The intake manifold for the M44 and some M42s that came with the 318ti expects a certain type of air-shrouded Bosch fuel injector that accepts a vacuum line. I have read this supposedly helps with fuel atomization but opinions are varied, and it may simply have to do with reducing emissions at idle. The air shroud give this Bosch injector a unique silhouette and requires a small nest of vacuum hoses to transfer negative pressure from the intake manifold to the injector body. These injectors use a Motronic connector type and are normally closed until the DME applies a ground signal to one of the injector terminals to move the injector solenoid and release fuel from the rail into the intake manifold.

Unfortunately I found no engines other than the M44 or M42 that use this shrouded injector type so when it came time to replace a failing injector I found these injectors are not made new anymore and even remanufactured and flow-tested injectors cost more than I was comfortable paying for a used part. I researched some online and found that these are the specs for the injector that are shipped with the stock the M44. It may be the same part number for some M42s but I can't say for certain, I only know that some M42 engines have shrouded injectors also. It may have been a US-spec thing for emissions reasons.

Then consulting this nice table of Bosch fuel injector specs I found a suitable replacement with matching flow and resistance specs.

This injector type was used in a wide variety of Ford V8 and V10 engines in trucks, cars and jeeps produced between '87 and '99. This is an EV6CL injector. And, importantly, I found a set of six brand new injectors for a mild $73.

The challenge with these new injectors is that they are physically shorter than the air-shrouded injectors I was replacing; in addition to the length discrepency the interface where the injector meets the manifold is 3mm bigger on the stock injector I was replacing. Evidently BMW made Bosch do things differently just for their cute little 4-banger engine.

Lacking access to any equipment to manufacture a serious adapter, I made 4 bootleg adapters that raised the manifold ports by 6mm and reduced the diameter of the injection ports from 17mm to 14mm out of a washer and a rubber grommet I trimmed with a dremel and sealed the thing with RTV. It fit well on the manifold and the pressure from the tightened-down fuel rail ought to keep it in place but to make sure I secured the adapter to the manifold ports with JB Weld #justmechanicstuff. Soon I will design a part in a CAD that can be machined, but lacking both relevant CAD experience and access to a lathe I could not machine these out of a metal as I'd prefer.

With the injectors replaced, I had a spare vacuum tree coming off the intake boot that needed to be handled. As this tree had no use I simply tossed it out and plugged the intake boot hole with a plastic PEX pipe cap I found at Lowes.

After taking it out for a careful drive with my fire extinguisher handy I noticed almost no difference in driveability except that the 3500 rpm range where my injector performance was previously degraded because of the failing cylinder 1 injector had disappeared (´• ω •`)

This could be used to upgrade the injector flow rate as many aftermarket injectors are 14mm diameter anyway, but probably don't turbo your M44! It's a small engine anyway; swap in the S50 or S52 engine if you're going after power and are really, really in love with the E36 platform, that's what the M3 shipped with anyway. With a new engine you get a new intake manifold that probably has more sensible injector interfaces.

I drove my car with this modification for about a year and it continued to run very well through that time. After almost exactly a year I pulled my bootleg adapters out and replaced them with machined adapters made by VAC motorsports, available here. I'm sure it would have continued to run fine, as my modification was very difficult to pull out, but the potential of one failing after many cycles, especially being exposed to operating temperatures in the bay, was not worth it to me. These could be machined on a lathe but this requires lathe operating experience (and access) that I lack currently.