It would depend on the existing exhaust size. For an undersized exhaust where backpressure overwhelms velocity at high exhaust gas flows, cooling and making the exhaust gasses denser would according to the theory decrease the backpressure but not be so dense as to reduce velocity. Again this assumes the cooling more than offsets the introduction of steam and the related volume.
Though increasing volume of water as it turns to steam could also increase the velocity...
A lot of variables here to keep track of and I don't have a model to relate them all to. Eric Fahlgren from Not2Fast likely could answer these questions more clearly than my stumbling through the concepts.
|