4-Stroke 2-Stroke

4-Stroke 2-Stroke

Post by Telus NEW » Tue, 06 Oct 1998 04:00:00



    Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
90?   Please?
 
 
 

4-Stroke 2-Stroke

Post by Ian Maclaughl » Tue, 06 Oct 1998 04:00:00


Quote:

>    Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
>90?   Please?

Okay, besides the obvious mechanical differences,
The 4-stroke is more costly
The 4-stroke weighs more
The 4-stroke sounds better

'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.
Ian
San Diego

 
 
 

4-Stroke 2-Stroke

Post by Paul McIntos » Tue, 06 Oct 1998 04:00:00


Some 2 stroke .60s weigh more than some 4 stroke .91s and vice versa.  Some 2
stroke .60s make more power than some 4 stroke .91s and vice versa.  There is
no cut and dried statements that you can make without some qualifications.

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
class.

Quote:

>     Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
> 90?   Please?

--
Paul McIntosh
Desert Sky Model Aviation
"Fast delivery, FASTER engines"
http://www.icatmall.com/desertsky
http://www.dancris.com/~warbird
 
 
 

4-Stroke 2-Stroke

Post by Dno19 » Wed, 07 Oct 1998 04:00:00


Quote:
>   Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
>90?   Please?

A 2 stroke 60 fires every revolution,draws fuel into the crancase where it is
forced through the ports to the cylinder above the piston, to becompresed, and
will turn a 11 or 12 inch prop from 11000 to 14000 RPM.  A 4 stroke 91 fires
every other revolution, has valves,cam, and pushrods, more like an auto engine,
and will turn a 13 to 15 inch prop at 8000 to 12000 RPM.  There are other
differences, but these are the basics.
 
 
 

4-Stroke 2-Stroke

Post by Ray Sheare » Wed, 07 Oct 1998 04:00:00


Quote:

>     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.

 
 
 

4-Stroke 2-Stroke

Post by Amar Sha » Wed, 07 Oct 1998 04:00:00


Quote:


> >     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.

What you said about the pattern flyers is not correct.

Until very recently, pattern flyers were limited in the maximum displacement of
the
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
substitute
for cubic inches, they refined the four strokes until they were clearly providing
more
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
abolished.
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
*** of
the maneuvers flown.  The OS 1.40 is slowly catching up, but it will not catch up

overnight.

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
you
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

 
 
 

4-Stroke 2-Stroke

Post by Wayne Patto » Fri, 09 Oct 1998 04:00:00


Ok the basics are a good start.

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
particular plane??

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

Quote:
> >   Anyone wanna explain the differences tween say a 2stroke 60 and 4stroke
> >90?   Please?
> A 2 stroke 60 fires every revolution,draws fuel into the crancase where it is
> forced through the ports to the cylinder above the piston, to becompresed, and
> will turn a 11 or 12 inch prop from 11000 to 14000 RPM.  A 4 stroke 91 fires
> every other revolution, has valves,cam, and pushrods, more like an auto engine,
> and will turn a 13 to 15 inch prop at 8000 to 12000 RPM.  There are other
> differences, but these are the basics.

 
 
 

4-Stroke 2-Stroke

Post by Cregge » Fri, 09 Oct 1998 04:00:00


Wayne Patton wrote

Quote:
>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
>particular plane?? (snip)

Obviously, there are no hard and fast rules or set criteria. I enjoy both
types of engines, but I am favoring four-strokes the older I get.  They are
"mellow" sounding, once tuned they require less fiddling, they are easy on
fuel and the mufflers work with the modeler instead of against him during
installation. At least in most instances.

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.

Ed Cregger