Small Fuel Injectos

[freqflyer]

I'm in the planning stages of second Mega Squirt project. What I want to do is to add a Mega Squirt to a BMW 328i in a piggy back fashion.

The BMW computer will run as normal handling ignition, cold start routines, and fuel etc. I am planning to do some modifications to the car that will add about 50 horsepower to it, maybe a little more. I know from others who have done similar modifications, that the factory computer will not handle the extra power without being 'chipped".

I don't want to chip the car. The chip programs are too expensive for it, and from what I've seen from dyno charts with AFR posted on other forums, they aren't a very good tune. The still let the engine go lean at high RPM. Some of these charts I have seen show as lean at 16 to 1 at very high RPM.

I want to add a second fuel rail with six additional injectors to be controlled by the Mega Squirt.

Here's the problem 50 horsepower equals 25 pounds of fuel more. This means I need roughly 30lbs more of delivery available to keep the injector pulse widths in the proper range. 30 divided by 6 equals 5. So I need six 5 lb injectors or at least something close to that.

Do they make injectors this small?

Where can I get them?

Am I going to be forced to go with one 30 pound injector located in a throttle body type setup?

If I have to go with one injector does it have to be a throttle body style injector if I put it just past the butter fly or can I use a port injector in that area?

I would appreciate any ideas and help.

[Matt Cramer]

You could run 14 lb/hr fuel injectors at a low duty cycle if you want; they're about the smallest commonly seen automotive injectors.

[freqflyer]

Here's what I'm worried about 14lbs injectors times 6 and 20% duty cycle is 16.8 lbs an hour. That would put me at about half my maximum needed.

How low of a duty cycle can I reliably run?

[Matt Cramer]

How low of a duty cycle can I reliably run?

Zero.

[Bernard Fife]

ff,

Matt is correct, you can reliably run 0% duty cycle - nothing will break. However, running very low duty cycles has a couple of problems:

- the effect of any discrepancy between the set opening time (including voltage adjustment) and the actual opening time will be a much larger percentage of a small pulse width, and thus the fuel may be very hard to tune - small increases to the fuel (with the VE table) in this pulse width range may result in way too much fuel, small decreases may result in no fuel at all if those changes span the actual opening time (and it can be quite difficult to determine the precise open time and voltage compensation factors accurately),
- the pulse width resolution is finite. So while you can run 0%, the next step up may be a big step. This is more of a problem with MS-I (0.1 msec/step) than MS-II (0.001msec/step). Again, this may make it hard to tune fuel precisely at low pulse widths.

In general, you will get further by thinking of the minimum pulse width rather than the minimum duty cycle (duty cycle is rpm dependent, so the same pulse width that gives 20% at 600 rpm will be just 2% at 6000 rpm).

There is more here: http://www.megamanual.com/v22manual/mtune.htm#idlepw

Lance.

[Matt Cramer]

Very true - the smallest pulse width, and the smallest increment you can change them in, are what counts. Many injectors behave a bit strangely (nonlinearly) at pulse widths close to their dead time. A system where the maximum duty cycle is 35%, however, doesn't worry me all that much - although, personally, I'd put the MS in control of the primary injectors.