Pre-Turbine Water Injection – Holy Grail for Turbo ICE?
June 6, 2025The benefits of water injection, especially on turbo engines is well known (although accepted standards according to engine size, boost pressure, power and so on is a bit lacking, at least for modern engines). Information of pre-turbine water injection is a lot more rare, but why?
Research – Pre-Turbine Water Injection
Here is article that goes deeper into the benefits of water injection with links to research. However all the data I could find relates to pre-turbo water injection or port injection of water at the inlet manifold. But I recently stumbled on this article which explores the benefits of pre-turbine water injection.
The article is focussed on emissions and fuel economy but it touches on something that could massively effect turbo selection.
Water expands by x200 to x400 times its volume in steam state.
Injecting water into the exhaust gas stream will lower the temperature of the exhaust gas and lower it’s pressure but the expansion of water into steam makes up for this loss of pressure many times over.
To give some hypothetical numbers.
15 PSI exhaust manifold pressure pre-turbine
700 Celsius exhaust gas
Flow of 250CFM
55mm turbine inlet diameter
Using the formula Total flow=Exhaust flow+x×Expansion ratio×Exhaust flow
We would only need to inject around 0.1% water by volume to exhaust gas flow, to be able to maintain the same pre-turbine pressure in the exhaust manifold for a 65mm turbine inlet size.
We can potentially drive something like a 20% (very approximately) bigger turbine, all else being equal, by simply adding 0.1% water (by volume) to the exhaust gas pre-turbine.
Why Don’t We See Pre-Turbine Water Injection (PTWI) Everywhere?
Because it brings with it a host of problems.
I expect the biggest issue is spraying water onto red hot metal. The fatigue must be immense.
There is an issue of corrosion in the exhaust system. For race cars a possible non-issue. For cars for the general public it may be an unwelcome cost.
Maintenance. If we decide to size our turbo for PTWI, and the water runs out, we could have a very sluggish engine. The user will have to refill an extra tank.
For products (vehicles) offered with long warranty, I can see why manufacturers would not want to include any sort of water injection on their product.
However, for the aftermarket and the enthusiast, on my brief look at it so far, I’m surprised pre-turbine water injection is not a lot more common. Perhaps the material heat fatigue/shock issues cost more to solve than the benefit the system gives. Or perhaps I need to do more research into the benefits, perhaps they are not as big as it seems….
