Two stroke bikes could have a sharp steep powerband because of a
transmission to help keep it "on the pipe". Outboards don't tend to be that
"peaky" as compared to bikes.

-W



"Greg" wrote in message
...
My RD350 (2-stroke) had
"toggle-switch power" - wimpy as hell under 5000, then...HANG ON! OTOH

Agreed
Same bike, same impression.
I have the same feeling about my 4 stroke Merc vs my old 2 stroke. I have
lots
of low speed torque swinging a 14" prop on a 60hp with the 4 stroke.

Anatomy of a hole shot:


Holeshot has a lot more to do with prop selection than 2 vs 4 stroke. I'm sure
that prop selection would be different between a 2 and 4.
It is really more art than science anyway.

Outboards don't tend to be that
"peaky"

2 strokes still seem to be fairly useless between fast idle and around 2000
RPM. That doesn't bother folks with planing hulls since that is not a good
speed for the hull either.
I still stand by my first comment. "go fast? buy a 2 stroke", "Go slow to
medium, buy a 4"

"RG" wrote in message
...


Hmmm. I'm the originator of this thread. I've posted the same query at a
number of boat/motor sites. The results were, at my last count..27 to 3
....
two stroke over 4 stroke.

I was specifically addressing the statement that "HP is HP". There are
other reasons that a 2 stroke might be better than a 4 stroke.

2 strokes, in general, weigh less. On a very small boat, such as an
inflatable dinghy, this can be the overriding issue. As the size of the
boat increases the weight factor is not as important. Also, as the size of
the motor increases the weight difference becomes less.

Recent advancements in small engine 4 strokes (outboards, motorcycle, etc.)
have managed to make lighter weight 4 cycles. The pressure to improve the 2
cycle emissions has made them heavier. The result is that the advantage of
2 cycles has dimenished somewhat.

Rod, is there any study, etc. that speaks to the "better thrust" factor on
4
strokes that you mentioned?

I can be more general in this philosophy.

Whenever you compare any two engines, look closely at the HP specification.
The horsepower should have an associated RPM that the horsepower was
measured at. If Engine "A" is rated at 50Hp @6000 RPM, and engine "B" is
rated at 50Hp @ 5000 RPM, engine B is going to out pull engine A in a tug of
war.

When thinking horsepower, think thoroughbreds verses clydsdales. A
thoroughbred is fast but it would never pull a plow. A clydsdale isn't
going to win any races but they can pull tree stumps.

Rod McInnis

"-v-" wrote in message
...


Then why do big 4 stroke OB's have to turn higher rpm to develop the same
HP?


Care to cite an example so that I know what you are talking about?

Be careful when comparing any two motors, no matter if you are comparing 2
cycle vs 2 cycle, 4 vs 4 or 2 vs 4. Too bad that the manufactures don't put
the torque curve in the sales brochure, then you could really see how the
motor rates!

Rod

"F330 GT" wrote in message
...


Two stroke engines have always made more low end torque, both motorcycle
and
marine. For one reason because the spark plug fires every revolution
instead of
every other.

If you compare engines based on displacement, what you say is true. If you
compare based on rated horsepower, it will be just the opposite. For a
given horsepower, a 4 cycle engine will have a lot more displacement to make
up for the extra two cycles it has to go through before getting to a power
stroke.


Engine horsepower is rated at the point of the torque curve where the hp
(torque x rpm) is the highest. Not at the maximum rpm. The torque (and
therefore the hp) ususally falls off very quickly at high rpms.

This is true.

The grind of the
cam on a four stroke controls the torque curve and the maufacturer can
move the
maximum torque up or down the band based on duration and lift.

If you consider destroying the low end torque so that you can have a lesser
peak at a higher RPM "moving the maximum" then I suppose this can be
considered correct.

A cam built for
low end torque will suffer at high rpms while a cam built for speed will
suffer
at low rpms.

This is a simplification of a much more complicated thing. The reason that
torque falls off at higher RPM is because the engine can't breath. One way
of getting the engine to breath better is to open the valves sooner, wider
and faster. It is the "sooner" part of that process that screws up low end.
It is usually the rate that the valves can open and close that limits the
upper RPM limit of a four stroke.


Marine four strokes are not cam'ed for low end since they are
mostly used at the higher end of the rpm band.

I doubt that. When you mess up the low end, you also mess up the idle
characteristics, which is important to most marine applications.


Two strokes generally have a much flatter torque curve.


???? What two stroke torque curve have you been looking at?

Find anyone who has ridden a two cycle motorcyle and ask them about the
acceleration at low RPMs vs being "on the pipe".

A conventional two stroke engine relies on the downstroke of the piston to
pressurize the crankcase to force the air/fuel mixture around the piston
into the cylinder. Reed valves are generally employed to prevent the
air/fuel mixture from backflowing through the carburator. At lower RPMs,
the reed valves are not as effective and the cylinder doesn't scavage as
well. As the RPMs increase, the exhaust momentum will actually help
scavange the cylinder and the engine "comes alive". In many cases, a 2
cycle is just coming to life at an RPM where a 4 cycle is starting to fall
flat.

Rod

Pitch might be different but "prop selection" remains generally the same:

If propping for normal use you prop it to make the median MFG suggested WOT
RPM.

If propping for "hole shot priority" you prop it for median RPM plus 500.

If propping for fuel ecomomy you prop it for the lowest end of the MFG
suggested RPM range.

-W



"Greg" wrote in message
...
Anatomy of a hole shot:


Holeshot has a lot more to do with prop selection than 2 vs 4 stroke. I'm
sure
that prop selection would be different between a 2 and 4.
It is really more art than science anyway.

I'll agree with that. Mine doesn't really shine till over 3000.

-W


"Greg" wrote in message
news:20031209141608.28615.00000536@mb-

2 strokes still seem to be fairly useless between fast idle and around
2000
RPM. That doesn't bother folks with planing hulls since that is not a good
speed for the hull either.

"Clams Canino" wrote in message
news:lvqBb.482583$HS4.3712334@attbi_s01...

I'll agree with that. Mine doesn't really shine till over 3000.

-W


"Greg" wrote in message
news:20031209141608.28615.00000536@mb-

2 strokes still seem to be fairly useless between fast idle and around
2000
RPM. That doesn't bother folks with planing hulls since that is not a
good
speed for the hull either.



My opinion. Buy a 4 stroke. My Yamaha T-8 is fantastic as a high thrust
kicker. But on the big HP engines, I still will go with a 4 Stroke. The
old Oil in fuel mixture OB's ran for ever, and had good lubrication. The
new DFI's? Where is the lubrication for the bearings coming from? Where is
the cooling for the pistons coming from? A lot less lubrication and less
cooling. More failures on the Ficht and Opti's. 4 Strokes have been made
for a lot of years. All the big dirt bikes are now 4 strokes and they seem
to last longer than my Kawi Greenstreak.

K Smith wrote:

: Make your own enquiries & decisions as always of course but, be aware
: this is from the same team who brought the world Ficht & they are now
: marketing under a different name, making all the usual BS claims about
: how "it's all fixed now".

Karen,

You should have a look at the latest version of "Power Boat Reports"
which has an article on their ongoing testing of the "fixed Fichts".

According to the article therein, they experienced "not one hiccup"
during testing so far in a variety of conditions. The motors (there
were two) were not babied.

PBR does not accept advertising from any manufacturers, so I don't
think this falls under the guise of "spruiking". They are generally
quite picky in their evaluations of marine products.

FWIW,

--Duncan