Comparison of the R2600-8 engine with the Hercules XI. The R2600-8 was developed at the same time as the XI, however for whatever reason the UK was not able or not willing to use it.
The max cruising power varied quite a bit with altitude. At the typical cruising altitudes of the Stirling between 15,000 ft and 18,000 ft, the XI had slightly more power. This was not by choice, it was meant to be higher but the aspect ratio and the excessive empty weight of the Stirling wing did not allow much higer. (Covered in another thread.)
The XI used quite a bit less fuel for all altitudes.
Fuel consumptions:
At 15,500 ft, both engines were at ~924 hp. The XI had a bsfc of 0.447 (FS) and the R2600-8 had a bsfc of 0.48 (MS)
At 19,000 ft, both engines were at ~876 hp. The XI had a bsfc of 0.448 (FS) and the R2600-8 had a bsfc of 0.54 (FS)
I was a little surprised for these two engines to be described as "completely different". My view is that they provide a reasonable comparison of very similar engines - except for the valve system. The power is rather similar, the big difference will come down to the supercharger. The fuel consumption favours the sleeve valve. As has been mentioned, these comparisons are only for this snapshot in time.
R-2800-8 info extracted from Avialogs SB2C1 manual.