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Injector positioning

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When changing injectors, using aftermarket manifolds or adding another row of injectors, often the injector nozzle location is not as it was before. Even the spray pattern may easily make significant difference in the fuel film and atomization.

What are the main aspects to keep in mind when changing or installing injectors?

What problems may occur, when injector sticks too far into the manifold or vice versa?

Ideally, you would replace injectors with different ones with similar spray patterns. If the spray pattern is different then a different location may be the cure. Or if a different location is used, you would need a different spray pattern. The only way to know is to think about it, and be prepared to test and swap parts if necessary. If you want to avoid that, only use combinations (based on other peoples experience) that are known to work.

I would not worry about injectors sticking farther into the air stream, but I would be concerned if if the spray pattern was modified by being to recessed into the mounting location).

Here's an example of an injector swap that took a couple of tries to sort out...

I worked on an F1 engine that was fitted with low-impendance injectors mounted above slide valve individual throttle bodies. The engine wouldn't run as the ECU fitted was built to only drive high-impedance injectors. We had a several month long test with various low-impedance injectors to find ones with the same flow (all so the engine would not have to travel back to the UK for dyno tuning -- and thus the ECU mapping would not need to change). While the selected injectors flowed the same amount, the spray pattern was different enough that when fitted the engine wouldn't rev above 4000 RPM (original injectors had better atomisation). The final solution was to put a hardware driver in between the injector outputs and the injectors that could drive low-impedance injectors, and use the original injectors. All that was required tuning wise was a proper barometric compensation (car was running at 6500 ft / 2000 m elevation). Engine now pulls strong to 11500 RPM redline.

Thanks for sharing your experience! Not revving over 4k rpm sounds quite extreme.

I'm not always confident on relying others experience as the level of refinement that qualifies as "working" is highly relative :). Also it would be great to know the main principles of what to be aware of. I'm not sure if I'm correct but for me it makes sense (contradictory to your experience) that we see a bigger difference at lower rpm with lower air flow.

For example we are sometimes struggling to get a good and stable idle afr. Sometimes it may be due to large injectors but recently I've been wondering if the problem may be caused or amplified due to an incorrect injector positioning? We've seen some cars with 1500cc Bosch injectors idling well at 0.8 lambda and unable to get a smooth idle with a leaner mixture.

Should the spray be into the intake port or should it be spraying partly on the runner wall? Up to what extent?

If there's two injectors per cylinder (staged injection), should the spray be different on the injectors e.g for better atomization on higher cfm?

Henrik, it may be down to poor atomisation of the fuel you mentioned - being largish droplets rather than fine - that is the problem, with increased tendencies for 'drop-out' and condensation on the port walls adding to the problem?

The location, and type of injector, would also be potential issues - often they are deliberately placed and aligned for placing the fuel in the highest speed airflow, and changing that could cause problems - some injectors will even have a sort of 'deflector' plate to aid this.

For peak power / high flow there is an advantage to having the injector mounted farther from the valves as this allows more time for atomized fuel to cool the intake charge.

Many early ECUs did not control the pulse width very precisely, so single large injector setups wouldn't work well, and staged injection allowed larger pulse widths for the primary injectors (usually mounted closer to the valves) for a better idle and low-load fuel control. When the throttle was opened sufficiently, then the upper injectors could be added, for race applications these were often mounted directly above the airhorns spraying right in the middle of the airstream.

I think spraying into the stream is preferred, but I wouldn't be adverse to spraying onto a port wall especially if it is particularly hot, so the spray would keep it cool. I think spraying into an manifold runner is the least desirable, especially with a large injector.