I did use one on my old S/P car as a favor of the shop owner, it was a fairly new one (maybe less than one year old ) from Australia, we tried some timing changes and changed one cylinder spark plug heat range to show a friend and fellow drag racer how sensitive E85 pump fuel is too the spark plug heat range, I put in one new Autolite 3933 which is one heat range colder than the 3934 I ran in that motor and it losts100 peak HP It didn't miss or make any noticeable differences in how it acted on that dyno other than losing that power, we put the old plug back in and regain the 100 HP That changed surprised me but made since after seeing those results
I had really fought finding out what that motor needed on tuning and spark plugs to make it run good on E85, it was very challenging That motor was around 14.to 1 compression ratio, I started tuning it with a Autolite 3932 and ten went o a Autolite 3931, that really hurt the performance so I went the other way up to a 3933 which helped it and then 3934 which help more and then tried 3935 which didn't help it so I ended up going back to the 3934 in it The last motor I put in that car with E85 had right at 15.0 to 1 compression ratio and it liked the 3934 plugs also
II whup it and learned a lot about staying with it until you get it corrected