water injection and turbo spool

[rallimike]

Does water injection lower exhaust temperature? Assuming it does, how does the lower temperature affect velocity and turbo spool?

[Brad]

I have never noticed an increase in spoolup time when using water injection. The most common reaction when done correctly is for the engine to acclerate quicker with the on set of water.
Water injection will lower the exhaust gas temperatue but the expansion of water to steam will increase the exhaust volume.
When tuning with water you can maintain a more aggressive timing curve that will increase spoolup and engine power.

[Forum Admin]

Power transition is a point that is especially prone to knock - i.e. during spool up. Standard tuning would run richest (relative to boost level - not in absolute terms) during this transition to boost to suppress the onset of knock during spool up. Enabling water injection during this transition will permit the tune to be leaner during spool up - I would actually expect post tuning with water injection for the EGTs to be the same or slightly higher during the spool up period from leaning.

This does not mean that spool up would be faster but I have not found it to be any slower.

The primary reason for the difficulty during transition is that boost is increasing faster than the reaction of the fuel system - so tuners tune ahead of boost build up. For turbos that spool fast once in their operating range (boost climbs quickly) this condition is especially true and where the knock suppression of water mixtures can help even more.