I am informed by a US engine shop which does a lot of Experimental engines (as well as certified engines) that 9.5:1 and below is fine on 91UL.
So why did Lyco play it so much “safer”? Do they know something? Are they assuming there are some pilots out there with no instrumentation who run at 550F CHT?
My IO540-C4D5D (8.5:1) is now certified for 91UL but it was more or less the last one Lyco did.
I know that the FAA has a historical obsession with turbocharging. They like to consider the presence of a turbo as something that requires a separate certification effort for just about anything.
One example is Camguard, it is officially not certified for use in turbocharged engines even though there is no reason for that (according to the manufacturer and the thousands of pilots that use it). He told me it’s FAA general position. No data you derive from a NA engine can be applied to a TC engine.
Another example is autopilots. You cannot get an FAA STC for an autopilot to cover turbocharged airframes unless you have done a completely separate certification for that.
I guess that might be behind this. Turbocharging above 31" might be something different but the same issue exists with turbonormalized engines and they never exceed MSL power so why should there be a problem?
