RE: Experts, please weigh in
in your extra yes you may have gained more speed. As other have said you need more thrust than drag. Your extra is a draggy plane and the 55 can only spin max size prop of 12x4-12x5 the 75 can go to a 12x6-12x9 prop.
Now for the Sundowner the 55 AX is 18.5 ounces and the 75 is 26.5 ounces. This is 8 ounces in the nose of that plane. add in the larger engine mount, and more fuel and you are looking closer to 12 ounces more foward of the CG. you will need around 4 ounces in the tail to balance this. You have just added 16 ounces or one pound to this plane with no other mods. yes the plane will be faster in a dive but then may have flutter or other problems. On the straight and level you will have to fly at a higher angle of attack to carry that extra weight and no you are just as fast as the lighter plane with a 55 AX in it. add to this the faster landing speed no you dont have as near a fun plane to fly...........
You can get the sundowners down to 5 lbs easily.
The mustang is set up so we can run inverted engines. this allows us to fully cowl the engine and have a streamlined fuselage. The world models have around 600 sq inches i think so they can carry more weight and not be affected as much.
On that 75 your bought the 95 AX is just a tad lighter and has two more mm in stroke which means alot more power in the same size package.