'Sounds better' is more than an aesthetic thing, and can be the
deciding factor, if your flying site has sound level constraints.
Four strokes in general develop their power at lower rpm. This,
combined with a firing rate that is already half that of a two stroke
means the fundamental frequency of the exhaust is much lower, in a
frequency range where perceived sound level (as measured by a sound
level meter set to the 'A' scale) falls off rapidly. Because of this,
it is not unusual for a four stroke to measure on the order of 6 dBA
quieter than a two stroke delivering the same amount of power to the
propeller. Propeller generated sound level is also reduced at the
lower frequency, assuming the blade length is such that tip speed is
similar or less. As a practical matter, it is fairly easy to meet a
.90 four stroke and judicious prop selection, but few .60 two stroke
engines can, regardless of how efficient a muffler is fitted.
The AVERAGE .60 weighs a bit less than the AVERAGE .91. Both make about the
same USEABLE power. The YS .91 is probably the king of brute strength in this
> Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
> 90? Please?
> > Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
> > 90? Please?
> The 4 stroke will cost more and develope about the same power as a
> 2 stroke BUT will develope the power at lower RPM. Thus you can
> use a larger dia prop with the 4 stroke and get better vertical
> performance. This is why almost all serious pattern fliers have
> gone to 4 strokes. The 4 stroke will also use less fuel so you
> can use a smaller tank for the same flight time as the 2 stroke.
> Ray S.
Until very recently, pattern flyers were limited in the maximum displacement of
engines that they could use. Back around 15 years ago, FAI decreed that the
maximum displacement allowable was .61 cu in for 2 strokes, and about double
that for four strokes (the four strokes of the time were not very efficient!)
Engine manufacturers said: "Right". Following the adage that there is no
for cubic inches, they refined the four strokes until they were clearly providing
power than the biggest allowable 2 strokes. And the pattern masses all switched.
Very recently (within the last year or so), the displacement limits were
Now you are seeing a number of pattern flyers at the top echelon migrate back to
2 strokes. An OS 1.40 produces more power than a YS 1.40, and is cheaper
and more dependable to boot! The only reason that the pattern crowd haven't
switched en masse back to two strokes is that there have been a few problems with
the OS 1.40 ... please remember that theYS engines have been refined over the
years to run dependably regardless of the attitude of the airplane or the
the maneuvers flown. The OS 1.40 is slowly catching up, but it will not catch up
I like the YS engines, but they are a (*&&* pain in the ass to keep running!
(Please don't bother posting how wonderful your particular engine has been. The
consensus of the pattern flyers around here is that every 25-30 gallons of fuel
can expect to rebuild your YS 1.20/1.40 ... and Futaba charges a small fortune
for the parts to do it!)
I will probably use a OS 1.40 in my new Python rather than the YS.
Amar Shan - pattern enthusiast
Talking about average engines: Why would someone choose a 91 4 stroke vs
a 61 2 stroke or vice versa for a particular plane? What do you more
experienced people use as criteria for one over the other in a
I have two planes. One with a OS 61 FX and the other (almost built) is
getting a Saito 91. I am trying both . . .
Wayne Patton wrote
In a model such as a Goldberg (CGM) J-3 Cub that calls for a .61 two-stroke,
the extra girth of a four-stroke is barely noticable. The wide fuselage
almost demands a rather largish propeller. Biplanes are another area in
which four-stroke engines enjoy an edge in this regard. However, if you
enjoy flying sleek pattern-like models such as a Killer Kaos, the two-stroke
would be my natural pick.