- The response - I concur
on the epoxy thing - I even like to have the secondary
transfers aimed at a very slight angle upward followed by
a flattening off the very edge (maybe just the liner -
depending on how thick), so that they compliment the
boost port to some extent and help with the scavenging
process. And another way to broaden the powerband is to
stagger the transfers. The secondaries can be brought up
a tad higher than the main transfer.
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- Trax310
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- Really? You let the secondaries open before the
mains? I've seen it the other way around in quite a few
motors though. I never tried that. But I'd be interested
to see the dyno result - a before and after test with it
done.
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- Rick
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I've found that productive also.
I will open the boost port about 2-4° earlier than
the secondaries, and the secondaries about 2°
earlier than the mains for more midrange (this after
achieving the target duration/area for the mains and the
objective powerband). In doing this you have improved
cylinder filling due to the slightly longer rear
transfers' duration. The mixture delivered through them
is improved more than you might think because they are
subjected to higher crankcase pressure than if the mains
were opened simultaneously. This is quite beneficial
since, usually by design, they are more restrictive than
the mains even after your best efforts (within cylinder
metal limits) to reshape them. This trick usually chops a
HP or two from the peak though, due to a bit too early
opening at higher rev's and a slight tendency towards
momentary stagnation or even slight reversion in those
rear ports. Also there may be a slightly increased
tendency towards "short circuiting" of the main
transfer's fuel/air stream since its' velocity has been
dropped ever so slightly, so a prudent balancing of this
technique is necessary.
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- For a more high RPM biased
setup, I open them all at the same time and lean towards
"just enough" tranny duration to do the trick. Inadequate
blowdown at high rev's will cut peak power since the
piston is moving much faster and time vs. crankshaft
degrees of rotation is decreasing.
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- For a lower RPM spread of
power, I delay the opening of the rear transfers, point
them slightly upward and also biased rearward. Same with
the boost port... upward sharply and late opening. This
compromises cylinder filling at upper rpm's but insures
against momentary reversion and dramatically reduces
charge loss out the exhaust when the piston is moving
more slowly and there is time available for this to
happen. In this situation, crankcase pressure has also
been "bled down" through the main transfers (which also
need to be pointed rearward sharply) prior to opening the
rears. The sharp rearward and upward biasing of these
windows costs heavily on area though, and it will limit
the top end drastically. As always, it depends on what we
are trying to accomplish. Anybody ever tried "external
transfers"?
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- High output
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- I can picture what the flow pattern would look like
by starting the rear ports first, and it sounds like it
would do just about what you
- describe. Sounds interesting. I have cut the area of
the piston away which is directly adjacent to the port I
want to try different timing setups on. It can save time
by not having to cut a barrel.
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- With some big bore kits, the timing gets mellowed out
too much from installing the larger sleeve and is almost
impossible to get back unless you add a spacer plate or
double up on the base gaskets. A little machine work on
the piston can make a big difference. I'm not a big fan
of resleeved big bore kits. Upping the displacement too
much on some motors - ones that have a small crankcase -
usually creates problems running at high RPM's because
they don't have enough crankcase volume to fill the
cylinder and tend to lean out and lose power. Real small
bore motors can have a lot of mixture left in the cases
during the scavenging cycle since they typically have a
tall piston. A hole or two in the piston - high up on the
skirt - lined up with a tunnel in the wall can release
more cool charge into the cylinder.
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- Some watercraft cylinders have more transfer port
area than I have seen on other types of engines. They go
out of their way to make the transfers wide - so wide in
fact that in some cases the piston ring end gap directly
crosses the port openings. This makes for transfer ports
all the way around the cylinder with very narrow
separations to support the rings. For what they are, low
RPM 2 strokes usually turning under 7000 RPM - they have
very radical port timing. Some stock motors have
194° to 196° exhaust duration with 126° to
132° transfer port duration. It is not uncommon for
some of these motors to have very poorly designed
transfer port tunnels. Though they are limited by the RPM
they can turn by their rev limiters they still produce
impressive HP numbers. Without the rev limiters the motor
would rev to the moon when the craft hit a wave and
became air born. Who knows what might happen to the crank
when an over-revving engine's impeller suddenly hooks up
again. Their multi-cylinder configurations and larger
cylinder displacement in each jug helps them get away
with the long port duration while they're exhausting it
all into a single pipe (compromised exhaust
consideration). These motors have to produce a lot of low
& mid RPM torque in order to move a fairly heavy
vehicle (5 to 600 lb..) through such a high drag medium.
It is very "testy" to make a watercraft motor produce
more low RPM power, and increase it's overall speed as
well. The extra power has to be transferred to the water
by additional impeller pitch to gain the top speed since
the maximum RPM is (usually) static.
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- ...And no I haven't done external transfers. Though
additional external exhaust I'm familiar with.
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- Rick
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- Pie cutting piston crowns is
something I love to do too. It is much cheaper to play
guesswork with pistons than a sleeve. Cutting the hole on
the piston's skirt is a great way to cool down the piston
a little more. It also can change the inlet duration if
the piston actually closes all ports off and calm a hyper
motor. On the big-bore thing, I concur - On some of the
older motors, primary compression was everything and the
motors produced some serious rpms but had very narrow
powerbands. The transfers were very short and when they
did open they packed a punch. However, there are better
ways now and primary compression is not needed as badly
as volume - although there is a equilibrium to be
established between the two. I have not done any external
transfers either, but would love to one day! High Output,
what types of motors benefit mostly from this?
Later,
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- Trax310
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