Choosing the engine for a J-3[The following came as an email in the Piper Cub Builders mailing list, written by John Renwick. -- Dan Ford]
I've flown my J3 with two engines -- Continental A65-8 and C90-8. The A65 was of course what it originally came with. The "-8" means the only "accessories" you can mount are magnetos -- no generator or starter, no electrical system, no transponder requirement if you happen to be based under Class B airspace as I am in Minneapolis. The C90-8 was the engine of choice when I upgraded because it's a nearly exact replacement for the A65; the only modifications needed were slight changes in the lengths of both exhaust pipes. An O-200 or C90-12 would have required relocating the muffler for access to the oil screen and to make clearance for the generator, had I installed one.
With the C90 I get a much better rate of climb than the A65 had, but not much more airspeed. Loaded at 10% over maximum gross weight, the takeoff and climb performance with the C90 is about the same as it was with the A65 flying solo. Solo with the C90, it can lift off from the three-point attitude, and it really wants to climb (I don't have a VSI, so I can't tell you actual rates). I find the C90 is a smoother-running engine than the A65.
I can cruise about 5 knots faster if I want, but with less range. With the C90 came a Marvel Schebler carb with an accelerator pump and mixture control in place of the A65's Stromberg, which was wired full-rich. The fuel consumption with the A65 was always 5.25-5.3 GPH, which is high for a Cub. The prop was a Flottorp 72A48, which I've learned was never certified for J3s, so maybe it's part of the reason for the high burn rate. I now have a 76" Sensenich. If I throttle back to about 2100 RPM for cruise at altitudes of 3500 feet or higher, leaned, I can get 5.1 to 5.2 GPH at the same 75MPH cruise speed I used to fly with the A65. So if I don't try to fly any faster, my C90 with the leaning carb and better prop is actually more efficient than the A65.
I have a 13 gallon wing tank in addition to the 12 gallon main, so range has never been much of a problem for me. A wing tank is absolutely the most valuable mod you can make to the stock J3, even if you don't install a bigger engine. You don't have to fill it if you don't need to, but if you do, it gives you important options. As a good friend of mine likes to say, there is no substitute for fuel.
I think the C90-8 is the ideal engine for a J3. The O-200 is almost the same engine, but its cam is set up to develop the same horsepower at a much higher RPM. More powerful engines like the O-235 would give even more impressive climb performance, but I'm guessing not much more cruise speed. It's just really hard to push a J3 airframe faster than 80MPH unless you're willing to burn quite a lot of fuel. The O-235 is popular on PA-11s, but that airplane has some aerodynamic improvements over the J3: pressure cowl, more streamlined windshield, streamlined bungee covers, and maybe other less obvious things. Without similar changes, I think more horsepower than 90 is just wasted on a J3.
That said, if somebody actually installs a PT-6 or smaller turbine on a J3 and gets it approved, I'd be willing to reevaluate the whole situation! :-) Didn't Sam Burgess put a turbine on a Bucker Jungmeister some years ago?
All of the above goes for the L-4, of course; there's really no difference.