SFC for Superior
For DA=8000 when Temp=57 your pressure altitude had to be 6615 as opposed to actual altitude of 5900. The equivalent MAP is thus 22.2.
For the MAP you may be getting with ram effect, using 1.0", your equivalent DA is 6900 (this assumes you get that boost).
For that DA your pct pressure is 77.5% of 29.92 but Superior and others count 100% at a lower altitude. They use 29.5. So 77.5% corrected for that difference gives you about 78.6% available air density. My aircraft actually gets even more boost so these are not exaggerated numbers.
The short version is that you have 5% more available power than you thought.
50.5 SFC / 1.05 = .48.
In my experience with the GRT, the SFC computation is just not truthful. It's useful, but not truthful. It is useful as a relative number. I can get mine to say .38 but it's not really doing that. That is simply because it is based on HP that is calculated without reference to some of the relevant factors.
But to make it even "better", Superior computes max HP at 2700 and you are running your engine somewhat faster. Kevin Horton's spreadsheets are likely the best for this, but just roughly, 2760/2700 = 102.2.
Going back to your earlier comment about going faster if you could slow the prop down a little - to 2700. That does not "work" for me and for the reason shown in my previous paragraph.
OK, now to contradict my earlier calculations:
Superior's "performance cruise" page (p.47 in the book mine came with) Shows SFC = about .525 at 72 pounds per hour at which the HP should be 138 or 76.6%.
If you don't have that page, PM me for a PDF of it.
IMHO, the Superior is actually a little better than the Lyco for stuff like this and some experts have told me it is also better on "volumetric efficiency". The guys at Superior, when I chat with them at OSH say that much of their charts are simply copied from Lycoming, though.
FYIO, I had my MAP readout checked when I had the airplane IFR certified so I am pretty confident in that. Fuel flow is another issue, though and it is worth checking. Also FYIO, when I try to check maximum performance I always use RPM as the final indicator, using my vernier mixture knob to get the RPM as high as it will go while on autopilot, just as you are doing. The EGT's usually come out where they should, but in my thought process, the maximum power is where the prop turns the fastest.
I may not know what I am doing here, the BSFC numbers do not make much sense. I can not get anywhere near the .48 BSFC on some Lycoming charts.
The density altitude was 8000' according to GRT EFIS, OAT 57F, WOT (actual altitude 5900'). The theoretical maximum power available is 75% due to atmospheric conditions. So here are the results.
At 100 ROP (fuel flow 12.7) My spread sheet computes a BSFC of .56 at 75%.
At 50 ROP (fuel flow 12.0) BSFC is .53
At peak EGT (fuel flow 11.4) BSFC is .505
The engine will run just fine LOP and fuel flow will fall off dramatically. I did not do that today due to high CHT's after running for a while for the gps speed legs.
Tomorrow I will see what speeds result from running 50 LOP and the fuel flow. That should bring BSFC down below .50.
This is not a Lycoming engine, it is a Superior clone. I wonder if it makes a difference?