I wanted to find high impedance injectors for a particular ECU application (more about ECU's later), so I did some research on this. Unfortunately it gets rather involved.
The intake manifold needs to be modified to fit 'modern' (high impedance) style injectors, where they mate with the small ports on the runners. That will involve machining and welding, plus more adapters and different seals. Then there is the matter of the length of the injectors. The design of our manifold, with the plenum directly over the fuel rail, does not allow for any additional length to fit under it. A lot of the high impedance injectors I came across were longer than our stock ones, with the adapters adding to the length. There are some "mini" injectors that might work, however their selection is even fewer (and cost higher). I also looked into using a different fuel rail but the same challenges existed, plus it also added to the overall cost.
The injector's fuel output rate was another issue. Most of the modern injectors are actually too big (flow) for our little engines. So finding the right combination of injectors (size, configuration and specs), adaptors, manifold modifications, seals, electrical harness connections, and getting the fuel flow rate needed all gets very difficult. Adding to the challenge, there isn't a ton of information available for the exact dimensions and other specs for many of the injectors out there. They are typically listed for replacements on certain applications. Trying to cross reference things got silly. Most of the sources I found had incomplete info and was contradictory with other sources. Frustrating and very time consuming with little results.
I'd love to find that unicorn injector part number, right selection of additional parts, and specific mods needed. Hopefully someone with more resources than me can come up with it. Although there is yet another factor to throw into this mix, cost. The price of modern injectors can be significant. Add to that the cost of the additional parts and services needed and it brings the already lofty price of the ECU, etc to new heights, far exceeding the worth of this to me.
There are a couple alternative approaches to this. One is to use a ECU that allows use of low impedance injectors. Many of the better ECU offerings have this capability. Unfortunately that comes at a cost, putting the overall expense back up there. Aside from the top of the line MegaSquirt products (also quite costly), the "affordable" versions (eg. MicroSquirt) do not allow for low impedance injectors.
The other option is to 'convert' low impedance injectors to high impedance ones by adding resistors. Several car manufacturers did this with their OEM FI systems for years. It allows for low impedance injectors (including the stock X1/9 ones) to work with lower cost ECU's that otherwise cannot run them. The drawback is it creates a slight time delay in the injector's response. But if you are programmed to run the FI with batch type firing, this really isn't an issue. Especially for a street driven car; it might be more of a factor if you are planning a extreme 'full race' engine build. And the lower cost ECU's like MicroSquirt will only run in batch mode anyway (the decision over batch vs sequential injection is another topic). In fact the MicroSquirt manual details the use of resistors for this purpose with the ECU.
Looking into the resistor issue I found a couple considerations. The impedance value of the resistor should be calculated for the specific injector to be used, such that it brings the total value close to the ECU's maximum capacity. The reason for this is to reduce the impact of time delay and to maximize the injector performance. Talking to the builders of the MicroSquirt units I was able to get some data to help with that. Running the ECU within a safe margin of total resistance, but closer toward the upper limits of that range, creates a slight amount of additional heat. So I purchased a aluminum heat sink to mount the ECU onto. Probably not necessary but why not.
The heat capacity of the resistor itself also needs to be high enough for continuous operation at that impedance value. The wattage rating and specific design and packaging of the resistor makes this possible, with the right choice. I've also purchased another aluminum heat sink to mount the resistors onto. Again, not necessary but I figure it will help with managing operating temps.
Finally the resistor needs to be a type that can conveniently be spliced into the electrical harness for the ECU.
Finding resistors that fit all of these requirements required some more research, but surprisingly the cost and availability of them wasn't bad once a specific part number was determined. The total cost for the resistors and heat sink was around $20.
My stock injectors have an unknown history and although inspection found them to be working, I decided to replace them for ones with a slightly higher output for my turbo build. Fortunately low impedance injectors are plentiful and very affordable. I found a direct replacement with 20% more capacity (perfect for my low boost turbo) at a very reasonable cost (around $20 each). Combined with the MicroSquirt's low cost, it puts the expense of using a aftermarket standalone management system into the realm of affordable for my low-budget build. This may not be the ideal solution. That decision will differ for each build's requirements, budget, application, and personal preference.