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11-23-2007, 05:55 PM | #1 |
Scooby Specialist
Member#: 114101
Join Date: May 2006
Chapter/Region:
SCIC
Location: Buena Park, CA
Vehicle:93 Impreza L Swap |
How to choose a WI kit
There are plenty of write ups that go into detail about the pros and cons of alky/meth/water injection, but none that explain how to choose a kit... so I thought I'd take a stab at it. I wrote this before Richard of Aquamist wrote his In-depth study of WAI injection systems, so naturally it will overlap and go much more in-depth in some areas than I will. For further reading, please refer to that thread. My goal here is to provide you with a basic, unbiased understanding of how these kits work, what the major differences are, and what each manufacturer has to offer. Fyi, I'll be using the word "water" here interchangeably with meth, alky, or whatever you plan on injecting. Please feel free to let me know (gently) if I've made a mistake.
STEP 1: single vs. dual stage. 1. Single stage (on/off): using a boost switch,these kinds of kits simply come on at a certain manifold pressure, and spray at a set flow rate until pressure drops below a certain psi. If you have your kit set up to spray 5 gph at 15 psi, you'll spray 5 gph at 15 psi and above, and not spray at all below 15 psi. Cons: water delivery will not adapt to changes below or above the preset pressure. It will not provide you with the knock prevention you'll need at anything below the pressure you've set, and will not provide increased knock prevention as rpm and pressure rise passed the set pressure. The effects of this varies with different setups (further explained in "MAP based controller"). Single stage kits: Coolingmist Aquamist System 1s Aquamist HFS-1 Devil's Own Snow Performance 2. Dual (progressive): these kinds of kits have variable flow rates, increasing and decreasing flow in accordance with your car's activity. These kits have controllers that vary flow in different ways, which I will explain in the following section. MAP based progressive controller: These kits determine when to inject water based on manifold pressure (just like single stage). The difference is flow rate is not static; it rises and drops in accordance with manifold pressure. The start and end points can be adjusted by the user. SMC's MAP-based controller: Cons: Flow should increase and decrease based on the volume of air ingested. Unfortunately, pressure is not always a good indicator of that. The volume of air at a given pressure increases with rpm, which pressure does not account for. If you hit your max pressure set at 20 psi and it tapers to 16 at 7k rpm (which is typical), you'll be spraying LESS water when you actually need MORE. Ideally, your engine should be combust twice the amount of air at double a given rpm. Fortunately, this is not always the case due to a decrease in volumetric efficiency. Smc, devil's own, snow performance and coolingmist all have MAP based kits. FIDC-based controller: Fuel injector duty cycle is arguably a better way to determine flow, since it rises and falls in accordance with total volume of air ingested. This solves the MAP problem I mentioned earlier. Labonte MotorSports, Aquamist, and Perrin (whose kit is made by Aquamist) make fidc based kits. They monitor IDC and determine how much water to inject based on that. Labonte's is actually 2d (see below). Here's a pic of Aquamist's: MAF-based controller: Since the ecu determines how much fuel to inject based on the MAF sensor, MAF-based controllers also have the same advantages as FIDC. Fortunately, Subaru's MAF sensor works just like the MAP (varying 0-5v analog signal), so some MAP-based controllers will work with the MAF sensor. 2d Mappable Controller: Some progressive controllers can determine how much to inject based on multiple (2 or more) inputs. Aquamist's MF2, Coolingmist's Smart Injection, and Labonte MotorSport's VC3G, Fjo's CWI0010 all have the ability to inject based on 2 or more variables. Any controller that includes on of these controllers will be able to inject based on multiple inputs. There are additional possibilities depending on what other hardware/software you have, but these are the most common. Dual stage kits: (For Aquamist and Fjo, scroll down) AEM Coolingmist Devil's Own SMC Snow Performance Labonte MotorSports (Controllers sold seperately) STEP 2: failsafes You need to find a good way to prevent damage in case of an emergency (aka your kit fails). Engine management can help, but only to a certain extent (again, I won't go into detail). It's highly recommended that you buy a kit with a good failsafe, or buy one separately. The reason is simple; if your meth kit fails and you're not adequately prepared... BOOM! There goes your engine! Imo, the failsafe is THE most important thing about a WI kit.Coolingmist's smart injection: How a failsafe works: A failsafe is a device that monitors an engine's and/or WI kit's activity through the interpretation of one or more inputs. Using these inputs, the failsafe device will determine whether or not the WI kit is working properly. Common inputs include flow sensors (offered by Aquamist, Labonte) and clog detectors (coolingmist). Upon sensing a fault, the failsafe will activate one or more outputs, which stop the engine from running under aggressive settings. The most common approach various companies have taken is boost cut. They use some sort of device to monitor flow, so if the line gets clogged or the tank empties, power to the boost control solenoid is cut, reducing boost to wastegate pressure (which is ~7psi stock). There are other methods, but I'll be here all day if I go into too much detail. Instead, here are some links. Fyi, smart injection is my favorite, with the DDS3 following in second place. Smart Injection Aquamist DDS3v8 AEM kit w/ failsafe Snow Performance Safe Injection Labonte MotorSports IFS-10 STEP 3: flow precision/dynamic range If you plan to go progressive, you'll have to consider how well the kit will respond to varying flow demands. Most progressive kits alter the speed of a pump's motor to vary pressure and flow. There are 4 main concerns with this method:1. Speed: Many are doubtful of the common shurflo pumps' ability to respond to the fast-changing demands of WI kits (pic below). The concern is that the shurflo's rotating internals generate too much inertia to be able to respond quickly enough to drastic changes, as demonstrated here. 2. Dynamic range: unfortunately, flow and pressure do not increase at the same rate. This means a drastic increase in pressure will only result in a minor increase in flow. 3. Atomization: a decrease in pump speed results in a decrease of pressure, which results in poorer atomization. Small droplets are needed to ensure the vaporization of all water injected. 4. Longevity: obviously, a pump whose motor speed is continually increasing and decreasing will not last as long as one that is used at a consistent rate. There are two ways companies have tried to remedy these problems: the high speed valve, and the electromagnetic piston (aka race) pump. Because of the speed and precision of their lightweight moving parts (which are kept to a minimum), both the hsv and the race pump can dramatically increase or decrease flow within moments. They also operate at full pressure 24/7, so atomization is not an issue. Here are some examples of kits with HSVs and race pumps: Coolingmist (HSV): Coolingmist S-HSV kits EDIT: Coolingmist's valve is quite different from other hsv's and injectors. It is a proportional valve, which works like a variable restrictor instead of a simple on-off solenoid valve (like Aquamist's and FJO's). Aquamist (HSV): System 2c System 2d System 2 HFS-5 Perrin (Race pump): PWI-1 Fjo Racing (HSV): Water Injection System To conclude, there really is no "best kit for the money." If you spend less, you'll get less. That's even true (to a certain extent) between different brands. It may seem like I favor Aquamist and Coolingmist (which I do), but their "premium" kits are almost double the price of everyone else's. You can get another kit that is stupid simple, costs much less, and get 80% of the benefits. You could even build your own if you want (which is actually what I'm doing). However, alky/meth/water injection is no joke. If you don't do things right the first time, you might not get a second chance. This isn't the typical "plug and play" mod for your typical suby folk. Yeah, it's a great power-adder, but proceed with a plan, proceed with caution, and above all, proceed with knowledge. If anyone has questions, feel free to reply to this post or pm me. Last edited by WReXd; 03-23-2008 at 05:10 PM. |
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