Increasing the HP is a popular option among all brands. The manufacturer's often did this as a means of product expansion, starting with a low HP model and introducing higher HP versions as the model matured.
The Grumman line is no exception. Starting with the 1969 model AA-1, the original 108 HP O-235-C2C was replaced on the AA-1C Lynx with the 115 HP O-235-L2C. The factory expanded the 2 place to a 4 place with the introduction of the Traveler in 1972, and upped the engine to a 150 HP O-320-E2G. 3 years later the Tiger was introduced with the 180 HP O-360-A4K.
Owners in the field were often not content to wait for the factory to increase the HP on their favorite model, and developed a number of HP increasing engine swaps and tweaks to existing engines. The following is information about MULTIPLE STCs that can be installed on additional airplanes without FAA field approval. There are many field approved mods, or one-time STCs that can not be duplicated without FAA approval again. Those projects are interesting, but not included here.
2 Place STCs
One of the earliest such mods was Jimmy Collier's O-290/O-320 conversion for the AA-1x series, SA2477SW. This STC was subsequently sold to FletchAir. FletchAir also has a companion GW increase STC for planes that have been modified with their O-320 conversion STC, SA4106SW.
Later Maynard Crosby received approval for a follow-on STC with some improvements, SA1208NW.
Maynard also received approval to swap the Lynx prop, spinner, and oil cooler onto the earlier 2 place series for higher engine Redline and improved T/O and climb performance, SA2169NM.
Ken Blackman, as Maynard's ex-partner, duplicated the O-320 conversion effort with some additional changes to receive SA4387NM.
Ken received approval to move the "Sparrow Hawk" high compression STC from Cessna 152s to the AA-1C Lynx, SA641NW.
Bill Scott received approval for a common mod to the low compression O-235s that machined the case for larger through studs and installed higher compression pistons, essentially converting the O-235-C2C to a conical mount version of the O-235-L2C Lynx engine, SA00985CH.
4 Place STCs
By far the most popular mod to the 4 place is the Bill Scott High Compression conversion for the 150 HP Traveler and Cheetah engines. Bill adapted a popular mod that was used on Cessna, Piper, and Beech trainers to the Traveler and Cheetah engine, and received the airframe and engine STCs, SE683NE, SA682NE. The basis for this STC is installation of the Lycoming 75089 piston as used in the O-320 B and D series engines that are rated at 160 HP. Due to lack of testing for EPA noise requirements, the HC STC limits the Redline RPM to 2650, which according to the Lycoming Engine Operator's Handbook for the High Compression O-320 series is equal to 157 HP, and it is not technically correct to call this an 160 HP STC.
Maynard Crosby received approval for an installation of the Tiger O-360-A4K in Cheetah and Traveler airframes, effectively converting the Cheetah to a Tiger, though the Traveler became a "Super Traveler" with the limitations in fuel capacity and CG limits from the smaller tail.
Peter Otten received SA5452NM to install the 200 HP IO-360-C1C6 and Hartzell C/S prop from a Piper Arrow onto a Cheetah. As part of the conversion, the option of increasing the GW by replacing the center spar with a Tiger part allowed recovery of the UL lost with the change in engine and prop weights. After years of keeping the multiple STC unavailable to the public, Peter sold the rights to FletchAir, and they eventually sold the rights to Scott Jernigan, who re-engineered the drawings and coined the term "Hyper-Cheetah", saying that if the 180 HP Tigerization made a Super Cheetah, the next higher superlative would be "Hyper", which coincided with his Hyperdyne Systems business name.
John Coze received approval to install a 260 HP IO-540-D4A5 engine and Hartzell C/S prop on his Cheetah, SA4966NM.
There are many other one-off conversions with 4 and 6 cylinder engines installed on the Cheetah/Tiger airframe, but none are multiple STCs or available to the public at this time.