I have a byron F-20 wih a OS 77 and byron fan,the jet is 16 pounds is this engine enough thrust to fly

off tarmac or grass?

tarmac only

yes you can but your pushin your luck. the o.s.77 was very popular when the byron jets come into the public but the f-20 didnt come around till later when other motors were available. but yes i have seen a byron f-20 fly with a o.s.77 and it was ugly but it flew the pilot did nothing but strugle to keep the bird in the air you would be better of with a .91 size motor that has more torque. the f-20 is a good flyer at 10 to 12 pounds. you said your f-20 weighs 16 pounds what all do you have in that jet? you cant put all the bells and wistles in a fan powered model and expect a great flyer.(tryed it with a byron mig-15scale gear,brakes,flaps,speed brakes,canopy opened,landing lightsetc...it flew like a brick not enough thrustwith a rossi 81) a fan model will onlly put out so much thrust so watch the weight if you cant trim some weight,i wouldnt try to fly with a 77 you better put some thought to getting a more powerful powerplant good luck with your project keep em level donnie

can i still use the same fan unit with a 91? will the 91 fly it with authority?

the only ones I have seen fly with "authority", were converted to turbine. You will have better luck with a .91, especially if you can put it on a diet.

I have one that had a 77,gone now, 91 flys it well.The ailerons are very touchy on mine so use expo if you have it.This one flew off tarmac first then grass! Ill check to see what the auw is on mine but I dont think it is close to yours.

Guy i bought it from said 15-16 pounds,I have not weighd it but feels very heavy.
This is my first jet set-up and dont have many local resources on this specific jet so
you guys have convinced me an upgrade is most nessasary.
if upgraded to a turbine is any reinforcement needed? The only extras on the jet so far
is air retracts but that will change soon so i'm getting the turbine and putting the f-20
through liposuction to losse the unessasary

if this is your first jet i would refrain from the turbine in a first jet the f-20 is a wonderful jet but not a trainer jet i would suggest a byron mig 15, yellow a-4, byron f-16, bvm f-86 something that will "float" or has some glide to it so you can learn the transaction from not relying on the prop if it has some glide to it you can get out of trouble a little better than having your jet just fall out of the sky just like most of us you dont learn to fly for the first time with an extra 300 you start with a trainer the difference between a prop and a fan is like night and day your byron f-20 is getting hard to find and i would hate to hear if you cracked it up trying to learn with it its a beautiful aircraft hope you have many flights with it hope this helps keep em level donnie