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Old 02-11-2004, 05:04 PM
boostm3 boostm3 is offline
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Join Date: Mar 2004
Location: White Plains, NY
Posts: 7
Default Location of Water Injection Nozzle relative to Intercooler

This is about a discussion weve been having on a bmw forum in the Forced Induction section. One of the most popular setups on our E36 M3s is a Vortech supercharger cooled by an rms Aftercooler which is a water to air unit placed inline between the blower and the throttle body, and an Aquamist 1s system with the nozzle placed right after the aftercooler core, just before the throttlebody.

I have instrumented results in the above setup, before the aftercooler was installed, with the water injection alone, with the nozzle placed between the SC discharge and the throttle body. I found that Intake Air Temps (IATs), prior to the boost kicking in, ie, cruising IATs ran Ambient PLUS 100 degrees F. Then, when the WI was triggered, the IAT at redline showed a 50 degree reduction. On an 80 degree day, part throttle cruising, no boost, showed IATs of 180 degrees. When the boost was invoked with a WOT run to redline, this went up.. And, the WI was triggered, at about 4 psi, or about 4500 rpms. IATs at redline had dropped to about 130 degrees, which is over a 50 degree reduction.

Now, when the aftercooler was installed, the nozzle was placed on the outlet side of the aftercooler, just after the core, and just ahead of the throttle body, in a similar location as to where it had been mounted without the aftercooler having been installed.. Cruising around part throttle IATs, no boost, with the aftercooler, now read ambient plus about 35 degrees, or, about 115 degrees, showing about a 65 degree IAT reduction (80 (ambient) + 100 - 65 (aftercooler) == 115 degrees) .. Going WOT and taking an IAT reading at redline, we see a new IAT of about 100 degrees (115 from above - 15 degrees from WI), showing that the water injection caused a further reduction of about 15 degrees over the temps I saw when cruising.. Had the WI been as effective as before when it was sprayed into the non aftercooled 180 degree air instead of the 115 degree air, now cooled by the aftercooler, wed see a new IAT of about 65 ( 115 minus 50 degrees). Why did the WI produce a minus 50 degree result without the aftercooler being installed and 'just' a minus 15 degree result With the aftercooler being installed? I believe its because the water evaporates much more efficiently in hotter air than in cooler air.

Given this observation, it kind of begs the question, why not mount the water injection nozzle BEFORE the aftercooler core, ie, between the supercharger discharge and the aftercooler, instead of the common way to mount it, after the aftercooler core? That is, why not take advantage of the fact that water evaporates more efficiently in the hotter air which
we have before the aftercooler core?

I would think that moving it to before the aftercooler core would be a no brainer. Except for one possible question, the answer to which I havent worked out yet. The real question, it seems to me, is does the aftercooler behave like the water injection, ie, does it remove more heat from air that is hotter, and less from air that is colder? Thats certainly how the water injection works... But since evaporation isnt a factor with the aftercooler, I didnt think so.. But... Now im not so sure.. While its clear that with the aftercooler evaporation isnt the factor at work, I nevertheless have noticed that as the ambient air temps are now near 50, I dont continue to see the same ambient plus 35 to 40 degree iats. Moreover, Im pretty damn sure that when ambient is 40 degrees, I wont see iats of 75 (without wi). In other words, the colder the ambient air, it seems that the less percent cooling we see from the aftercooler. When ambient is 25, will we see IATs, without wi, of 60 degrees? I think we reach a point of diminishing returns. So, perhaps it does act similarly to the way the water injection behaves in the face of falling temps in the immediate environment. I would think this might be the decisive factor in making the decision to move the nozzle location or not.

Id love to hear your comments, experiences, etc, with this issue. Richard, if you have any theoretical info to help clarify this problem Id love to hear.. Thanks...
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Paul E -
'99 White M3; < 69K miles; Dinan SC kit with 6" RMS crank pulley and water/air Aftercooler: 11 psi-367 SAE rwhp/304 ftlbs; Custom Tuning by Nick G; AA Aquamist Water/Methanol Injection and AA exhaust; Fikse FM-10s;
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