[Richard L]

RX7 engine in a clamped engine bay can really do with any form of water injetcion cooling. You are right

In most cases, a simple on/off water injection system will be good enough, triggered abive a certain boost. Injecting at a rate of 10-15% water to fuel will not require a tune. Based on your total fuel flow of 2860cc/min, you can inject between 286 to 429cc/min.

Looking ahead of future modifications where fuel injectors and turbo will be replaced. Once you have a larger turbo, your power range will be broader. For this reason, you need a progressive system, preferrably follows the boost pressure and RPM.


WM system options:
1. Single stage system (SS-S): full spray after triggering
2. Double stage system (DS-S): A second nozzle is activated at a higher engine load
3. Progressive pump speed system (PPS-S): A motor speed controller progressively increasing pump speed to attain high line pressure to increase flow.
4. PWM valve system (PWM-V): Same principle as a standard fuel injection system, flow is controller by an inline valve.


WM system availability from WM manufacturers:
5. Single stage: All manufactures make them
6. Two stage system: Often needs a second pressure switch added to the single stage system
7. Progressive pumps system: all manufacturers except FJO and Aquamist
8. PWM valve system: FJO (cell by cell) and Aquamist (IDC tracking) only.


Not all the progressive systems are the same
9. A PPS system normally costs about 30% less than the PWM valve system.
10. A PPS (progressive pump speed) system cannot be compared to a PWM valve (similar to an OE fuel injection system) system. At best the dynamic flow range of a PPS system is x2. This calculation is based on a pressure change of 60psi to 240psi. For a "pressure change" of x4, you will only get x2 flow change.
11. For "PPS" system to work as well as a "PWM valve" system (at worse x10 dynamic flow range), the PPS system needs to have a pressure change of 60 psi to 600psi!!!
12. The other important factor most people missed is the response time to a load transient. A rotation mass as in a PPS cannot suddenly speed up and slow down. This leads to a delay in a "ramp up" and a delay in "ramp down". A PWM valve system response to load change in a "thousandth of" a seconds compare to "tenth-of" seconds. This leads to inconsistant AFR during load or gear change.


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