View Single Post
Old 06-17-2013, 01:03 PM   #3 (permalink)
t vago
MPGuino Supporter
t vago's Avatar
Join Date: Oct 2010
Location: Cedar Rapids, IA
Posts: 1,766

The Karen-Mobile - '05 Dodge Magnum SXT
Team Dodge
90 day: 26.72 mpg (US)

Fiat Dakota - '00 Dodge Dakota SLT RWD Quad Cab
90 day: 16.67 mpg (US)

The Red Sled - '01 Dodge Durango SLT 4WD
90 day: 16.96 mpg (US)
Thanks: 799
Thanked 681 Times in 437 Posts
Originally Posted by NachtRitter View Post
Does having a returnless system mean that different amounts of fuel can be injected with the same length injector pulse?
Yes. In a returnless system that uses a constant-pressure fuel regulator, fuel injector pulse time is one of two variables that determines the amount of fuel to be delivered. Returnless systems, that vary the voltage of the fuel pump to vary the fuel pressure, are likely not affected, but I don't know for certain.

In a returnless system that uses a constant-pressure fuel regulator, the amount of fuel to be injected is now also affected by the pressure differential across the fuel injector itself. In fact, it will vary as the square root of the pressure differential. Earlier systems used a fuel pressure regulator that was indexed to intake manifold vacuum, thus providing a constant pressure differential across the fuel injector. These earlier systems would reliably squirt the same amount of fuel for a given fuel injector pulse, whether at idle or at cruise or going up a hill or at WOT.

Given that my fuel pressure regulator regulates to a constant 58 psig, regardless of intake manifold vacuum, and that my intake manifold vacuum could vary from 5 to 14 psig, that means that the fuel injectors could themselves see anywhere from 63 to 72 psig of differential pressure. This translates to about a 7% variation of the amount of fuel that may be delivered, for a given length injector pulse. The engine computer knows this and will compensate, but the MPGuino does not (and cannot, in base form).
The Fiat Dakota

The Karen-mobile

The Red Sled
  Reply With Quote