injector experts, please help...

[sfwilliams@comcast.net (Scott F. Williams)]

Joshua asked:
> What duty cycle should I use in my calculations? What fuel pressure?

Shoot for an 80 - 85% dutycycle. Yes, you can be like some infamous folks on
this list and peg your injectors at 100%, but that is not safe; nor will it
likely permit you to realize your maximum power potential.

As for fuel pressure, keep in mind that most Bosch injectors that you'll
come by are best used between 3 and 5bar (which is the maximum that Bosch
recommends for the majority of their products). Also realize that fuel flow
is a function of injector flow capacity, fuel pressure, and the duration of
time that the injector stays open.

Also remember that while running very high pressures on a small injector
increases the amount of fuel that can be supplied at WOT, it also increases
the lowest possible amount of fuel at idle. Here's an example: Suppose that
the minimum amount of time the injector can complete an on/off cycle is 1ns.
And, suppose that during that 1ns timespan at 3bar fuel pressure the correct
amount of fuel is supplied.

Now suppose that you turn up the boost and start to run lean at high rpms.
You don't have enough flow up top so you decide to increase the fuel
pressure to 5bar. What will happen? You'll solve your problem up top but
will run rich at lower speeds. So, there is a practical range that you must
work with as far as fuel pressure, ECU calibration, and injector size are
concerned.

So, the goal here is to match your injectors to suit a hose of factors.
Along with other concerns, you must consider the size of your engine, power
goals, brake specific fuel consumption, engine management system, fuel
quality, air temperature, compression ratio, and probable intake
temperatures.

> I'm trying to piece things together from commonly available junkyard
parts.

Well, let me know what size motor you've got, what your engine management
system is, how much power you're looking to make, and how you're planning to
do it. I'll let you know what JY injectors make sense. Or, depending on what
you say, I might have something in my inventory for you right now.

> Also, will an ABA (late model 2L 8v) fuel rail be properly spaced for a
16v?

The spacing is correct, but they don't reach deep enough through the intake
manifold to reach the injector tops.

> And if so, what FPR's can I run on it?

You'll probably find your best deal with a Ross Machine Co. fuel rail and an
adaptor to fit a Bosch regulator of the sort that came on the Corrado G60,
Porsche 944 Turbo, etc.

> Will I need to find a way to step my fuel pressure down from the CIS-E
> 80+ psi?

That's what your fuel pressure regulator does. The extra volume of fuel will
be returned to your tank just as it is done right now. By the way, there are
lots of hotrodders out there tuning nutty RX-7s, WRXs, Supras, and other
fast cars and they're using CIS pumps. :^) Our fuel systems run a *lot*
higher pressures than most modern cars, so we don't have to upgrade our
pumps like they do.

Anyway, fire back those details and I'll walk you through your injector
selection.
--
Scott F. Williams
NJ Scirocco nut
'99 Subaru Impreza 2.5 RS
Mazda 323 GTX turbo "assaulted" vehicle
Golf GTI 16v "rollycar"
ClubVAC: "Roads found. Drivers wanted."