Originally Posted by acs0424
Seeing as you're getting a Diablo Tuner this may be irrelevant, but run mid-grade fuel. I switched from 87 octane to 89 octane at the advice of a friend who was a mechanic for years and knows a crap ton about Dodge. I gained a noticeable MPG improvement in the city and highway, enough so that the extra cost of mid-grade was off-set by the MPG improvement, and there's a slight performance gain. So yeah, run mid grade fuel. That's what the manual recommends anyways.
The benifits that you experienced are because the engine (Hemi) is designed to run on mid grade (89 octane) fuel not 87 octane. It is perfectly fine to run 87 (per the manual) but you will see a slight decrease in performance.
You're truck is now running as it was designed to do, performance and all.
By running lower octane fuel than the engine calls for, the engine will pull timing to prevent pinging which reduces power.
I don't think that you were implying this and I hope your mechanic friend wasn't either, but running higher octane then the engine requires will not increase performance. For the best performance you want to run the lowest octane that you can get by with. The higher the octane the more resistant it is to ignition which will allow you to run more timing, thats where the performance gains come from (and or higher compression etc)
Edit: I may have typed all that up for nothing as I just saw your last sentence