There are a number of mods I would like to try on my M-4, most of which involve equipment common among experimental flyers, such as electric prop, composite blades, ignition, fuel injection, avionics and my own ideas about improving the wing,

Any one have some experience with doing this? What does it take to put the airplane into experimental category?

It is easily done, but there are restrictions. Look up experimental exhibition category, that is probably where the FAA will put you.

Suggest reading AC20-27G, you are required to design / fabricate / build over 51% of aircraft for normal experimental certification. There is a checklist listed in the appendix which must document this. When using production, certified parts from a production aircraft, they can not contribute to the 51% rule even if you rebuild. I would also suggest you talk to whoever the FAA would assign as your inspector, as his decisions will greatly influence whether you can even do this starting with a production, certified aircraft. It can be done as I've done it myself, but a lot depends upon FAA Inspector's interpretation of rules and his attitude toward such a project. Some parts of the country I think it'd be just about impossible to do... The restrictions and limitations placed on you for the exhibition category are pretty hard to live with as well. Usually restricted to area of 100mi or less unless traveling to airshow, flown locally for currency only, etc, etc... Would probably be much easier to just start with a kit such as bearcat in these FAA times as the rules on what you can do keep getting tighter and tighter!! Good luck!!!

What he said. I looked into it and talked to the FAA and the EAA. If you get the checklist for E-A/B and look at it you will discover that you will either need to scratch-build a fuselage or scratch-build a set of wings to get to 51%. I would say use a Maule fuselage and build you a long set of pa18 wings with big flaps and double tanks.

Okay, not wanting to earn a repairman's certificate, so not sure the 51% rule would apply.

I know that type certificated airplanes are often put into experimental category to develop/test other equipment. Micro VG's come to mind, as a Luscombe I knew of was operated under experimental for some time while developing and testing them for approval. Same for a V tail Bonanza a friend owned after King or Apollo was done with it for the same purpose.

Is there not another category of experimental?

Again, depends upon where you're located and local FAA's interpretation, but I have done full experimental with original wings, and fuselage twice, and met 51% to their satisfaction... This was probably 10 years ago and don't know if I could do it in today's environment with FAA, as the whole nature of their mission, inspector GA experience level, have all changed drastically. Frankly, think it'd be very difficult to impossible! With that set aside, airman repair certificate has nothing to do with it, as if you build it, you're able to work on it without that certificate. When developing an STC, yes, you can get aircraft put into experimental category for your test flight program which is usually quite limited on limitations. Think you'd have problems doing multiple mods, just to fly around in to see if they work at your leisure unless you were seriously developing STC's and doing all the paperwork and engineering, etc etc as the test flight program is usually last thing done in the proving process.... This will sometimes require FAA Test Flight crew to fly as well, and would probably be quite costly to get to that point. This topic has been much discussed on this forum if you check back thru the history. I guess what it boils down to is for what purpose you want to do this, what exactly you want to do, how much $$ and time you want to spend, etc...