I had been running my 04 DHS on 87 octane without problem but after carefully reading the wording of the OM, I switched to premium. Yes, the computer will retard the timing and make all of the adjustments to use regular gas but boy did my car wake up with the use of premium gas.
I can't be too surprised that a 10:1 engine would perform more efficiently on a higher octane fuel than it would on 87 but what surprised me was how much better it performed. On 87 I had done a couple of WOT to pass another car and quite frankly was not impressed with the results although I would rate the passing efforts as "adequate" overall. Moving to higher octane gas opened for me, a whole different perspective of the N*. I tried it today on the hwy between towns (6mi, no traffic to hit) and a stomp from 80kph had me up to 140kph in a few short seconds with noticeable push back into the seat and a beautiful, previously unheard song from under the hood.
For the difference of a very few $ per tank offset by a + in the fuel economy, I'm sticking with the good stuff, not the "bar stock" for my Cad.
Opinions please ???