Anatomy of a hole shot:

I'm using a 6 cylinder 2-stroke so I get 6 firing pulses per single RPM.
Mine seems tuned so that the " best powerband" is from 3200 - 5500 rpms.

When I punch it from a dead stop. I use a "quick 2-step" hole shot.
The 1st movement of the lever advances the spark from idle to WOT advance
(in my case 21 degrees BTDC), As I do that the motor starts to come up to
2000+ rpms. Then, I hammer it the rest of the way and throw open the
butterflies in the carbs.

Now, all this happens inside of a second and is actually a faster hole-shot,
because if you just dump it to WOT - there is a "bog" caused by the sudden
loss of vaccuum in the throttle bodies. If you let it spool up a little on
the advance first, it then takes the carbs opening with glee.

Soo.... once the carbs are also at full WOT, the RPMs rises rapidly from
about 2000 to 4500. At some point during this - the boat actually starts
making some real forward headway, quickly jumps on plane, and reaches 5500
as the boat speeds up. By the time all this has gone on (assuming you're
pulling a squeaktoy behind the boat) it's time to pull back on throttle to
the 4200 rpm cruise.

BUT , if the boat is overpropped, overloaded, or both, I can see a distinct
"hesitation" at about 3000 RPM where the motor "struggles" to make more
RPM's. Once it passes 3200 I know it'll make it out of the water, and the
RPM's climb much more quickly from there. So I conclude that somewhere above
3000, the motor hits the powerband and starts making its best power. I
noticed the same characteristics with my old 80hp six, except that it also
had a distinct "bark" as it made it over 3200 and got up "on the pipe".

At the other end..... it pulls hard to 5500 and then the curve drops off a
bit. It takes "a while" for it to get from 5500 to 6000 as the boat speeds
up slowly. I'm told that on a racing hull they'll pull to 7000 and then
drop way off. The experts conclude that it makes little usable power over
7000.

Sooooo, I conclude from all this, that I make the "best power" from
3200 - 5500.

-W







"Lloyd Sumpter" wrote in message
news
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