Injector supply voltage & positioning.

[Dale Ulan]

> On Wed, 21 Dec 1994, Lou Faustini wrote:
> 
> a PFI application is this:  The injector is only open a short time and 
> must deliver a highly precise amount ofd  fuel.  The throttle body 
> systems have much larger orifices and are typicaly open longer than the 
	      ____ yes ____________                       **nope***

> PFI injectors.  The PFI system must have a good pressure to atomize the 
> fuel as it doesn't have a lot of distance to travel to the cylinder... 
> Anything still liquid just washes oil off the cylinder walls and is 
> detrimental to performance and emissions.

Best performance seems to occur when airflow is minimal in the intake
port. I don't know why, but timed injection systems appear to avoid
injecting while the intake is open. This is probably to minimize wall
wetting due to reversion.

TBI injectors are open for *shorter* than PFI injectors. PFI injectors
have up to 720 degrees of crank rotation to complete their injection.
With this amount of time, a low flow rate injector can be used, for very
precise flow control. The high fuel pressure is more to do with the fact
that port injectors run hotter than TBI systems, so the high fuel pressure
is needed to prevent vapor formation in the fuel injectors. PFI injectors
do not have the appropriate structure to vent vapors formed within.... all
vapor must exit through the output port.

TBI systems can run a lower pressure because they generally have a very open
design, and bottom-up fuel feed. This helps control vapor venting. Lower
pressure means one thing only: it's cheaper. The regulator is cheaper, as
are gasket materials used everywhere. And most importantly, the fuel pump
is cheaper. Take a look at the TBI paper presented by GM in 1980.
Also, in a V-8 2-injector TBI, each injector must open four times in
720 degrees, or 180 degrees maximum opening time (minus closing/opening
times). Thus, the TBI systems are actually open for shorter periods
of time.

-Dale