Currently I am modifying a NA EFI gasoline (non-diesel) engine to a turbo application.  I have succeeded the first time, but now I am going into uncharted territory (larger turbo).  I have been using an Aquamist Water injection system to help deter knock.  Now I have seen the use of of Propane in some applications to be successful, but most of the applications have been diesel engines.  So here is the questions I have for you automotive experts:

1.  Can a combination of water and propane injection be used?

2.  Where would be the best place to inject propane (from my experience would the in the intake runners for better dispersion into each cylinder)?

3.  From my reading on other applications, once you use the propane injection you are able to run more boost (to a certain extent) on the same fuel system (i.e., same amount of fuel being used)?  Propane's A/F is 15.7-15.8, so does that mean I would need to inject more gasoline to make up for the lean A/F of propane?

4.  What are the pros and cons of running propane?  What are the pros and cons of running water+propane (if it is possible)?

AFAIK the propane injection for diesels is added before the saftevalves and its combustion is to help mix the diesel and remaining air, so you can add more fuel making more power. I don't see gasoline engines improving in the same fashion because all you'll end up doing is displacing gasoline, which will put out more power anyway. As for anti-knock, I don't see a small amount of propane helping too much, and a larger amount will probably lose hp to a lower-boost all-gas setup.

I think you are estimating the criticalness of propane based on diesel use. There, a tiny bit too much liquid propane will, for sure, cause detonation, and very likely, a damaged engine. In this spark ign application, the addition of liquid propane will increase the octane by the high octane of the propane itself plus the octane enhancement of a richer mixture. The worst I can see for a too much propane condition is drowning the engine in too much fuel.

A question that comes to mind, though, is how best to use the latent heat of the liquid propane. Should it be introduced early in the intake stream, right at the valve or somewhere in between?  

You really do not need to run a richer mixture for a turbo, although adding propane vapor will provide more boost by continuing to burn when the exhaust valve opens (higher octane, longer burn duration).  But this comes at the cost of power and exhaust system life, not the thing that is really necessary.

Adding extra fuel for cooling is common to essentially all piston aircraft engines. For very high powered applications, the richness is increased well beyond best power to allow higher boost and higher net power. Increased richness tends to increase the effective octane considerably.

There is LITTLE OR NO correlation between octane rating and flame speed. Of course, if octane is not sufficient and DETONATION occurs, that is a quite different phenomena than normal flame front combustion and produces very destructive local forces - like TNT would.

Combustion cannot continue after all the oxygen is reacted, so continued burning through the exhaust event should not happen. I have often observed the common feather flame that is present on high powered piston aircraft during rich takeoff and climb. But, that is the external air reacting with the hot exhaust.

In the case of adding propane for enrichment, the exhaust system should not be damaged by a cooler exhaust unless the metal is sensitive to a carborizing flame and not an oxidizing one.

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