hungryandbroke

How much better fuel economy would I get using an OS FS-70 ll Surpass 4 stroke compared to an OS 61 FX 2-stroke if they were to be compared on the same plane?

5%, 10%, 25%, 50%

please reply

charlie

scratchonly

A rule of thumb is 1 1/2 oz of fuel per minute per cubic inch at WOT for a 2 stroke, 1 oz for a 4 stroke.

carrellh

For what engine size does the rule of thumb apply?

It cannot be a blanket rule. My .25 two stroke should burn less fuel per minute than my .61 two stroke.

MinnFlyer

My Feedback: (4)

Carrell, Look at the equation again:

1 1/2 oz of fuel per minute per cubic inch

In any case...

hungryandBroke:

I'll give you my perspective on this subject. If I have a plane that calls for a 60 2-stroke, I use a 90 4-stroke... And I don't go through nearly as much fuel as I did before i got rid of all my 60 size 2-strokes

carrellh

I missed that part. DOH!

Sport_Pilot

The rule of thumb it 1 ounce of fuel per minute for a .91 two stroke. The same for a 1.4 four stroke. For a .61 two stroke that should be 2/3 ounce per minute, thus almost 18 minutes for a 12 ounce tank though actually less because you can't use the last ounce or two, say 15 minutes for 10 ounces. I know I used to get 14 15 minutes running mostly wide open with a 12 ounce tank on a .60. For a .70 four stroke thats about 20 minutes.

hungryandbroke

So I am assumimg that if I compare a .61 2 stroke to a .91 4 stroke the fuel comsumption would be about even for the amount of run time?

Also, what size engine compares to a .91 4-stroke? A .61 or more like a .75?

I'd like to know.

thank you,

charlie

Sport_Pilot

No actually the rule of thumb is getting a bit antiquated. Four stoke engines have become more powerull since this was done so the .91 may use a bit more fuel but even more power. Still I think it is close enough to use. The old rule of thumb is that a four stroke must be 1.5 times the displacement for the same power, but It is a bit below that now. So I think the .91 four stroke is now closer to the .75 than the .60, but still fairly fuel efficient so the fuel used is between that of the .60 and .75.

charlie

MinnFlyer

My Feedback: (4)

Power-wise you may be right, but it still seems like my 94-stroke burns less than a 60 2-stroke does (But I haven't checked in years so it is only a guess, but a good one)

Something else is that with a 4-stroke you will find MUCH less slime left on your plane at the end of the flight. There are MANY times when I don't even bother wiping them down before I throw them in the car at the end of the day (In fact, I RARELY clean them)

WMB

My Feedback: (1)

My Saito 65 gets 20 minutes from 10oz with reserve fuel left. And the lack of slime after flight is great. It may lose it's job to a 60 Webra, the plane could use a lot more poop. I'd like the Saito 100, not quite ready to spend that much at this time. I think the 65 would be nice in an LT40.

Sport_Pilot

I am going on my on experiance, but then I tend to use more nitro on the four stroke. That may be the differance.

hungryandbroke

Upon further thought, the extra money for a 4 stroke is well worth it when it comes to cleaning all the oil off from a 2 stroke. I just flew a .61 2 stroke and it had so much oil all over it I was about to throw the whole plane in the trash can. The money you save in paper towels makes up for the difference in price for a 4 stroke. Well, almost!

charlie

WMB

My Feedback: (1)

I took the Webra 60 powered plane to the field today. I usually use ~ 30-40 oz per flying session with the Saito 65. Today I went through 5 oz with the Webra, it clearly uses less fuel. And all 5oz needed to be wiped off the plane when I was through for the day. To get this king of mileage/session, merely be unable to tune to fly reliably. Of course, I couldn't get it running reliably at all and didn't want to try a flight at full throttle only. I also destroyed two new plugs in the process.
So, the Saito 65 has earned it's postion back on this plane for the time being. The Webra has earned a spot on the engine stand. Funny, this is the only engine I have used that didn't first do time on an engine stand. Talk about lessons learned the hard way, they are never forgotten.
MikeB