the throttle opens,
except to the extent that the wide throttle results in a longer pulse-width.

   BTW: With this set up, you may be able to get away without a fuel return
   line.
Don't try it ! You won't like the results. One of the side benefits of
regulating the pressure at the injector rail is that there is a constant flow
of fuel through the rail. This fuel is coming from a cooler environment than
underhood and therefore cools the fuel rail. Without this flow, and this
does happen in some old TBI designs, is that the fuel will actually vaporize
in the rail, giving you wildly varying fuel delivery and, at it's most
benign, a rough idle (this is mostly an idle problem since that's when
underhood temps. are at the peak and the fuel pressure is at it's minimum)

This is based on many hours in the hot arizona sun, trying to cure
hot fuel problems on a carburetted turbo about 15 years ago. 
The final solution, at idle and hot start at least, was to add an electric fuel
pump to the tank so that fuel was always being pushed, and a return line
from the mechanical fuel pump on the block. A return from the carb was the
preferred solution, but it wouldn't pass crash testing.


ed
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