It's interesting that you would say that Saabtuner. I've reading about air density and humidity. Surprisingly humid air is often stated as less dense. Let's take a different look at this, not from a throttle/turbo response view, but an efficiency point of view.
At the same temperature, water+air should be more dense than dry air alone--a simple more molecules/cubic area argument. I think the reason for the humid air being seen as less less dense than dry air is a misunderstanding of temperature, hot air being significantly less dense than cold air--hence the hot air ballon--and hot air holding more water--higher saturation point (the mid-west is summer).
If we picture a droplet entering the center of the impeller, it is immediately whisked centrifugally outward and "backward," evaporating along the way, perhaps explosively. Locally, liquid water is being transformed into gas, along with the air being compressed. On a purely local level there are more molecules in a gaseous state at the end of the compression, but since the liquid evaporated heat was absorbed.
If we imagine a model were droplet size reduces exponentially as it traverses the impeller to expeller length, at the beginning the density of the air with or without the droplet is nearly the same, however, at the end of the traverse (notably the point of most leverage) the air + droplet gas will be much more dense since the gaseous contribution of water will be huge.
I think what we will see by injecting water is that there is a slowing of the impeller wheel that results in the same compression of air at that slower speed plus the added benefit of reduced temperature of that compressed air. It my case, turning water at 5000 RPM, we may shift the impeller speed back to where it was at 4000 (and 70% efficiency), as we increase RPM on up to 6000, 7000 while increasing the water injection correponding, we can try to stay at the same 4000 RPM, 70% efficiency range.
This is one way to look at: view it as creating a static impeller speed of high adiabatic efficiency (e.g. the 4000 RPM equivalent impeller speed, say 120,000 impeller RPM) by injecting more water to slow the impeller down. But we don't want to do that, we want the impeller to continue it's upward climb in speed so that it can compress more air. The reality is most likely in middle, slowing the impeller some (while cooling the air that is compressed), but allowing for more compression with increasing speed (but still cooling the air compression). There is no way density of water + air is going to completely overcome the torque of the exhaust side.
Now, coming back to the rapid throttle/turbo response view. The quickness of the throttle is going to be mainly a function of the rate of air and fuel entering the engine. Early in the turbo's speed range, slowing the turbo's ascent in speed will be a disadvantage, prior to peak compression, in fact the impeller is an impediment to air flow if the wheel is not turning at all. Once a decent efficiency is reached though, any further cooling of the air being compressed will result in more air in the cylinder for every turn of the impeller, thus the rate of air will be faster at that instant, and the rate will increase with increasing water injection linearly. I think that is where you could see an increase in throttle response over no WI pre-turbo, but it would depend on maintaining the temperature difference as the compressor continues to spool.
I can't believe we stumped Richarl Lamb with this stuff. To his credit, it is more turbo theory than WI theory. We are all learning something I guess.
Thanks to John A for the measurement. I'll have three 0.5 mm jets, so I don't think I'll achieve his volume as it is distibuted between the jets unless I add another pump or greatly increase the pressure.
In the end, it's going to come down to testing it out. :wink:
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