Gas vs Glow
 

Running old school FS-120 on glow. How much bigger 4 stroke Gasser would it take to fly the same plane?

Are you talking about a really old style F120? Chances are a modern 120 converted to gasoline, or a purpose built gasser fourstroke (say, a Saito FG21, or such) would allready achieve the same output hands down.

But for engines in equivalent state of tune, rule of thumb is that you need 25% more displacement on gasoline compared to glowfuel (so a 150 converted to glow should by and large equal a 120 running on glowfuel).

Hope that answers your question?

Yes, I'm talking about the (pre Surpass) OS FS-120 with 2 valve covers bought back in the early 80's. I flew a 1/4 Cub with this engine and it was perfect. I'm looking to fly this plane again and I'm wanting to go with a modern 4-stroke gas engine. I was looking at the FG21 specifically and was hoping it would at least have as much power as the older 120 on glow.

The FG21 most likely will even outperform that old OS.
I do not know what size and weight your 1/4 scale Cub is, but if I look at the one as sold by SIG (105"WS, 14~16 lbs) I am fairly sure that if the plane is kept light, that OS will allreay fly that plane on gasoline with sufficient power for safe flight, albeit not overly powerful.

I know that because about 5 or 6 years ago or so, for a friend I modded one of those pre-surpass 120's to run on gasoline, it powered a 15 lbs Fokker F VII of about 100" wingspanwith power to spare.

Not to encourage you to convert your current engine, just to confirm that the FG21 will with certainty be able to power your Cub.

If you're interested: That engine found its way to me in horribly seized state (20 years of storage, drenced in coagulated Castor residue will do that) and it took me a week to only disassemble it.
Once cleaned, overhauled and fitted with new bearings, I went to modify the carb (only thing I did to it technically) and ended up with this:




Engine was still running stock carb, no pump, no regulator, just modified fuel linearity. Mounted inverted behind the dummy radial, it never showed any tendencies for critical behaviour.

So bottom line, that FG21 is a safe bet in my opinion. I measured that old OS at about 1 hp on gasoline (which means it should not get much above 1,3 or 1,4 hp on glow), the FG2 does much better than that, at 1,8 hp @ 8800 RPM.