Water injection in exhaust to reduce backpressure

[robbilau]

I wanted to throw this out there because I know it's done in the marine/boating world. By injecting into the exhaust the effective exhaust length is tuned.

I'm wondering if it might help spoolup by reducing backpressure after the turbo. The injection pattern would be the reverse of usual, say only at WOT or high throttle and tapering off with rpm. Not sure if this could be done in the downpipe since it would travel through the catalytic converter or if you'd have to inject after the cat.

Has anyone heard of this being done on cars?

[Roger G.]

Looking at the flow wouldn't WIS in the exhaust side increase the backpressure ? If so then it's probably better for a straight open exhaust to spray water in at lower rpms to increase backpressure and therefore toruque in the low end. I'd also would say that this doesn't help in spool up time for a turbo as the pressure differential before and after the turbine wheel causes it to spool. The less restriction after the turbo the better the spool up.

[Charged Performance]

I think the initial thought relates to cooling the exhaust and by increasing density through the cooling backpressure would be reduced. However, someone that is much better with steam tables than me would need to evaluate whether or not the denser exhaust gasses would more than counteract the expanded water volume.

As a side note - I still can't come fully to grips with the way the term backpressure is used with regards to exhaust. Backpressure in the exhaust does not help at all. If the real object is scavenging - then it is exhaust gas velocity not backpressure that produces this low end toque benefit. To the extent that a larger header primary reduces velocity of the smaller volume of exhaust gasses at lower rpms then scavenging suffers - adjusting the diameter of the header primary to maintain a good exhaust gas velocity while not overwhelming the system with backpressure is where exhaust tuning comes in. Off course the pulling effect of exhaust gas velocity is relevant to maintenance of scavenging at the header collector all the way until the muffler tip so tuning needs to be done all the way through. But it is the maintenance and balance of velocity not backpressure that is beneficial.

Once exhaust gasses have gone through the turbo they no longer have any significant effect on scavenging. Ideally you want to decrease back pressure as much as possible in order to increase the pressure differential on the turbine increasing its effectiveness. In on car applications you generally would not be able to utilize a fully optimal design, since you have to evacuate the exhaust gasses quite a distance from the turbo. So you still want to maintain exhaust velocity sufficient to assist pulling the following gasses through the system and keep back pressure low. Generally post turbo you want to reduce backpressure at all operating points as much as possible without totally stalling velocity until the exhaust tip.

Thoughts from someone who almost understands the science of this stuff but always remains ready to be corrected and learn.

[JohnA]

To me this looks like the opposite of what should be targeted.
cooler exhaust gases after the turbo will have lower pressure and move slower. We want them to get the hell out of the way, to make space for the next ones!
Exhaust wrapping aims to help in that respect, and it works (proven on the dyno)
How would this idea produce any gains, if it does the opposite? :?:

[Charged Performance]

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.

[Warren_from_PINZ]

It seems to me that introducing water into the exhaust environment would create a lot of steam which would increase exhaust back pressure. I'd be astounded if the cooling effect was (even nearly) as significant as the steam expansion.
Warren

[Dust]

I'm sorry to bring this back from the dead, but I will have a free o2 sensor port in a few months. Wondering if this is something I should look into or not.