Rod McInnis" wrote in message
...

"Lawrence James" wrote in message
nk.net...

50hp is 50hp.

Not exactly.

What gets you moving is torque. Horsepower is torque times RPM.

In many cases, a two cycle engine is capable of running at a higher RPM.
If
you can get 10% more RPM out of an engine, you can often get 10% more
horsepower. That doesn't mean you can use that horsepower. If you can't
get the engine up to that extra 10% it will never be used. Unfortunately,
it was at that higher RPM where the engine earned its rating.

If there is a concern about getting on plane, then low end torque is king.
This is where 4 cycles usually have the advantage over two cycles. On the
other hand, if you can get planing okay, and need the best top end, 2
cycles
usually come alive at the higher RPMs and can provide more top end.

Rod McInnis





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.

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.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. A cam built for
low end torque will suffer at high rpms while a cam built for speed will suffer
at low rpms. Marine four strokes are not cam'ed for low end since they are
mostly used at the higher end of the rpm band.

Two strokes generally have a much flatter torque curve.

Be very careful believing everything you read here, you may end up sorely
disappointed if you buy a four stroke. They are heavier and produce much less
low end torque. The four strokes are selling based on their fuel economy and
emissions, not on performance.

Barry

The four strokes are selling based on their fuel economy and
emissions, not on performance

Also because of the smooth power curve. I said real early in this thread that
if you are a go fast guy you need a 2 stroke, for lots of reasons. 4 strokes
make more sense for guys who go slow to medium speed (3000-4000RPM) a lot with
occasional WOT.
At WOT or close, it makes a lot of sense to have fewer moving parts.

On Tue, 09 Dec 2003 02:29:34 +0000, F330 GT wrote:

Rod McInnis" wrote in message
...

"Lawrence James" wrote in message
nk.net...

50hp is 50hp.

Not exactly.

What gets you moving is torque. Horsepower is torque times RPM.

In many cases, a two cycle engine is capable of running at a higher RPM.
If
you can get 10% more RPM out of an engine, you can often get 10% more
horsepower. That doesn't mean you can use that horsepower. If you can't
get the engine up to that extra 10% it will never be used. Unfortunately,
it was at that higher RPM where the engine earned its rating.

If there is a concern about getting on plane, then low end torque is king.
This is where 4 cycles usually have the advantage over two cycles. On the
other hand, if you can get planing okay, and need the best top end, 2
cycles
usually come alive at the higher RPMs and can provide more top end.

Rod McInnis





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.


Nice theory, but it hasn't been my experience. My RD350 (2-stroke) had
"toggle-switch power" - wimpy as hell under 5000, then...HANG ON! OTOH, My
Virago (4-stroke) has lots of low-end grunt at 2000, and tops out around
6K.

I think is has to do with efficiencies - a 2-stroke doesn't run well at
low rpm.

Lloyd

"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

"-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

There has been a lot of good information posted here but I have one thing to
add. You mention brand names and with four strokes in particular, that can be
misleading.

I have a 115 Mercury EFI four stroke that has a Yamaha powerhead. I also know a
few people who grabbed some great deals on "OMC" four strokes after they filed
bankruptcy. Evidently these were all made by Suzuki so the parts and service
are still available.

The bottom line is, find out who the real manufacturer is.

Dan


RG wrote:
I have the opportunity to add a new engine onto an older, re-built 16 ft.
flats boat with a tunnel hull. It will be a 50 h.p. according to the ratings
etc.. I've read a lot recently about the new 4 cycles not having anywhere
near the "pep" that the 2 cycles do. The weight difference is 50 lbs between
two choices of engines, and the engine will be hung on a jack-plate. As a
comparison, I'm looking at 2 and 4 cycle Yamahas, though the final choice
might be Johnson/Evinrude or Mercury. The cost difference is not the
over-riding issue this time.

The typical running pattern of this boat will be short, quick runs between
various flats. Longest run will be a few miles at a time. Getting up on
plane and racing a few hundred yards will not be unusual, with maybe a dozen
such moves over a half-day fishing. I might be working birds, which, of
course, requires getting up and "over there" as quickly as possible as an
important consideration. Any comments comparing the 2 vs. 4 strokes based on
power and pep? I know all about the discussion about pollution, banning 2
cycles, etc. and that is not part of my concerns today.

Thanks for your help. RichG

I have 2 neighbors with 2000 4 stroke OMC/Suzukis The guy with the 70 is only
complaining about all the paint falling off. The guy with the 40 is still in
bankruptsy court trying to get his fixed.
The biggest problem with the 40 seems to be he can't keep temperature sensors
in it. They turn white and rot out. The top one causes a false overheat alarm
and it goes into limp home mode. I suspect his other "drivability" complaints
are related to that.