90% Isoprol Alcohol knocks more than 70%--Why?

[dsmtuned]

I'm driving a 1st generation DSM running 21psi on the stock intercooler with 91 octane gas. I have a two stage homebuilt WI system running Aquamist nozzles.

I have been running 50/50 water and 70% isoprol alcohol. Recently I changed my mixture to 50/50 water and 90% isoprol alcohol. My car had an increase in knock with the 90%. Why would that be:?: It seems like with the higher % of ETOH, the benefits of WI would be increased. I put in some more 70% and the knocking went away.

Any thoughts? :?: