- The Response - Big bore/stroked motors are
very cool. I like them a lot - for certain applications.
But is it the best choice for getting from point "A" to
point "B" the quickest? There are several things to
consider with a bored motor/stroked crankshaft
combination pertaining to high power applications. A
stroked crank will increase the displacement of the motor
and provide a slightly longer power stroke. Increases in
the piston size also increase displacement. Bigger is
better right?
-
- There is a limit as far as how many feet per second
(FPS) a piston can travel without destruction.
[Please note that this should
be FPM feet per minute - Rick] This limit has
been established at (an average) of about 4000 FPS for
high output motors - 5000 FPS for short drag racing (rare
indeed) - even more rare are the to fuelers who go as
high as 6000 FPS. Increases in the stroke make the limit
closer and have to be compensated for in other ways.
-
- Increases in the stroke will decrease the motors
upper RPM limit. It increases friction against the
cylinder walls because of its steeper angle at 90°
ATDC and makes pistons wear out sooner. A longer rod (say
the 1987 or later model year +5 mm) may help a lot.
Increasing the bore additionally adds more heat and
friction.
-
- Consider a stock 72 mm stroke motor at 8000 RPM. Its
average FPS is 3773, at 8300 is at 3914 FPS, and at 8900
is at 4197 FPS. All respectable numbers and very
attainable with the TRX motor. It will live a long life
with these numbers since it will not see 8900 RPM for
extended periods of time. Change that stroke to 78 mm and
at 8000 RPM we have 4093 FPS (already over the 4000 FPS
rule), at 8300 its at 4246 FPS, at 8900 its at 4553 FPS.
Very high numbers - too high. In fact in order to stay
below 4000 FPS you would have to limit the RPM to about
7600-7700.
-
- A bored/stroked TRX will launch HARD. It will have to
be ported and piped for the lower RPM and geared this way
too. If configured properly it could be fast - over a
wide but lower RPM range. It may well be the best all
around dune motor you could make. But it will make a
short lived drag race motor.
-
- Depending on what the purpose of the motor is - there
are several ways to go with these parts. For a dune
"chugger" motor I'd use the maximum of everything - big
bore (72 mm) and the long stroke (78 mm). With 330 cc's
and a decent port/pipe arrangement it could be very
potent - though it will never live long in the upper
stratosphere. I'd keep the revs under 7800. There is
simply too many things going against it.
-
- 1) The average FPS at high RPM
- 2) The heavier piston (72 mm Wiseco about 250 grams
vs. stock TRX Wiseco 205 grams) slows down piston
acceleration.
- 3) Additional crank angle increases friction and
decreases mechanical leverage.
-
- For a high output motor that will live a long life
you might be better off with the shorter stroke (72 mm)
and the larger bore (72 mm) piston. Though you'll still
give up some rev-ability because of the heavier piston
and its associated parts. If its ported and piped
correctly it will yield a very fast combination. Expect
about 59 to 63 (or more) rear wheel HP @ about 8100 RPM.
This combination launches very hard but allows very
little over-rev - maybe 8300 max.
-
- My choice however (based on my experience and a bunch
of 2 stroke engine modeling software) would be the even
lighter 69 mm (Pro-X - about 220 grams when properly
modified) or 69.5 mm (Wiseco) piston in combination with
the short rod/short stroke (for rev-ability) and to pipe
and port for this combination. To gain additional mid
range switch to the longer rod (same stroke) but it will
give up some top end revs. Expect about 57 to 61 rear
wheel HP @ about 8500 RPM. This combination launches
hard, revs quick and has a real good top end with over
rev to well over 9300.
-
- For a good article on strokers please see -
http://www.groupk.com/yamstrokers.html - It's a piece
about Yamaha watercraft motors (701 cc twins) but don't
let that fool you. There is much good information in that
piece.
-
- Rick
-
-
-
- Interesting reply regarding the
maximum average piston velocity to balance engine
performance/reliability. May be an error with your units
though. Working through the math leads me to believe that
this target value should be expressed as feet/min NOT
feet/sec. Here's the conversion:
(8000rev/min)*(2strokes/rev)*(72
mm/stroke)*(1in/25.4mm)*(1ft/12in) = 3780 FPM. Similarly,
computation of the maximum piston velocity at 8000RPM and
a 72 mm stroke [(8000rev/min)*(2PI/rev)*(72
mm/2)*(1in/25.4mm)*(1ft/12in)] yields 5937 FPM.
Intuitively, this seems to be a bit low. If you see an
error in my math, please post the correct
solution.
-
- Texridr
-
-
-
- You are absolutely correct. Thank you for pointing
that out. It is mean feet per minute NOT feet per second.
Though the peak number goes far beyond 4000 FPM at or
near 90° ATDC it also comes to a full stop at
180° ATDC and at TDC but it is the mean FPM we are
looking at. Here is a simple formula for calculating it -
from Gordon Jennings 1973 book Two-Stroke Tuners
Handbook:
-
- Cm = .166 x L x N
- Cm is mean piston speed in feet per minute.
- L = stroke in inches
- N = RPM
-
- Rick - uses 2 stroke software to get these numbers
anymore...
-
-
- The only other thing to keep in
mind is that 250R 2 stroke pistons are heavy on the
exhaust side. Just look at one and you'll know why. The
larger you go in bore size, the more it becomes offset
balanced between the exhaust and intake. This causes the
piston to wear much faster. Another thing is that when a
motor is stroked, the cylinder must be raised with a
spacer plate because the piston sits higher at TDC. What
happens is that the exhaust roof is raised also. So what
you end up with is a motor that should rev less - but
instead it wants to rev more.
-
- I run right at 331 cc right now
and it is all bore. The piston has the gudgeon pin's bore
offset to the exhaust a tad to relieve some of the
off-balancing. My rod is '86 stock length and I want to
change to the 5 mm longer rod one of these days. While I
do agree with Mac on the 4000 FPS [4000 FPM -
Rick] rule, it is cc
independent to a degree and there are always exceptions.
An extremely large piston (i.e. Big bore in stock jug)
will surely cause major overheating problems(which kills
reliability) at high rev because of the swept surface
area it obtains in a jug designed for much less. A
stroked motor will do the same(if you can get the revs:))
for the same reasons. So, while the rule is a general
guideline (and a good one at that!) you must still figure
into the equation that "HONDA" or whomever did not make
it that way for a reason. Wiseco makes some good light
pistons(Pro-lite, Mini-lite, Micro-lite, etc.) and NIKS
makes CRAP!!!! Go with the Wiseco Pro-Lite. The
Mini/Micros are for "tear down after every race" pistons.
The larger bore pistons can be cut and lightened very
easily. In fact, Wiseco now makes 5 degree crowns. This
is much lighter than the round crown piston you are used
to seeing. But, the head really needs to be cut too.
Langbolt, in my experience with the 250R motors, I can
honestly say that the 330 big-bore is the best thing I
have ever come across and I just happened to be lucky
enough to own it. I have seen/ raced against everything
that exists - I think ;) and have not had to do a rebuild
in over a year. I am getting ready to re-piston/bore my
motor only because the rings are worn slightly and I DO
NOT wait on my motor to TELL me something is wrong. I
would DEFINITELY recommend going large bore. When I ran
the 310 cc, I was disappointed to some extent because a
WAY modified 250 cc could keep up. Well, now, with this
330, that AIN'T gonna happen. I ported her for hi-rev and
I DO hit 8100 still pulling! Just go with the 72 mm jug
and stock rod, then you can always get the 76 mm sleeve
pressed in.
-
- Trax310
-
-
-
- I'll agree with most of that. However I have had VERY
good luck with Pro-X pistons, in fact prefer them over
Wiseco in almost every instance - especially where the
owner doesn't want to open the motor very often. They may
have had a quality control problem at one time
(especially with the manufacturer of their rings) but I
have not had problems with their stuff for a long, long
time. You can run them tighter without worry of scuffing
or seizure and when properly modified are very light too.
Their cam & taper is a lot less than a forged piece
so less warm up time is required as well.
-
- I have made a few TRX cylinders using an older model
DR250 piston (Yamaha 70 mm piston) with good results too.
Bore out the original sleeve and weld up the corners of
the the exhaust port. Remove the exhaust bridge. Resleeve
the barrel and give it a single large exhaust port. Port
and pipe for this combination to have a very fast 275 cc
revver. The piston weight is light as well.
-
- For such a long time 2 stroke motors were square (68
x 68) or over square (70 x 64). Their power was made
through higher revs, not longer power strokes. Honda came
up with the under square combination 66.4 x 72 for their
250 cc motors several years ago. Other manufacturers had
been playing catch up. Take a look at all [most]
current 250 cc 2-stroke motors, they're all 66.4 x 72.
This combination yields a long power stroke without the
need for unnecessary revs - though they still rev very
well. A long power stroke is preferred in most
instances.
-
- I have run the 76 mm 330 set up too, and in my
experience found it to launch hard, but not rev very well
- shutting down much sooner than I wanted. This is due to
growing the motor too large and having limited room for
transfer ports - they simply can't be made large enough
without making their timing too radical. Doing so results
in too much short circuiting due to not enough time for
blowdown. The 76 mm piston is bored into the same barrel
the 310 kit uses and for that matter every other size
piston Pro-X makes. All the way from 250 cc to 330 or 350
cc with a stroke increase. That's a pretty broad range -
too broad.
-
- Going too large with the bore results in problems
associated with igniting the mixture well enough too. The
stock ignition system was not intended to fire the gasses
of the +10 mm bore. Replacement of the ignition is a must
when going to 76 mm.
-
- I like the idea of a 76 mm bore long rod motor, using
the late model 76 mm piston (if you can get it) to avoid
using the spacer plate (its pin is located 5 mm higher).
I like the 72 mm sleeved motor better though - bored to
maximum 73.5 mm and running its lighter pieces. It revs
better, launches just as hard and is matched better to
the Pro-X port tunnels.
-
- Rick
-
-
-
-
- Texridr, you can't do it like
that! And MacDizzy, 4000 ft per minute calculates to only
45 miles per hour! My piston goes MUCH faster than that!
The piston's speed is directly related to the cranks
rotation and it is NOT linear in any way. Just like you
cannot take the derivative of a discrete function, you
cannot use linear formulas to calculate non-linear
movements. The piston moves only along the cranks "y"
axis if plotted. However, the crank moves along both the
"x" and "y" axis. What this means is that when the crank
is near TDC and BDC, the piston is not moving very much
in relation the the cranks speed. I have heard of the
4000 feet per SECOND rule and have always thought that a
250R could come no-where near that limit. The high
revving motors that hit around 18000+ rpms are the ones
that I have always thought the rule applied to. Take for
instance - we KNOW that the piston's top speed IS just
before(because of rod angle) the crank is at 90 degrees
(TDC being 0 degrees). If we use simple proven math
techniques, we can determine how fast the piston is
traveling at any given rotation angle and speed of the
crank. The crank pin draws a circle as it travels and we
need to use the circumference (pi*diameter) of this
circle. So 3.14*72 mm = 226 mm traveled by the pin in one
revolution. Now at 8300 rpms, the crank pin travels 226
mm/(25.4mm per inch) * 8300 = 73914 inches per minute.
This can be broke down "per second" by dividing by 60 and
we get 1232 ft/sec. The piston will almost NEVER go
significantly faster than the crank (Only on very long
strokes or short rods is it much different) . The formula
we can use to obtain the speed of the piston at any
rotation of the crank is cos(angle)*1232 ft/sec. Now if
you want to know how fast the piston is moving at say 45
degrees of crank rotation, just substitute it into the
formula. It would be traveling around 870 ft/sec. But,
this is all an approximation because the angle of the rod
actually accelerates the piston as it approaches 90
degrees. It is not that significant on most motors.
However longer stoked motors suffer more. On a 250 cc
motor with a (around)125 mm rod and radius of pin to
crank center = 36 mm, the angle of the rod when the crank
is at 90deg is 73 degrees. If we wanted to calculate this
into the equation, it would be using sin(). The formula
would be (Piston FPS obtained earlier)/sin(73). Example:
1232 ft per sec/sin(73deg)= 1288 FPS. As you can see, not
much change,but the smaller the angle gets (longer
stroke), the faster the acceleration of the piston. I
will not get into differential equations and such to
calculate the mean velocity, but this should show you the
worse case scenario and that's what you really need to
know!
-
- Trax310
-
- In your example - [3.14*72 mm = 226 mm traveled
by the pin in one revolution. Now at 8300 rpms, the crank
pin travels 226 mm/(25.4mm per inch) * 8300 = 73914
inches per minute. This can be broke down "per second" by
dividing by 60 and we get 1232 ft/sec.] Isn't that
answer in inches/sec? You'd need to break that down even
further by dividing it one more time by 12 to get it to
ft/sec (102.6 ft/sec [69.95 MPH] and 107.2 ft/sec
[73.09 MPH] at 73¡). Any rate what you have
figured out is the speed of the crank pin.
-
- Yes, 45 miles per hour SEEMS slow - but what we're
talking about here is the distance the (surface of the)
piston travels in one minute. It is its linear travel
that we are concerned with. We are not talking about
piston acceleration. That is totally different - and MUCH
higher indeed. I think we're starting to mix up the cats
& dogs. We need to define a couple of things to be
sure we're talking about the same thing.
-
- Piston speed is used to define the maximum distance a
piston will travel in a minute. We need to know the
actual piston to cylinder wall surface to surface touch.
The mean piston speed expressed in feet per minute (peak)
- is used as a boundary. Piston speed is simply a
benchmark to design motor parameters by - a guideline to
determine piston stress,but it comes in handy when
thinking of the other engine pieces too. It would be
easier to understand if it were called "piston travel"
per minute. The stock TRX power peaks at 7500 RPM and
revs to 8500 which is 4014 FPM. The 4000 FPM rule is a
rule that has been around for a long time and for the
most part is still VERY relevant.
-
- Personal watercraft motors still use 3500 FPM because
of their unique environment and the possibility of
cavitation. Automobile drag racers frequently go FAR over
this rule for their 1/4 mile at a time bursts. If a 4"
stroke big block drag race motor spins at 8000 RPM all
the way through the 1/4 mile and finishes in 6 seconds it
would only have rotated its crankshaft 799 times (the
formula indicates a 5312 mean piston speed).
-
- Piston velocity is the "current" velocity (usually
expressed in feet per second) of the piston at a given
crank angle. It is a number which can be very hard to
understand because of the 2 start/stop cycles involved in
each revolution. I have seem 3 different programs for
calculating this figure. Each one gave me different
results, it depends on how they designed the program.
Some programs make the velocity symmetrical - that is to
say that there is the same value at 45° ATDC as
there is at 315° ATDC. Some programs have a number
higher after TDC (going down) than before TDC (going up)
depending on whether the motor is compressing or
expanding gasses further complicating things. Though peak
velocity should occur at or close to 90° ATDC
because the mechanical leverage is greatest - it is
actually about 5° - 10° or more before that -
according to the software.
-
- Piston acceleration (usually expressed as maximum
feet per second squared)is more of a reflection of the
time the piston takes to travel a given distance. In the
TRX example at 8300 RPM, that's 138.33 revolutions per
second. How fast does it need to accelerate from a full
stop (though it has the inertia of the 8300 RPM's to keep
its acceleration high) to travel the 72 mm ? The formula
for determining maximum piston acceleration is as follows
- again taken from Gordon Jennings 1973 book Two-Stroke
Tuners Handbook (that darn book is SO handy).
-
- Gmax = N^2 x L / 2189 * (1 + 1 / 2A)
- Gmax is the maximum piston acceleration in feet per
second squared.
- N is crankshaft speed in revolutions per minute.
- L is the stroke in inches.
- A is the ratio of the connecting rod length (between
centers) to stroke.
-
- For the TRX (answer taken from software) the number
is 114864 ft/sec^2 for a 72 mm stroke (maximum) with a
125.3 mm rod at 8300 RPM. Change the rod to 130.3 mm and
the number moves to 113880 ft/sec^2. The longer rod slows
down the acceleration because it allows the piston to
dwell longer at TDC and BDC. When I did the calculation
manually (ugh) I came up with slightly different numbers.
I got 114623 ft/sec^2 for the 125.3 mm rod and 113669
ft/sec^2 for the 130.3 rod.
-
- Piston acceleration relates more to the ability of
the rings to seal the bore than anything else. If you go
too high with this number, the rings will "flutter" lose
their ability to seal near TDC. With the kind of piston
acceleration numbers high output 2 strokes make it is
still possible for modern thin rings to lose their seal.
The piston is traveling VERY fast at its peak - then has
to come to a full stop at TDC. As the piston quickly
slows down, its rings may very well become air born and
fly off the bottom of the ring land where it is sealing
the pressure - breaking the seal - even with combustion
pressure trying to push the rings against the bore. This
information is useful to help establish a rev limit.
Heres one chart that seems ok though it represents a
symmetrical "ideal" example.
-
- Stroke = 72 mm (2.834 in.)
- RPM = 8300
- Rod length = 125.3 mm (4.933 in.)
-
- Degrees .....Piston Velocity ......Piston
Acceleration
- ATDC
............ft/sec...................ft/sec^2
-
- ------- --------------------------
-----------------------------
- 0: 0.00 f/s ( 0.0 MPH) 114820.18 f/s/s ( 3588.1
Gs)
- 10: 22.87 f/s ( 15.6 MPH) 112017.76 f/s/s ( 3500.6
Gs)
- 20: 44.62 f/s ( 30.4 MPH) 103780.62 f/s/s ( 3243.1
Gs)
- 30: 64.22 f/s ( 43.8 MPH) 90623.65 f/s/s ( 2832.0
Gs)
- 40: 80.75 f/s ( 55.1 MPH) 73409.50 f/s/s ( 2294.0
Gs)
- 50: 93.51 f/s ( 63.8 MPH) 53323.97 f/s/s ( 1666.4
Gs)
- 60: 102.06 f/s ( 69.6 MPH) 31797.54 f/s/s ( 993.7
Gs)
- 70: 106.28 f/s ( 72.5 MPH) 10363.01 f/s/s ( 323.8
Gs)
- 80: 106.33 f/s ( 72.5 MPH) -9534.61 f/s/s ( -298.0
Gs)
- 90: 102.63 f/s ( 70.0 MPH) -26735.15 f/s/s ( -835.5
Gs)
- 100: 95.82 f/s ( 65.3 MPH) -40516.10 f/s/s (-1266.1
Gs)
- 110: 86.61 f/s ( 59.1 MPH) -50658.61 f/s/s (-1583.1
Gs)
- 120: 75.71 f/s ( 51.6 MPH) -57410.09 f/s/s (-1794.1
Gs)
- 130: 63.74 f/s ( 43.5 MPH) -61359.15 f/s/s (-1917.5
Gs)
- 140: 51.20 f/s ( 34.9 MPH) -63264.52 f/s/s (-1977.0
Gs)
- 150: 38.42 f/s ( 26.2 MPH) -63888.50 f/s/s (-1996.5
Gs)
- 160: 25.58 f/s ( 17.4 MPH) -63874.89 f/s/s (-1996.1
Gs)
- 170: 12.78 f/s ( 8.7 MPH) -63686.98 f/s/s (-1990.2
Gs)
- 180: 0.00 f/s ( 0.0 MPH) -63595.09 f/s/s (-1987.3
Gs)
- 190: 12.78 f/s ( 8.7 MPH) 63686.98 f/s/s ( 1990.2
Gs)
- 200: 25.58 f/s ( 17.4 MPH) 63874.89 f/s/s ( 1996.1
Gs)
- 210: 38.42 f/s ( 26.2 MPH) 63888.50 f/s/s ( 1996.5
Gs)
- 220: 51.20 f/s ( 34.9 MPH) 63264.52 f/s/s ( 1977.0
Gs)
- 230: 63.74 f/s ( 43.5 MPH) 61359.15 f/s/s ( 1917.5
Gs)
- 240: 75.71 f/s ( 51.6 MPH) 57410.09 f/s/s ( 1794.1
Gs)
- 250: 86.61 f/s ( 59.1 MPH) 50658.61 f/s/s ( 1583.1
Gs)
- 260: 95.82 f/s ( 65.3 MPH) 40516.10 f/s/s ( 1266.1
Gs)
- 270: 102.63 f/s ( 70.0 MPH) 26735.15 f/s/s ( 835.5
Gs)
- 280: 106.33 f/s ( 72.5 MPH) 9534.61 f/s/s ( 298.0
Gs)
- 290: 106.28 f/s ( 72.5 MPH) -10363.01 f/s/s ( -323.8
Gs)
- 300: 102.06 f/s ( 69.6 MPH) -31797.54 f/s/s ( -993.7
Gs)
- 310: 93.51 f/s ( 63.8 MPH) -53323.97 f/s/s (-1666.4
Gs)
- 320: 80.75 f/s ( 55.1 MPH) -73409.50 f/s/s (-2294.0
Gs)
- 330: 64.22 f/s ( 43.8 MPH) -90623.65 f/s/s (-2832.0
Gs)
- 340: 44.62 f/s ( 30.4 MPH) -103780.62 f/s/s (-3243.1
Gs)
- 350: 22.87 f/s ( 15.6 MPH) -112017.76 f/s/s (-3500.6
Gs)
- 360: 0.00 f/s ( 0.0 MPH) -114820.18 f/s/s (-3588.1
Gs)
-
- Average Piston Velocity = 65.34 f/s ( 44.5 MPH)
-
- Average Piston Acceleration = 142.29 f/s/s ( 4.4
Gs)
-
- Rick
-
-
-
- You guys must be wondering what
kind of sh1t I've been smoking. I left out the divide by
12 for feet (and I'm a math minor - go figure), but the
formulas are correct. This stuff is just right up my
alley and I love to compute stuff like this. Texridr, it
is OK to obtain the MEAN the way you did. We know that
the piston has to travel a given distance (72 mm) twice
for a single rpm. So it is A-OK to simply multiply 8000
rpms * 2 * 72 mm to obtain the MEAN distance traveled by
the piston in one minute. Mac, the 4000 feet per minute
you were talking about was a MEAN and I did not realize
that - (I just noticed that when checking the post to
mesg Texridr) I thought you were talking top speed. 45
MPH for mean speed of a piston sounds about right.
So anyway, sorry for screwing that
all up...
-
- Trax310
-
-
-
- No problem. I noticed that math error as you'll see
in my response to your post. I only wish I had seen it
sooner because I was scratching my head trying to figure
out why the numbers didn't look right. Here's a few of
the other more commonly used formulas. I had to use some
of these a lot lately. I finally decided to open up my GP
760 motor.
-
- [Please see The Formulas
Relating to Engine Building page for more -
Rick]
-
- Engine Tuning and Diagnostic Formulas
-
- References:
- "Two Stroke Tuners Handbook" by Gordon Jennings
- "TSR Software" by Tom Turner
-
- Predicting Power
-
- BHP = PLAN/33,000
-
- P is brake mean effective pressure, in PSI
- L is piston stroke, in feet
- A is the area of one piston, in square inches
- N is the number of power strokes per minute
-
-
- Brake Mean Effective Pressure (BMEP)
-
- 2-Stroke BMEP = (HP x 6500)/(L x RPM)
- 4-Stroke BMEP = (HP x 13000)/(L x RPM)
-
-
- L = Displacement in Liters
- i.e., 80 cc = .08 Liters
- 1 ci. = 16.39 cc
-
-
- Piston Speed
-
- Cm = .166 x L x N
- Cm is mean piston speed, in feet per minute
- L is stroke, in inches
- N is crankshaft speed, in RPM
-
- Piston Acceleration
-
- Gmax = ((N^2 x L)/2189) x (1 + 1/(2A))
-
- Gmax is maximum piston acceleration, in feet per
second squared
- N is crankshaft speed, in RPM
- L is stroke, in inches
- A is the ratio of connecting rod length, between
centers, to
- stroke
-
-
- Piston Stroke Motion
-
- S = R cos X + L cos Z
- S = the distance piston wrist pin is from center of
crankshaft
- R = the radius of the crankshaft wrist pin
- L = the length of the connecting rod
- X = the angle of the wrist pin
- Z = the angle of the connecting rod
-
- or
-
- sin X = R/L sin Z
-
-
-
- Piston Travel vs. Crank Rotation
-
- d = ((S/2) + L) - (S/2 cos X) - L sin[cos-1 (S/2L
sin X)]
-
- S = Stroke (mm)
- L = Connecting Rod Length (mm)
- X = Crank Angle Before or After TDC (deg)
-
- Note: (L) Rod Length is usually 2 times the (S)
Stroke
-
- OR
- For Spreadsheets and some Calculators
-
- HT = (r + c) - (r cos (a)) - SQRT(c^2 - (r sin
(a))^2)
-
- r = s/2
- dtor = PI/180
- a = d x dtor
-
- HT = The height of piston
- r = The stroke divided by 2
- c = The rod length
- a = The crank angle in radians
- d = The crank angle in degrees
- dtor = Degrees to Radians
-
-
- Port Open Time
-
- T = ( 60/N ) x ( Z/360 ) or T = Z/( N x 6)
- T is time, in seconds
- N is crankshaft speed, in RPM
- Z is port open duration, in degrees
-
-
- Compression Ratio
-
- CR = ( V1 + V2 ) / V2
- CR is compression ratio
- V1 is cylinder volume at exhaust closing
- V2 is combustion chamber volume
-
-
- Exhaust Systems Tuned Length
-
- Lt = (Eo x Vs) / N
- Lt is the tuned length, in inches
- Eo is the exhaust-open period, in degrees
- Vs is wave speed in feet per second (1700 ft/sec at
sea level)
- N is crankshaft speed, in RPM
-
- Length of Curved Pipe
-
- L = R x .01745 x Z
- L is length
- R is radius of the pipe bend
- Z is the angle of the bend
-
- Diffuser Proportions
-
- D2 = SQRT( D1^2 x 6.25 )
- D2 is the diffuser outlet diameter
- D1 is the diffuser inlet diameter
- 6.25 is the outlet/inlet ratio constant
-
- Baffle Cones
-
- Lr = Le/2
- Lr is mean point of the reflection inside the baffle
cone
- Le is the length of the baffle cone
-
- Rick
-
-
-
- Mac, that was some pretty
awesome and thoughtful iinformation you just left. I do
hope you have OCR & scanner and did not have to type
all that in. This is all some pretty cool stuff to think
about. I do agree 100% on the piston traveling its
fastest well before 90 degrees. I would think that when
the angle of the rod is exactly tangent with the crank,
it is at its fastest. And that would happen well before
90 degrees. I have printed out your posts and am going to
have a look at the stuff in more detail.
Again, Thanks for the great
info!
-
- Trax310
-
-
-
- Thanks - this has been a very good thread. I have
enjoyed it a lot. It has made me go places with my
thinking that I haven't had to go for quite a while. I
have those formulas on a FileMaker database so I just
copied and pasted them in there.
-
- Rick
-
-
-
- I thought about what was going
on with the mechanics of the rod and stroke a bit more,
came up with some formulas to describe their motion and
threw them into a spreadsheet. The results follow:
-
- 1) For a given engine speed
(RPM), piston speed varies directly with stroke length
(longer stroke = higher piston speed). Also, the maximum
piston speed varies inversely with rod length (longer rod
= lower max piston speed) yet, average piston speed is
not affected.
-
- 2) Maximum piston speed does
not occur when the crankshaft is 90 deg BTDC as intuition
suggests, rather it is dependent on the ratio of the rod
length to the stroke length/2 and occurs nonlinearly as
follows:
-
- BTDC (deg) rod/stroke
ratio
- 65 1.7
- 70 2.6
- 75 3.8
- 80 6.0
-
- Thus as the ratio of rod length
to stroke length/2 increases, maximum piston speed occurs
closer to 90 deg BTDC. Note that this max occurs at the
same angle ATDC as well but NOT when approaching OR
leaving BDC as one might guess, though I don't know why.
Another interesting phenomenon that I can't explain
occurs around TDC and BDC. It seems that the piston
decelerates/accelerates more quickly as the piston
approaches/leaves TDC than BDC. This also varies with the
rod/stroke ratio.
-
- I was glad I dug into this a
bit more as I discovered for myself some interesting
things about engine mechanics. If anyone can further
enlighten, it would be appreciated. Please note that my
analysis only dealt with speed since the derivative of
the velocity function to yield acceleration was getting
kind of ugly (if you know what I mean). Also, to keep it
simple and within my scope, I assumed a constant engine
speed and neglected crankcase and cylinder pressures as
well as any effects of inertia. The spreadsheet along
with charts is available if anyone is
interested.
-
- Texridr
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