WI based on Fuel delivery or maniflod pressure?

[FastAOD]

Ive noticed with a few progressive controllers out that base the water delivery on injector cyle/fuel delivery. This seems like a great idea. But i am wondering if injecting the water based on maniflod pressure would be more ideal?

I would think being able to inject water, progressivly, based on maniflod pressure would be the way to go, since the need for water is directly based on additional cylinder pressure?

Just wondering what peoples general opinions are on this.....

[slostar]

injector d/c is alot better, as it will follow load and rpm. for example you could have 10psi manifold presure at 4000rpm, and the same 10psi manifold pressure at 8000rpm. in a perfect world you'l need twice as much water at 8000rpm as 4000rpm. since the motor is spining twice as fast. injector d/c will usually closely follow how much air the engine is flowing.

so end of the day its better to have it follow flow, not presure

[FastAOD]

I see.... thanks for the info...That makes sense.

[hotrod]

I still think there is some merit to manifold pressure triggered systems. Detonation is very closely tied to cylinder pressure but not closely tied to engine rpm.

For example in the high rpm example given you would not need 2x the water for two reasons in my estimation. One since at high rpm your cylinder burn time is only 1/2 as long, engines are much more prone to detonation at moderate rpm and high load than they are at high rpm. In fact Honda found in their F1 engines that if you twist the engine fast enough detonation almost goes away as an issue since the shorter combustion time does not allow the high temperature decomposition of the fuel air mix to take place.

Also as most engines lose volumentric effeciencey at higher rpm, for a given manifold pressure cylinder pressures will be lower at high rpm than they will in the same engine at an RPM near max torque where VE is best.

You need the most water near max torque rpm at high load than you do at higher rpm.


A combination of the two systems is probably the best to get the most detonation protection for the least use of water.

Larry

[FastAOD]

Interesting reading...Thanks for the info Larry...

[NAnderson]

You need the most water near max torque rpm at high load than you do at higher rpm.

A combination of the two systems is probably the best to get the most detonation protection for the least use of water.
For most smaller displacement engines, namely the turbocharged 4-cylinders most all on this forum run, max torque and thus max cylinder pressure will fall somewhere in the 4000-6000 RPM range. Delivering the highest volume of water at this point and then tapering off as RPMs rise will give you your best results.

An MF2 set up with a 3D boost and RPM map comes to mind. :wink:

[hotrod]

A single fixed flow spray jet also naturally tapers water flow per power stroke with rpm.
If you turn it on just before you need it, you also end up with max flow at midrange rpms, and as engine rpm increases the effective flow per cylinder drops as the air flow increases but the fixed nozzle continues to provide a uniform flow.

I set mine (2002 wrx) with the turnon point at 10 psi and a fixed nozzel. My next setup will be slightly different, as I will use a double nozzle setup and turn them on in cascade. For example the first nozzle will turn on at 10 psi, and the second will turn on at 14 psi or there abouts.

This will give an effective fuel flow that ramps up quickly at peak torque and then tapers off as engine rpm increases to redline.

Larry