Yes,No, it will not effect the manifold, the only problem with alcohol is that it is harmful to pump/brass nozzles/parts,IE injection nozzles. If you are running injection, you need to clean the system if you leave it set for more than 2 weeks without running the engine (it makes green slime) i donot worry about it myself, but those are recomodations of top tuners. I just go to the track and run it, it runs like crap the first pass but , by the second pass it cleans up and runs the same as always. Ofcourse you can spend lots of time flushing the system, after each trip to the track and it's ready to run like always the first trip down the strip. Bottom line it's up too you .
JMO
Zip.