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Originally Posted by Greenv8s
It's a crazy idea, but bear with me.
I'm running a low boost supercharger (0.4 bar) with a relatively long inlet manifold, probably five feet or so total length. On boost I run upstream water injection, just a 0.7mm jet connected straight to the pump.
With the water off I get slight detonation on boost, by 'slight' I mean that there's a quiet rattling noise in the background but no really heavy bangs. With the water enabled it seems to pull better, the background rattling noises seem to stop but sometimes I still get very loud detonation.
I'm wondering whether the water injection might be causing this in some way, and I've got a theory that explain how it might.
According to this theory, it is very important how much of the injected water evaporates upstream of the intake valve. Water that evaporates upstream cools the charge and enables me to get more charge into the cylinder. Water that evaporates downstream of the valve cools the flame front during the combustion process and suppresses detonation. According to this theory, cooling that occurs upstream might actually be promoting detonation by increasing density (and hence peak temperature and pressure) in the chamber. Hopefully the amount of water that gets into the cylinder before it evaporates will be more than enough to offset this, but I don't see anything fundamental to ensure that it will be.
What do you reckon, am I barking up the wrong tree?
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Are you only spraying with only water? I have tried water, a mixture of isopropyl alcohol (which lead to more knock), and a mixture of methanol and water. The latter has worked the best to provide good cooling and knock suppression.
-Craig