Having mounted throttle bodies on a ST170 manifold I have a choice of fitting the injectors close to the head or in the bodies themselves. I don't need to fit two sets yet. I will have less than 200bhp when all the mods are done. Before I drill the fuel rail I need to decide as the spacing is different. Mostly road driving with some track days.
Rescued attachment inj.jpg

I used the St170 manifold injector holes on the same setup as yourself. My mate who did a gas conversion said the instructions for it said the additional injector should be mounted as close to the head as possible. Also All car applications that I have seen always have the injectors close to the head.

Steve

Injector position



The position of the injector in the inlet tract has a noticeable affect on the way the engine runs, it can affect economy, transient throttle and power output. It is generally accepted that injector positioning close to the inlet port gives good economy, transient throttle and idle together with good emissions and that injector positions further back in the inlet tract improve power at the expense of these criteria. Ultimately for the best power output the injector should be sited as far back as possible, I.E. in the trumpet or air-horn. Siting the injectors here does give a big problem at low throttle openings and low RPM since the fuel hits the butterfly; it can also cause fuel to be bounced out of the trumpet by the shock waves in the inlet.



Dual injector systems


Dual injector systems attempt to exploit the benefits of the close to port injector while also gaining from the power gains to be had from having the injector in the trumpet. The way this is done is to fit two injectors, one close to the inlet port and one in the trumpet. The EMS controls these two injectors using the near injector for part throttle, low RPM and transient and switching to the second trumpet mounted injector when the engine is at WOT (Wide Open Throttle). Some systems switch from one injector to the other immediately a certain set of conditions is reached, other system go 50/50 between the injectors or grade one injectors usage down while ramming the others up. This system if implemented properly gives the best of both worlds.

Twin injector systems


Twin injector systems are normally used when the size of injector required would be very large and might affect the metering and atomisation capabilities at low RPM and idle, typically on a turbocharged engine where fuelling requirements vary enormously from transient to wide open throttle. The fuel can be metered through one injector when requirements are low, and through both when requirements grow exponentially, or it can be metered through both at all times. Often a second set of injectors are fitted by after market tuners whose modifications may require fuelling beyond the capacity of the current injectors, this is most likely to happen in turbo or supercharged installations


Hope this helps.

[Edited on 10-2-06 by BKLOCO]

If you want to read the whole article where the above info was "stolen" from It's HERE A lot of it is very basic stuff but it does have useful info both for Engine management newbies and old hands.

Ah, the mystery unfolds.
It's not magic after all.
Cheers BKLOCO.