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We really need Brad to respond.
The T7 limits the 'torque' by limiting the AF charge. With the T5 that limited the pressure, water injection could increase charge density and octane. So T5 power could increase with water injection even though the peak boost was not changed.
So some feel that many changes to the T7 are futile as the ECU will limit any gains. The T7 will limit increases in AF induction. But the idea that this relates to engine output torque is not quite true. The T7 controls inputs to the engine, but the output torque is not fully determined by this. The torque is limited by the backpressure from the turbo's turbine, as controlled by the wastegate. By reducing intake and exhaust restrictions, the turbo compressor works less hard and the WG will be more open and the backpressure is reduced. The exhaust backpressure can easily be two times greater than the boost pressure. So at 17 PSI of boost, the exhaust backpressure could be 35 PSI. Anything that you can do to reduce that will produce more power, and will reduce exhaust gas pressures and heat on the exhaust valves and combustion chamber in general. Brad reported a couple of weeks ago where a larger IC in one application lowered back presssure by 5 PSI.
When you reduce intake and exhaust restructions and thereby reduce exhaust backpressures, the turbo compressor is working less hard. So there will be less heat for the IC to reject. And the compessor will be working more efficiently as well, which also means less heat as well. So the benefits are self applifying to some extent.
As for the effects of water injection, this will cool the air charge and will increase the charge air density. I don't know to what extent, as water vapor will add to the air volume. If the net air flow is more dense, this represents a compression, a strange concept I know, but this will reduce the work that the compressure needs to do and the exhaust baack pressure will be reduced again. To what degree I don't know.
The added octane effect of the water injection and alchohol enrichment also allows the ECU to increase air flow and fuel if the system was otherwise octane inhibited. In any case, the ECU may be able, with water injection, to use more agressive ignition timing and deliver more shaft HP from the same amount of air and fuel. And higher efficiency means lower exhaust gas temperature.
So the issues are complex. We really need some real life impressions or dyno results for water injection on T7 systems. The benefits may also be more pronouced for systems with more aggressive ECU software that run into temperature and octane limitations. Adding water injection might therefore not be a wise first engine modification, but perhaps it might be for the for the 9-5 Aero, but a modified ECU will probably remain the most cost effective first mod (with intake and exhaust changes as required).
For those at higher altitudes, or in general where pump gas octane sucks, water injection may be required even in stock trim, to get good stock performance. I am gathering more and more evidence that 92 octane is not quite good enough in my 03 Aero. For those near sea level with 93-94 octane, thats a great place to be.
posted by 207.43.195...
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