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? :?:

[hotrod]

Isopropyl alcohol does not behave like methanol or ethanol. At that higher alcohol percentage you were in effect increasing your fueling and reducing your WI cooling. Same thing happened to WWII fighter aircraft. When they substituted Isopropyl alcohol for methanol or ethanol without changing injection amounts they blew up engines.

Isopropyl alcohol in high concentrations acts more like a fuel than a coolant.
I suspect if you go to the 90% concentration you would need to change your injection rates, or increase your water percentage, to maintain knock resistance. High concentrations of some alcohols have been rumored to be sensitive to pre-ignition as well.

You get maximum cooling effect with 100% water. You need to add some alcohol to compensate for the dropped combustion temperatures. It takes somewhere between 10% and 50% methanol or ethanol to get max power depending on fueling and ignition settings.

Larry

[dsmtuned]

So what are most people using as a mixture % of Isoproyl ETOH? Is there a formula to figure out the best mixture, or is it strictly an experimenting situation?

Thanks for the reply!

-Craig

[joesmoke]

Some buick guys swear by straight methanol for max power and detonation suppression!