| Maker | Bristol Mercury IX | Bristol Pegasus X | Bristol Pegasus Xc | Bristol Pegasus XI | Bristol Pegasus XII | Bristol Pegasus XX | Cirrus-Hermes Major |
| Arrangement | Radial | Radial | Radial | Radial | Radial | Radial | Radial |
| Cooling | Air | Air | Air | Air | Air | Air | Air |
| No of Cyls | 9 | 9 | 9 | 9 | 9 | 9 | 4 |
| Bore/Stroke | 5.75" x 6.5" | 5.75" x 7.5" | 5.75" x 7.5" | 5.75" x 7.5" | 5.75" x 7.5" | 5.75" x 7.5" | 4.72" x 5.12" |
| Displacement | 1519 cu ins | 1753 cu ins | 1753 cu ins | 1753 cu ins | 1753 cu ins | 1753 cu ins | 386.1 cu ins |
| Comp Ratio | NA | NA | NA | NA | NA | NA | 5.80:1 |
| bmep Cruise | 159 | 159 | 137 | 159 | 159 | 161 | 102 |
| Blower ratio | N/A | NA | NA | NA | NA | NA | NA |
| Cyl Material | Steel | Steel | Steel | Steel | Steel | Steel | Steel |
| Valves per cyl | 1 Inl 1 Exh | 1 Inl 1 Exh | 1 Inl 1 Exh | 1 Inl 1 Exh | 1 Inl 1 Exh | 1 Inl 1 Exh | 1 Inl 1 Exh |
| Valve Arr. | OHV | OHV | OHV | OHV | OHV | OHV | OHV |
| HP max - not Take-Off | 840 | 915 | 790 | 915 | 915 | 925 | 148 |
| @ RPM | 2750 | 2600 | 2600 | 2600 | 2600 | 2600 | 2450 |
| @ Altitude | 14000ft | 6250ft | 5500ft | 6250ft | 6250 ft | 10000ft | Sea level |
| HP Take-Off | 730 | 960 | 910 | 960 | 960 | 830 | 135 |
| @ RPM | 2650 | 2475 | 2475 | 2475 | 2475 | 2475 | 2100 |
| HP Cruise | NA | NA | NA | NA | NA | NA | 109 |
| @ RPM | NA | NA | NA | NA | NA | NA | 2200 |
| Fuel Octane | NA | NA | NA | NA | NA | NA | 70 |
| Prop Drive | Geared | Geared | Geared | Geared | Geared | Geared | Direct |
| Dry Weight | 980 lbs | 1005 lbs | 1015 lbs | 1005 lbs | 1005 lbs | 1015 lbs | 325 lbs |
| lb/Cruise HP | 1.17 | 1.09 | 1.28 | 1.09 | 1.09 | 1.09 | 2.98 |
| Carb(s) | NA | NA | NA | NA | NA | NA | 1x Claudel-Hobson |
| Ignition | Mag(2) | Mag(2) | Mag(2) | Mag(2) | Mag(2) | Mag(2) | BT-H Mag(2) |
| Electric Starter | NA | NA | NA | NA | NA | NA | BT-H |
| Length | NA | NA | NA | NA | NA | NA | 42.32" |
| Height/O.D. | 55.30" | 55.30" | 55.30" | 55.30" | 55.30" | 55.30" | 29.41" |
| Width | NA | NA | NA | NA | NA | NA | 17.05" |
| Bed centres | NA | NA | NA | NA | NA | NA | 17.05" |
| Mount Dia | NA | NA | NA | NA | NA | NA | NA |
Let me know if any of the above does not read true, or if you have information that does not agree. Wherever possible, I will update these pages on a regular basis as data comes in.