I get significantly better MPG with the high compression engine (and tight double quench pads and more airflow) than the stock one, it costs about the same per mile and is more fun to drive, quite a bit further range, more power everywhere in the RPM range, a lot less downshifting to maintain speed up hills, very snappy reving.... I have not found a downside since I had the heads laying around and the engine apart anyhow. If your really concerned about the compression ratio you could run a 6.1 cam to bleed a little cylinder pressure or you can run the thick head gaskets like everyone else does but the reduced quench is gonna make it even more detonation prone.
I do understand the relationship between compression and efficiency. The same issues were in play in the early 70s when ratios dropped from the 9.5 to 10.5 range down to the low 8s. Less squeeze, worse power and mileage.
I've never heard my engine detonate using 87 octane. Not once. I have noticed better mileage when using 89.
I'm keeping the MDS so any non stock cam is out.
It would be great to just swap in a later model 6.4 but with any 2009+ engine I'd expect there to be differences in wiring, fuel flow rates, computer, etc.
If there was a published example of this being done without major fabrication, I'd love to know about it.
Regardless.....
If I do need to bore this block, this would be a great excuse to use the better heads.
