So we inadvertently thread-jacked another thread when this topic was brought up, but i figured it was an interesting enough topic that it deserves its own thread.
Basically, what wet compression is, is injecting WMI before the compressor to artificially increase its efficiency. Anyone who is familiar with boost knows how it works: You bring in a large volume of air, compress it, "witchcraft happens, and horsepowers are made" (in the words of Jeremy Clarkson). However, lets dive into that a little deeper. When we compress air we are improving density, but are also creating heat which acts to reduce density ... in a sense, the compressor is fighting itself.
Yes, we have an intercooler to bring some density back to the charge air, but anyone who has spent any time around FI knows one thing for sure, compressors get HOT.
What if there were a way to increase that efficiency? To basically make our compressor act like it is much larger in size than it actually is? Basically, by injecting WMI (water-methanol injection, for those that havent caught on by now) before the compressor, you are preventing that heat from ever occurring and removing some of that density from your charge air.
The effects here are two fold. When the WMI is injected into your intake, the methanol tends to evaporate almost instantly at near freezing temperatures. This not only improves the quality of air going into the compressor, but the freezing phenomenon creates a low pressure zone basically sucking even MORE air into the system. Think of how it gets really windy when a cold front comes though... Not only is your compressor receiving better quality air, its getting a hell of a lot more of it. It is not uncommon to see very small maxed out compressors shift their efficiency islands significantly to the right when injecting WMI in this fashion.
Lets look at some numbers here: we're going to ignore the intercooler for the sake of making things simple right now. I am an "engineer" by hobby, not profession, so a lot of this i have had to google and understand as i'm going along the years so there may be some slight errors.
-The specific heat of air is about 1.01 Joules / Gram. That number doesn't need to mean anything, we're basically just going to use it as a comparison.
-The specific heat of water on the other hand, is about 4.179 Joules / Gram, or about 4.13x that of air. This means it has the capacity to absorb roughly 4 times more heat (at standard pressure, sea level, blah blah blah, a lot of scientific contingencies).
-The specific heat of methanol is ~75% that of water, so we'll call it right around ~3.13 Joules / Gram.
So, now we get to the nitty gritty of it. A lot of people have reservations about spraying liquids before their compressor, which i cannot say i blame them. Cast wheels tend to stand up better, but with the popularity of billit ever increasing this may be a dying art. Methanol evaporates almost instantly however, which is good for the sake of compressor wheel longevity, it means nothing but a cool fog will be making contact with the wheel. The downside here is, it does not absorb as much heat this way... A lot of people make the mistake when comparing Alcohol to water that Alcohol absorbs more heat because when you pour both on your hand the alcohol feels colder. This is not true as we have seen above, what you are instead feeling is the alcohol evaporating at a much faster rate, and pulling heat much faster, whereas the water will sit there all day before it absorbs enough heat to evaporate. For this reason, the more maxed out your compressor is, the more water you may want to see about adding. In the thread i linked below, we saw our best results with 80% methanol and 20% water by volume.
You can see here, that we are able to remove a substantial amount of heat from the compressor, with very little WMI injected. On my old platform, we actually did extensive testing on a car with a built motor by just running the piss out of the tiny stock turbo and seeing what we could get. The results were pretty astounding, we ended up setting the stock turbo record by about 15 or so whp... pretty cool. Here is that thread for those interested.
Built and Bolted... Stock k04. Possible record? - Mazdaspeed Forums
*The point of that thread is to cover the bit of longevity testing we did with this method, as well as improved compressor efficiency. Most fully bolted mazdas will see 330 whp on E85 (not counting torque since our motor was built), and some of the more heavily modified ones have seen as much as 350 though that is much more rare. Almost none are seeing over 17 psi at 6000 rpms... ours was still as high as 20.
Its a very interesting concept i have tried to talk several procharger guys into trying since the things get so damn hot at the track, but no one has bit the bullet. I cannot say i blame them, but it is something i would be interested in if anyone is feeling frisky. Its a neat topic, and hopefully this thread will peak some discussion with the amount of FI interest popping up on this forum.