What's your air:fuel mixture ratio? Anyone running 13:1 A:F on 91 unleaded?

The air/fuel mixture will depend upon the load and is not necessarily octane dependent.  With a good tune, the idle and light cruise will see 14-14.7:1.  Full throttle will see something on the order of 12.4-12.7:1.

Lorena, Texas (South of Waco)

Unless your running some weird ethanol blend it could be good to try to clean it up some. whats afr under load? stoich is 14.1 for e10 and 12.99:1 for e30. being a little lean at idle isnt bad either so 14.7 wouldnt hurt anything on e10 pump gas which should alway be less than 10%

Seth

My concern is avoiding blown head gaskets running 91 octane with a DCR of 8:1 and non-posted G heads.

You should be fine with 8.1:1 DCR and premium grade fuel. assuming grade 8 head bolts and all surfaces were reasonably straight during the engine assembly.  Only when the tune up gets sloppy should you have any concerns.  Detonation is the biggest concern when it comes to head gasket failures so if the timing and fuel mixtures are in acceptable ranges, then the only other concern is the quality of the fuel.  If fuel octane is on the low side, then backing up the ignition timing to give an additional margin of safety is an option.  Electronic ignition goes a long way in insuring that the tune up stays reasonably stable over the long haul.

While having a DCR that’s on the high side does make for a narrower tuning window, the fuel efficiency as well as overall performance improves.  The narrower tuning window just dictates that the quality of the fuel (octane), the fuel mixture, engine temperature, and ignition timing attributes are more critical.

Also keep in mind that as the engine gets more miles on it, the timing chain does stretch and that by itself will lower the DCR as time goes on.  Most initial stretch takes place in the first 12K miles and then after that, it’s more gradual.

Lorena, Texas (South of Waco)