No Muffler = Power? ACK!

Okay, so maybe

It IS only a tiny 49cc, two cycle scooter, but I am not sure how to fix
this problem?

There was a problem with the muffler and when it was off of the bike,
the thing REALLY cruised. That 49cc was FAST and it could not only go
up the hills, but accelerate while it was doing it!

I put the muffler back on, and now it is pretty poor in the power
department. Doesn't go near as fast and barely makes it up the hills.

So my obvious question is, how can I keep the muffler on it AND have
the power at the same time? *Blink*

Thomas
www.thomasamckean.com

 

Do you know what an expansion chamber is? It's an exhaust pipe that's
shaped like a wasp's abdomen. It usually has a very small stinger
coming out the end of the wasp's belly and makes an awful loud noise
when it runs. Expansion chambers work by trapping hot exhaust gasses
inside and letting them out at a slower rate than the exhaust goes into
the pipe...

You could have 50% more power with an expansion chamber and all your
neighbors would hate you when you annoyed them with the awful racket on
Sunday morning when they were trying to sleep off a hangover...

But you can build a quiet expansion chamber with an inverted stinger so
you can feed the exhaust into a rather larger muffler can. The inverted
stinger takes the exhaust out of the center of the chamber instead of
the end of the wasp's abdomen, and that really quiets it down compared
to the noisy expansion chambers we used to have on our dirt bikes.
There is a classic book called "Two-Stroke Tuner's Handbook" by Gordon
Jennings. He told all about how to modify 2-stroke engines to produce
more power and he told about how to build expansion chambers.

Try googling for "expansion chamber" and how to design one...

 

okay, mr. krusty kritter, i demand to know how you know
so much

irax.
2002 CBR 600F4i

 

The Encrusted One usually hits, sometimes he misses.

"Expansion chambers work by trapping hot exhaust gasses
inside and letting them out at a slower rate than the exhaust goes into
the pipe..."

His discussion of airbox snorkels and resonance was good, probably
should have used the same thinking here.

 

Cool? I suppose so.
But what do you think happens to the gas pressure when it expands?

I'd bet the exhaust gasses exit the pipe at the same rate they enter it!
The unmentioned idea is that a reflected pressure wave pushes intake
charge back into the cylinder.

Yes, thanks crusty, I have learned several things from you. But *nobody*
is spot on all the time.

 

Muffler, or the whole exhaust system?
______________
/ \ _______
____/ \___| |_
____ ___ _
\ / |_______|
\______________/ Muffler/
Expansion Chamber Silencer

Check that the silencer is not blocked -- I've known wasps to
build their nests in there! The silencer also tends to block up with
unburnt oil. If there's a removable baffle you can clean it off by
burning it with a blowtorch, or just throwing it in a campfire. If
there's no removeable baffle, you might be able to burn it off by
starting the oil burning with an oxy-torch, then flowing oxygen-rich gas
down the pipe (or maybe even just compressed air) -- make sure all
rubber, etc., seals are removed before trying this!

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD, DT175MX "You Porsche. Me pass!" DoD #484 JKLO# 003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

Which matters more? Whether everybody keeps "score", or if newbies get
their motors runnin' and get out on the highway?

Did you ever read Gordon Jennings' book, "Two-Stroke Tuner's Handbook?"

Back in the 1960's, a pair of Japanese engineers (Naito and Nomura)
investigated the expansion chamber that had already been invented by
the German Walter Kaaden who designed GP racers for MZ. In the late
1950's, the Japanese factories bought rights to a bunch of patents for
pre-war German
2-strokes, so that's why the architecture of all the 1970's Japanese
2-strokes is so similar.

I still have copies of the scientific papers that Naito and Nomura
wrote.
But Gordon Jennings' literary style is much superior to such dry
academic works. Jennings picked up on that work and wrote the classic
TSTH that occupies an honored position in my library of reference
works.

If you buy TSTH, you can use it to answer a lot of newbie questions
about
airboxes, expansion chambers, carburetors, compression ratios, and
ignition systems, and spark plugs, and combustion phenomena, and that
little red-and-white paperback treasure house of information is only
156 pages long!

Gordo (that's what the other speed tuners called him, and everybody who
mattered in motorcycling seemed to know each other in the late 1960's
and early 1970's) was the kind of Dirty-Handed Da Vinci who was an
engineer and a hotrodder and a moto-journalist all wrapped up into one.
I think I was even an ASC, since I often notice his name on 1950's
movie credits. I wish Kevin Cameron (who is a very knowledge
technofreak) was as highly-educated as Gordo was. We lost a genius when
Gordo died of cancer a few years ago.

If you ever rode a two-stroke with an expansion chamber you may have
noticed that it seemed to need to "pressurize" before it did anything
for performance. It needed to get hot, too. The speed of sound in a gas
depends upon the temperature of that gas, and the formula for flask
resonance applies to an expansion chamber just as it does to an airbox
or a 2-stroke crankcase that's pumping air.

The expansion chamber as been described as "just a pressure bleed
restrictor" by some motojournalists and I used a similar statement in
describing how an expansion chamber works in the first thread in this
post because the original poster may never come back and even read the
short answer I gave him. That's the waste of NG's, a lot of questions
get asked, a few get answered, but sometimes the newbie doesn't even
remember all the places he posted to.

Yet the echoes do coming roaring back, as people who just LOVE to argue
find something to nit-pick on and argue about in order to re-affirm
their own self worth.

I won't go much deeper into what Gordo had to say about expansion
chambers, other than the fact that the diameter and length of the
stinger is critical for trapping pressure and heat in the pipe so it
will resonate at the right frequency without melting the piston, and
the angle of the reflector cone is important for controling whether the
engine can be over-revved past the power peak (important in road racing
where the rider often needs to stay in one gear all the way around a
long corner with the engine screaming near redline) or whether the
powerband will have a sharp cutoff at a certain RPM.
Moto-X expansion chambers of the 1970's would have wide angled
reflector cones for that purpose, but road racing expansion chambers
would have narrow angle diffusers and reflectors to make it possible to
over-rev the engine.

 

The "score" matters! As you note further down, the newbie often never
returns to find his answer anyway.

Meanwhile, you're quite a storyteller for the lurkers. And there's that
aura around here of 'What krusty says is truth! We love krusty, and we
want HIM to be right.'

You sidestepped "So my obvious question is, how can I keep the muffler
on it AND have the power at the same time?"
Explaining how expansion chambers work is nice, but won't help him.

Nope! I've read lots of his magazine articles over the years.

And that's what I found lacking here. Your recent airbox/snorkle
discussion was darn good! The expansion chamber explanation was not.

Translation... you don't tolerate correction well.

Neither do I! That's tough s**t for us both, wrong information ought to
be corrected wherever and whenever.

See? Now you've done better. Readers got more than "Expansion chambers
work by trapping hot exhaust gasses inside and letting them out at a
slower rate than the exhaust goes into the pipe..."

 

Expanding gas slows, increasing pressure.
I know, I hate it too, I want expansion to result in a pressure drop

Smaller section increases gas velocity, reducing pressure

Krusty can explain diffusers and nozzles in turbine engines, because I
may be lying to you.

 

Maybe it will, if he ever returns. My take on the question was that the
OP's motorbike had a muffler clamped onto a header pipe, so I figured
he didn't have what most of us would call an "expansion chamber", even
tough that's what a 2-stroke "muffler" usually is. I figured that the
OP was just running a straigh pipe about 12" to 18" long...

Dr. Ivan D. Reid's take seems to have been that the OP's motorbike had
a more obvious-looking expansion chamber with a silencer on it. He
apparently thought the OP was running without the silencer, which would
make the expansion chamber flow exhaust more freely...

The quietest and best performing expansion chamber possible would have
an inverted stinger. As I recall, Hoyt McKagen was saying that the best
inverted stinger wasn't just a straight pipe stuck inside the reflector
cone, but should have a flare on that end. This means that an original
equipment expansion chamber would need some more extensive surgery on
it...

Then the expansion chamber should have the largest possible silencer
attached to the 1"~2" long stub end end of the stinger

But you got your money's worth the first time, didn't you?

I am preparing your new bill now. How would you like to pay this,
Mr. Sparkle?

 

Illustration, please.

 

http://www.suzuki-rg500.com/exhaust.htm

It look like this guy used inverted stingers on his top pipes that he
made for his RG-500 Gamma. His top pipes are the lower ones in the
photo.

Back in the 1950's, when MZ was racing 2-stroke GP bikes, they decided
to run the exhaust pipes out of the rear of the cylinders because a
forward turning crankshaft puts a side thrust on the rear of the
pistons. This would offer an exhaust port sealing advantage. The
expansion chambers were probably too long, and that's how inverted
stingers were *probably* invented, though I don't have any pictures of
such exhaust systems.

Then along came Gordon Jennings in the late 1960's and he knew a lot of
good stuff about the pioneering work of Walter Kaaden, et al.

This was back In The Day when American roadracing pioneers couldn't buy

a really fast, race-winning motorcycle, they had to build everything
themselves, inspired by what they were reading about European GP
racing.

Jennings had a rider named Tony Murphy who is probably
little-remembered. Murphy's most enduring statement was, "Everybody
wants to be a racer, but nobody wants to go fast." This was before the
invention of real brakes and real tires, so I can't fault Murphy for
wanting to win races at the slowest pace possible on a motorcycle that
didn't turn or stop well, races were won by dragracing the other rider
on the straights.

Jennings had a Bridgestone GTR-350 air-cooled twin with rotary valves,
and he was going to make it into "Son of Secret Weapon" (Secret Weapon
had been a roadracing flathead Harley Davidson, as I recall).

First step on Jennings plan was to turn the cylinders around backwards
by put the right cylinder where the left belonged and vice versa, to
get the exhaust ports on the backside and take advantage of the
crankshaft thrust sealing the exhaust port.

I don't know if Bridgestone was tricky enough to do what Yamaha and
Suzuki knew to do. Those companies were simply offsetting the wristpin
one millimeter toward the rear of each piston to get improved exhaust
port sealing without turning the cylinders around backwards.

In order to turn the cylinders around, Jennings had to do a bunch of
welding and machining on the lower end of the transfer ports, the part
that was in the upper crankcase half.

(I walked into my local Kawasaki dealer once, trying to talk him into
letting me race his Norton Manx, and he showed me the upper crankcase
half of his 250cc A-1R road racer. He was trying to emulate what
Jennings had done with Son of Secret Weapon.)

Anyway, Jennings got his cylinders mounted backwards on the engine, and
fabricated his expansion chambers, but the problem was that they were
too
friggin' LONG! The stingers stuck backwards part the rear wheel, and
the AMA rulebook said that the exhaust system could NOT protrude that
far back...

So Jennings just slid the stingers INSIDE the convergent or "baffle"
cone.
And Son of Secret Weapon was very fast and Jennings was surprised how
QUIET the expansion chambers were. He didn't know why the Bridgestone
was fast, it just was.

Later, he got the chance to do some dyno work with adjustable sliding
internal stingers. In Two-Stroke Tuner's Handbook, he said that there
was, surprisingly, NO DIFFERENCE in power with the internal stinger
pushed all the way into the chamber to a point about 1' ahead of the
beginning of the baffle cone (the exhaust sound was a lot quieter
though)and there was NO DIFFERENCE in power with the internal stinger
pulled all the way out to the normal position, but there was a
noticeable drop in power when the stinger was pushed in to the halfway
point of the convergent cone.

This halfway point is the same as the "mean point of reflection" of a
*complete* baffle cone with a pointed end. The "mean point of
reflection" is the end of the expansion chamber's tuned length. If the
stinger takes pressure from that point, it steals heat and pressure
energy that's needed to make the expansion chamber "tune" at the
required frequency to be in "synch" with the exhaust port opening and
closing.

On his RG500 Gamma webpage, Rob Koopman mentioned the possibility of
building an expansion chamber with the baffle cone coming to a point
(like the wasp abdomen I mentioned earlier) and recovering ALL of the
heat and energy in the exhaust gasses, and then just bleeding a small
amount of exhaust out of a stinger that came off the fat belly of the
expansion chamber.

The whole concept of a side bleed stinger makes me wonder if the
motojournalist's description of an expansion chamber as a "pressure
bleed restrictor" isn't actually TRUE.

And it makes me wonder if expansion chambers couldn't just be big fat
UGLY cylinders with flat ends like SAAB's mufflers for their 2-stroke
triple automobiles of the 1960's. It would be simple, but it wouldn't
be as pretty as those sinuously-flowing stamped sheet metal production
expansion chambers we've been seeing for the last 30 years.

 

Yes. We all get our money's worth here!

My RT2 is your's after I die.