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Injection angle/ITB's with injector above throttle plate

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The next subject in my education on EFI involves the injection angle. I understand the concept/process and settings. There are a couple complications that I don't see much or any comments on

1) There seem to be two schools of thought, whether to spray on an open or closed valve. Are there any 'typicals' or thoughts on why one or the other is better for given applications?

2) The systems I am working with have the injector installed above and spraying onto the throttle plate thru most of its use. With ITB's and normal street driving, the throttle plates are open typically between 0-30 degrees so most of the spray hits the plate then is dripped/blown/sucked into the runner and subsequently thru the valve. Does this totally wipe out the concept of injection timing? or is there still a value in it? Curious if anyone else runs into this and what any thoughts may be



Hi Paul,

My experience for 1) is that both statements are true, it just depends on the conditions that the engine is running in as to which is better. For setting up the injection timing, I run the engine in various load conditions and then adjust the timing until the mixture richens up, I then note the degrees that this occurs at and then adjust the timing further until it leans out again. I then set the injection timing angle to a point midway between those two positions. This is done over a few points until a curve can be extrapolated and built. The end result is that I'll typically have injection at low load occuring with the valve closed (this also stabilises the pressure differential across the injector) and as the load comes up more fuel is injected with an opening inlet valve and the higher air mass flow assists in mixing the fuel blend. The presumption made here is that End of Injection timing is used. DI is different as it uses Start of Injection.

Some inlet runners position the injectors (non ITB) in a way that the injectors are spraying onto the opposite wall of the runner, building a Fuel Film that is used in the transient response of the engine. Other runner designs place the injectors so that they are spraying directly into the inlet valve recesses so having the fuel spraying on the throttle plates is probably not much different from spraying on the wall. Various motorbike engines that I have worked on have the injectors above the throttle plate from factory, though this can also be to ensure that there is a constant differential pressure across the injectors making the fuelling more consistent, without needed to plumb a manifold connected FPR into an ITB equipped engine.

With an ITB, you are dealing more with the speed of the airflow across the throttle plate rather than the pressure in the plenum, so when the fuel comes off of the back of the throttle plate at narrow throttle openings, it tends to be dropping into a rapidly moving stream of air that is also at a lower pressure at that point due to the venturi effect in place in the choke point. This increase in speed, combined with the pressure drop assist in atomising (and vapourising) the fuel, so that in combination with the turbulance created, the mixture can be quite homogenous when it is ingested by the engine. The point that this fuel is injected can be more critical with the ITB's to ensure that most of the fuel is passing through the venturi point when it is at its maximum mass flow rate. optimising the homogenisation of the mixture.

My thoughts - take them as discussion points, rather than specific advice.

1/ I don't see any reason to spay onto a closed valve, except when it's semi-sequential. If I understand the concept correctly, other than the previous, the idea is that the fuel will evaporate on the back of the warm valve and so improve mixing while at the same time removing some heat from the inlets. I don't recall any empirical testing either way, though. I would expect the best result to be to spray into the moving air stream, which should also help fuel economy and reduce emissions as raw fuel can't be carried out the exhaust on initial opening.

2/ It sounds like you're working on very high rpm engines, such as motor-cycles or full on race auto' engines. As you say, under light loads and/or lower rpm the butterflies are mostly closed - especially if they're mounted higher on the runners and larger for a 'taper' affect. Worse, the pressure drop may encourage fuel 'drop out' and there may be increased fuel puddling (wetting) on the runner and port walls.

You may find some improvement in torque, and response, by playing around with the timing of the injector.

Depending on what you have available, you may be able to substitute injectors which have better atomisation at low flow values, but a better option may be to weld bosses to the runners to allow placement closer to the cylinders - maybe use staged injection/

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