Propane injection.

[Zeitgeist]

Here's a kit that (sort of) injects liquid propane:  
http://www.delucafuelproducts.com/

It's vacuum activated, but has a manually adjusted flow metering system.  It converts the liquid to vapor, so it's probably not what you're trying to locate.

The Powershot is nice, but when new, it's too expensive for most DIY types (like me).  I picked it up for less than half price, otherwise I'd still be looking for another solution.

BTW:  Have any of you tried nitrous, yet?

[Zeitgeist]

bump:

Any new info on this topic since we last explored this in 2006?

[trav1856]

I would go with water/methanol injection way before propane.  You are going to get more power and it drops your exhaust gas temps.   Snow performance makes great kits.  I am going to be putting a kit on my 93 in the new year.

What about an HHO bubbler? The end result is water, which will both increase expansion on the power stroke, and lower EGT's.

[foxracer1]

I had a valve failure in my 1.6td and have since gone TDI. Well currently in the swap stage. I still plan to look into water and propane injection.

[blackdogvan]

water emulsion fuel makes huge sense in a mine.

http://www.energy.wsu.edu/ftp-ep/pubs/renewables/PuriNoxl.pdf

[R.O.R-2.0]

I had a valve failure in my 1.6td and have since gone TDI. Well currently in the swap stage. I still plan to look into water and propane injection.

TDI is much more friendly to propane injection, being that there is no pre-chamber to grenade the engine if you shoot too much propane..