Hello Aleatorylamp,
As I said before, you can obviously build whatever pleases you.
<sigh> Basic research is important. Some reference are reliable. Some are not as you already know.
As I stated in Post #8, The FAA Type Certificate for the Stearman 75 is A-743.
I believe the data there is about 98% reliable in the Military context and almost 100% reliable in the specifications context.
You should read it.
I wish I had this kind of data for projects I work on.
Your use of the AT-9 Jeep engine specifications for the Stearman 75 is mostly incorrect for the following reasons:
The Lycoming R-680 B series engines as used in the Stearman only had a 5.5:1 compression.
That is why they could get away with 65 octane fuel.
That series engine also had no ability to run a constant speed propeller, but they had adjustable pitch props at times probably as a modification.
The higher horsepower Lycoming R-680s had 7:1 compression and need 87 octane fuel.
The fact that your other engines use a constant speed propeller confirms that they are not the same series of engine regardless of designation.
The octane labelling is something the FAA seems to take pretty seriously.
I read an article about their requiring octane labelling even for a Fokker Eindecker replica that needed 60-something octane fuel.
The author thought it was pretty ridiculous because finding something that low is pretty much impossible, but the FAA makes the rules in this country.
The Type Certificate lists PT-13, PT-13A, PT-13B, PT-13C, and PT-14D as all requiring a minimum of 65 octane fuel....
Perhaps it is incomplete, but I am much more inclined to believe that it is correct with the information presented thus far.
The Type Certificate (and other places) also gives a lot of Propeller specifications for the various engine models INCLUDING some retrofits.
You AT-9 propeller has both an incorrect diameter AND incorrect pitch, and I know how those propeller tables were derived.
With that many data mismatches, one has to wonder how many conclusions from this testing are reliable.
My apologies.
- Ivan.
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