- The question - I
think this topic came up before but I am back debating on
it again - The tin cans that surround the crank halves. I
am wondering if removing them helps or hurts. I have ran
a crank(where they were removed) and I am currently
running with my crank that has them. I could not really
tell because with the "removed" crank, I blew the clutch
basket to pieces. I don't feel like the crank was the
reason because it was only a matter of time before the
stocker went Kaput.
-
- Anyway, the gains I can see from removing is that the
turbulence would be up because of the indention that the
cast crank halves have. And, the volume of mixture
readily available would be up. Also, the crank would
become lighter. On the downside, primary compression
would be at a huge loss. And, is the added turbulence in
this area really a benefit or will it possibly slow down
the velocity of air to the transfers because of the
change of momentum needed to get the gases flowing in the
correct direction. Lastly, could the turbulence also have
a slight effect on the crank's wanting to spin as freely?
Keep in mind that I do have a 76 mm piston and it may be
beneficial in different ways to different cc's (primary
compression probably would not be way off from a stock
250 - on the 330 with cans removed). I know that these
coverings are a rarity and Tecates (which we all know
FLY) did not have them. If you have any opinions or have
heard of anything on this, please let me know.
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The response - Ahhhhhhh, I
remember, I just had gotten my crank re-built and you wrote
about this and it set me off wondering. Well, it hasn't came
apart to date yet. I've thought about the gains or losses
associated, even though I'm still learning diligently. I'd
be very interested to see what comes out...... Can't say
that I'd go out and tear my motor apart tonight--dune trip
coming soon, but if I can help in any way please
ask!!!!
I've even heard of "turbo-finning"
the crank, some people say it's a 600 dollar hand grenade
mod, you just don't know when it's going to go off.....
Maybe they haven't found a design that works best
yet......
Backcountry
- Haven't been in here in
ages..... still mega-buried @ shop. I know you know all
the theory involved, so I won't bore you with
speculation. I would run some simulations for you
sometime if you care to cc the bottom end and come up
with a current primary compression ratio without the
"cans" and with the bigger motor. I need a TON of other
information to set it up and run it, though. And time is
something I have very little of lately. I can e-mail you
the wave simulation requirements if you're interested. We
would also have to determine the volume change when the
"cans" are reinstalled. I can give you some tips on that
if you wish. Let me know when you get a minute.
High Output
-
-
-
- It has been my experience that the real effect of
small changes to the case volume (ratio) of modern
engines is hard to measure either "seat of the pants" or
otherwise. I have not seen a modern 2 stroke that was
severely mis-cased. Years ago however it was very common.
I remember stuffing (lots of epoxy) piston port cases and
using sealed main bearings to decrease crankcase volume
on some motors. When thinking about the "cans" a couple
of things come to mind. A larger case volume holds more
mixture so in theory it can deliver more total mixture.
Honda and the other manufacturers go to great trouble to
establish proper crankcase volumes. They plug the big end
pin of the connecting rod (though the plugs seems like
they should be put in facing the opposite
- direction) but interestingly enough don't plug the
wristpin. It seems that they want the case volume - the
mixture - accessible, available and not far away from
where it needs to go. They fill the area as much as
possible directly around the crank and create case volume
above it. Removing the cans will create additional space,
but at the time when the piston will be nearing BDC to
open the transfers the new space isn't very accessible -
it's nearer to the bottom of the case. This may be
compensated for by using a properly tuned exhaust.
-
- Base plate spacers which increase crankcase volume
keep their added volume high in the case - where it seems
like it can do the most good - right at the opening into
the transfers. Because the primary compression ratio gets
lowered when using one there is an additional small
benefit - there will be a slightly decreased pumping loss
due to the decreased pressure. This includes sucking in
the mixture and pumping it out. There may be an advantage
of a slightly cooler mixture as a result of the larger
volume as well. The loss of suction on the intake stroke
may be more of a disadvantage though - the reduction in
amplitude of wave activity - reducing the ram effect. The
change in the volume will change the power delivery of
the motor (independent of any increase in turbulence). By
using the right exhaust pipe you could get it to suck all
the mixture out of the case - to below atmospheric
pressure in many cases - over a certain RPM range.
-
- Turbulence is an interesting thing. When you spin a
propeller in water you can see its act very clearly. It
makes things move around a lot. In a 2 stroke motor the
volume of mixture isn't going to be sitting idle in the
case so the crank can whip it up. The direction of the
incoming charge is not towards the crankshaft counter
weights, it is more inclined to follow the piston - going
up.
-
- If I understand turbo veining correctly it is a
modification which increases the volume in the crankcase
by its nature as well. The only one I've seen had
material removed from the crankshaft. It won't suck in
mixture - though it looks like it can. I doubt it does
much to direct a charge up the transfer ports either - it
may even create an eddy or barrier at some RPM's. It may
be USEFUL in creating turbulence for perhaps a hundred
degrees of crankshaft rotation (in the power band). The
change in turbulence would be mostly static since the
mixture is in the cases for such a short period of time
(a single revolution) unless there was substantially more
mixture in the cases at a particular RPM range (on the
pipe). There would be an effect from the increased
surface area of the counterweights as well - perhaps even
promoting attachment of mixture to it at lower RPM's -
which would not be good.
-
- I wonder if they tested this modification back to
back with properly sized cases (one case set compensated
for any difference in volume) to establish a scientific
baseline - to see if any changes in power were the result
of the crank alone or because of any change in crankcase
volume or friction. It may be true that slightly roughing
up the surface of the stock cans and counterweights would
provide much of the same benefit - if any is
measurable.
-
- When removing the cans on the TRX crank it would be
an advantage to test the motor with the same case volume
as original (like above) to isolate any measurable
changes that would be a result of the singular act. It
would be virtually impossible to measure frictional
losses due to the increased turbulence in this area
unless you design a test devise for this. It may be a
project that could be very interesting to do.
-
- I've seen some veined squish areas on cylinder head
domes - intended to promote direction of the charge and
create turbulence. They were made by pressing a tool
against the combustion chamber producing indented veins
there. This indenting also raised material away from the
plane of the squish area of the head. It made me wonder
if this could create hot spots (inducing detonation) on
the edges of the raised veins since it in effect made the
head closer to the piston. It seemed like there would
have to be compensation for this modification by making
the squish distance further - which defeats its purpose.
Later I saw them made by machining the metal from the
head - a better way to do it as long as the chamber
volume is maintained. I question any real advantage over
a properly designed normal, narrow angle parallel, tight
squish setup. I remember when 4 stroke motor builders
dimpled their combustion chambers to promote turbulence.
I haven't seen that done for quite some time.
-
- It seems that if there's a way to make people spend
more money on a part - someone will make it. Some parts
look SO cool. Isn't it funny how some parts can LOOK
faster? But looks are one thing. I've always been
interested in those trendy parts but rarely have I made
the plunge into being another machinists "rolling guinea
pig." I have tried a few things which were a waste of
time/money/effort but I have been lucky enough to "tune
to or tune out" just about any mistake I've made or been
smart enough to know when I can't.
-
- If you do remove the cans and CC the cases with and
without them I'd be interested in knowing the difference
in volume and the difference in the weight of the
crankshaft.
-
- Check out what 2 stroke guru Kevin Cameron (last
months Personal Watercraft Illustrated magazine - March
1999 - a must read article) wrote about crankcase volume
- the more volume in the case - the more power the motor
can make. This would include base plate spacers as well
as reed cage spacers.
-
- Rick
-
- Rick, your post on 2-stroke
engine crankcase volume contains lots of interesting
information. Thanks!
-
- I ask your help explaining to
me something apparently contradictory in the post. You
mentioned "stuffing" 2-strokes with epoxy, using sealed
bearings, etc., to REDUCE crankcase volume. "I remember
stuffing (lots of epoxy) piston port cases and using
sealed main bearings to decrease crankcase volume on some
motors." Elsewhere in the post, you state the maximum
power potential of a 2-stroke resides in INCREASING
crankcase volume (as base plate and reed spacers do).
"the more volume in the case - the more power the motor
can make."
-
- Resolution of the apparent
paradox (i.e., decreasing crankcase volume increases
performance; also, increasing crankcase volume increases
performance) may be a simple matter for someone
understanding 2-strokes. I ask your help explaining the
significance of crankcase volume to
performance.
-
Jim
-
-
-
- Yes, I guess I could clear that up a bit... Older 2
stroke engines were not very high tech, in fact there
were some pretty poor designs. Most were piston port
design - no reed valves - and the philosophy of
extracting more power from them was more of a mystery,
than a science. A mystery made even more difficult
because 2 stroke tuners had to figure most of it out by
themselves. There was very little technology available
for 2 stroke tuners to help them out. Much of it was
"seat of the pants" and "see if it helps."
-
- When carefully examining (some of) the old 2 stroke
motors, it was clear that the cases were poorly designed.
There were lots of spaces where mixture could become
trapped and never become part of the power making
process. Voids, pockets and terrible castings were the
"norm." Additionally, some engines were designed with a
tremendous amount of space between the main bearings and
the seals. The bearings need lubrication, but there is a
point where you look at them and the space behind them
and say to yourself " they must have had a bunch of these
bearings laying around and they needed to get rid of them
- or maybe it was the seals they were trying to unload."
Mixture can not be available if it's trapped down low,
behind the main bearings or in a void or an area that
causes a serious air flow trap. Increases in power output
were realized by fixing some of these imperfections. In
some cases epoxy was used to fill voids and help with
streamlining.
-
- Modern case designs are different. The cases are
compact and streamline. They have no (very little) wasted
space, yet their volume is UP! Where the improvement (and
volume increase) came from is as I stated in my earlier
post. The new design is to make the mixture available.
It's not down low out of the way - it's in the way -
ready to go to work. It's high in the cases above the
moving parts, right at the opening of the transfer ports.
With such a short amount of time available for the
mixture to reach it's destination, designers are not
willing to give up any of it to obstacles or distances
that are too far to travel during the scavenging
cycle.
-
- Technology has come a long way in the last 20 years -
especially with 2 stroke cycle engines. Modern pipes will
suck the mixture right out of the cases and cause the
reeds to open (again) when the motor comes on the pipe.
This is an improvement which is further enhanced by the
mixture being ready and waiting so it can do what its
supposed to be doing - making power.
-
- Transfer port shape, size and aim, crankcase volume
and the technologies surrounding the scavenging process
are more responsible for the increases in the overall
power output and increased power band width than any
other design factor of the modern 2 stroke engine.
-
- Rick
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