Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: Rota Ash on March 03, 2010, 11:17:13 PM
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Hey lads i have an 02 Rm 250 with 25 hours on it, a Gnarly pipe and a cut down stocko muffler, the bike has decent bottom end and a bucket load of midrange which dies off like its hit a brick wall in the top end, im gonna throw a new ring in it in another 10 hours and am thinking of touching up the port's while it's apart, thing is , is that this is my first 2 stoke and i would like to know how porting affects these things, so do u guys use a degree wheel to measure port timing, do u usually just widen the port an smooth things out, if i smoothe things out and lift the exhaust and inlet ports by a couple of mm will this give me a bit more top end without sacrificing the bottom end it already has, i love the power the bike has just wish it wouldn't hit the brick wall as hard,
any advice would be greatly appreciated ;)
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Before you get into porting, try a different pipe. The Gnarly is meant for bottom end and midrange only and flattens out a lot on top. I have one on my YZ250 for the woods, and a Fatty for the track. I also still have the stocker, it is a compromise between the two.
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Rota Ash, porting is a mixture of science and art.... personally I would not suggest that as a beginner you attempt to change port timing, which you note is suggesting.
I would ensure that your power valve is working correctly and is not all "gummed" up... this could easily cause the problem your experiencing...
And as MXLord327 is suggesting it can also be the pipe... but for my money I'd begin with the power valve...
Let us know how you make out..
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I agree with the other two responses and would add that your "cut down"
silencer will also boost low end at the expense of top end power. If you
look at SX specific silencers, they are all shortys to help provide that big
hit at the bottom. Conversly, an "outdoors" style silencer is normal length
for better top end. If you like FMF, I recommend a Fatty or SST pipe and
a Powercore 2 silencer. The right pipe and silencer combination along with
a good functioning power valve will wake up the already stout RM power
band.
dogger
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what's the best reed valve to get, should i get a new reed valve assembly like a v force, they seem expensive, are they worth it, anyone know any links to test comparisons on reed valves?
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Reed valves, power valves, silencers, who needs them? I don't get why the good old rotary valve didn't catch on. They provided so much power, and the throttle response was better than a reed valve engine (which has a bit of hesitation compared to piston-port or rotary valve). And, it was a spinning piece of metal instead of some bits of fiberglass bending back and forth. Unbreakable! Ever wonder how Kawasaki could make a 100cc flat-tracker that would pull massive wheelies with full-grown men onboard? Rotary valve!
That's my rant for the day.
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what's the best reed valve to get
It comes down to what part of the powerband you want to
enhance. For better low to mid and improved throttle response,
The VForce 3 is the ticket. For better top end, the Rad Valve
is the choice. I have used both over the years and now use
the VForce exclusively for 250 cc and up and the Rad Valve
on 125s.
dogger
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Rad Valve pinner to winner don't send your money to Italy keep it here!Boysen
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I copied this article from TSR. It's not on their website anymore, so dont bother looking for it. This is for a 92-95 RM250, it might work for newer RM's if the cylinders are similar.
TWO-STROKE TUNING WITH COMPUTERS
BY ERIC GORR
The design evolution of two-stroke engines was greatly accelerated by the
proliferation of computers. Tom Turner is one of the most experienced two-stroke
tuners in America. He markets his own line of design software. Some of the programs
are based on Dr. Blair's work (but "Improved Greatly") but most are TSR original designs. Turner
receives feedback from other tuners who use his programs for applications ranging from mx,
road racing, watercraft, ATVs, and snowmobile racing. He uses this feedback to
continually fine tune the programming code for better engine performance.
I use Turner's Programs everyday in my tuning business. Here is a case study of how I used
computer design software to improve the performance of a Suzuki RM250 (1992-95)
ANALYZING THE STOCK ENGINE
The design of the cylinder head, port timing, and pipe must be coordinated to
peak simultaneously in the RPM band. There is a proper order to the design
process. First I start by measuring the engine specifications and inputting
the dimensions into the TSR Computer Programs, to look for shortcomings in design.
The cylinder's exhaust port time area peaks at a relatively low 7,200 rpm. The
transfer port time area is just the opposite. The ports are very wide and have
high time area and a high rpm peak. However they don't start to flow properly
until about 5,000 rpm. The problem with the RM250 is that the exhaust port is
too small and the transfers are too big. That is why the stock engine hits hard
in the mid-range, because the exhaust and transfer ports come into sync right
when the exhaust valves open. However the transfers are too large to flow
adequately at low rpm to give the engine tractable power and the exhaust port
is too small to give the engine any over rev. This is an easy problem to fix.
REDESIGNING FROM THE INSIDE OUT
This is the proper order for redesigning the engine specs; exhaust port time-
area, blow down time-area, transfer port time-area, cylinder head, reed valve,
and pipe.
EXHAUST PORT
The chordal widths of the exhaust and transfer ports, and
the port opening timing were input into Turner's PORT2000 program to determine
the rpm band width of the stock ports. Time-area is a calculation
based on the width of the exhaust port, the point when the port opens, and the
total port open duration in relation to RPM. It is a very difficult
calculation to do on a hand-held calculator but the computer performs it fast,
accurate, and easy. The program enables the tuner to adjust the exhaust time-
area for the power band we are tuning for. On the RM250 the exhaust port's
timearea was too low and the exhaust opening timing and width had to be increased
to allow the engine to peak at 8,800 rpm.
BLOW-DOWN TIMING
With the exhaust port in the correct position, we
need to set the opening time of the transfer ports and time-area so the cylinder has
adequate blow-down time area. The blow-down phase occurs from when the
exhaust port opens to when the transfer ports open. Too little blow-down will
cause burnt mixture gasses to flow into the crankcase when the transfers open and
pollute the un burnt gasses. Turner's PORT2000 program has an
automatic "FINDIT" feature that is used to determine the optimum blow-time-area
of the blow-down phase. FINDIT tells you the distance from TDC where the transfer
ports should open. Now that you have the opening timing of the transfers, you
can use the PORT2000 program to find the optimum width dimensions of the
transfer ports. The RM250 transfers were way too wide and the original
transfers didn't peak until 10,000 rpm. I narrowed the widths of the rear
transfer ports with epoxy in order to reduce the transfer port time-area to
peak around 8,800 rpm.
CYLINDER HEAD
Once you have determined the optimum opening timing for the
exhaust port, you will have the effective stroke length. That is the distance
from TDC to when the exhaust port opens. That is the key to designing the
combustion chamber because it is linked to the compression ratio. Turner's
COMPRESS program enables you to design the combustion chamber with variables
http://tsrsoftware.com/compress.htm (http://tsrsoftware.com/compress.htm) such as; outside air density or altitude, peak rpm,
fuel octane rating, squish band area, and compression ratio versus bore size.
The output of this program tells you the trapped volume and the squish band width
so you can cut the head to specifications on a lathe. The SQUISH program enables you to
calculate the Maximum Squish Velocity (MSV) of turbulence in the combustion
chamber. This is an important design tool for finely targeting the "hit" in
the power band of mx bikes. I have experimented with this program extensively
and determined a link between power band and fuel. For example, a high rpm
road race engine requires a hemi-shaped combustion chamber with a narrow
squish band and unleaded fuel of about 104 octane. The MSV of this head design
will be about 13 meters per second (m/s). That contrasts greatly with a
super cross engine which demands an explosive mid-range power band and an MSV
rating of 23 m/s. Two-stroke engines that burn methanol require even higher
squish velocity because methanol is a relatively slow burning fuel.
EXPANSION CHAMBERS:
Turner's NEWPIPE program is a combination of an
expert system with look-up tables for specified applications and a mathematical
calculation program. These features enable you to specify certain variables like; the
type of induction system, engines with exhaust power valve systems, exhaust gas
temperature, metal thickness, the width of the tuned rpm band, port timing, port
diameter, etc. The program draws the pipe on a graphic screen, prints the dimensions
of the pipe, and a companion program (LAYOUT) prints the dimensions of the
individual cones. This feature makes it easy for a custom pipe builder o fabricate
a pipe. A third program that is part of NEWPIPE package (SCRNCONE) will
quickly give you cone angles off of any pipe you can measure lengths and
diameters. A forth program (VELOCITY) does a study of the exhaust port
diameter to rpm, to see if port is sized right to do what you want. The NEWPIPE program
is very accurate and corresponds closely to actual inertia-dyno tests. To date this
program has made several large company's LOT$ of money!
REED VALVES
Run REEDVALV.exe and test the frequency range of
reed valves and petals. The effective area of the reed valve ports, the petal
material and thickness determine what RPM range that the reed valve will be
best tuned for. The 1993 RM250 reed valve has a 30 degree angle which helps
low end power but the reed material (fiberglass) is heavy and flutters at about
6,000 rpm. Carbon fiber reeds are lighter and stiffer so they resist
fluttering. The program allows us to quickly check the affect of different reed
materials. Often times the motorcycle manufacturers design the reed valves for
the wrong range. For example, in 1993 I computer-designed an engine for top
American enduro rider Kevin Hines. The 1993 Husky 250cc WR has a giant reed
valve with carbon fiber petals. Kevin wanted the engine to peak at 7,000 RPM
with a "four-stroke" type power band. The stock reed valve has a frequency
range of between 8,000 to 13,000 RPM. Using the computer program, I determined
that I needed to fill epoxy in the outside corners of the reed valve to reduce
the total area and install #615 Boyesen Dual-Stage reeds from a 1982 RM500
Suzuki. This simple mod dramatically improved the low to mid-range power to
make the engine more suitable for enduro riding. Yamaha uses a common reed
valve for the 125 and 250 engines. The reed valve is actually designed to work
on the 125. When installed on the 250 cylinder, the reed stop plates block-off
the rear transfer port. Installing the Boyesen RAD valve on the YZ250 yields more
top end power because it is properly designed for the requirements of the engine.
Plus RAD valves don't use reed stops so the crankcase transfer ports aren't blocked.
CAPTIONS FOR PHOTOS AND DRAWINGS in Erics Report (you can see pictures on Eric Gorrâ??s Web site).
1) This is a calculation print-out of the stock RM250 exhaust port's time-area.
The port is maxed out at about 7,000 rpm where the BMEP reaches 150. This port
must be raised and widened in order to push the rpm peak to 8,800 rpm.
2) This is a calculation of the transfer port time-area for the stock RM250. The
TSR programs enable a tuner to examine the effective rpm range of the
transfers. The stock ports are too wide causing the transfers to peak at 10,000
rpm. A 250 mx or enduro bike needs to pull from about 3,000 rpm and peak around
8,500 rpm. The rear transfer ports were narrowed using epoxy to shift the rpm
range of the transfers lower in the rev range.
3) This is an example of TSR's COMPRESS program, used for designing cylinder
heads. This program gives helpful design information that enables a tuner to
adjust a cylinder head for any application, even for air density/altitude. This
program saves time in trial and error machining and testing.
4) This is an example of TSR's NEWPIPE program. It enables tuners to design
expansion chambers that correspond closely to changes in other engine
components like a tuned cylinder. The program even calculates the dimensions of
individual cones of the pipe. ALL New â??NEWPIPE software in 2002â??.
5) This is a drawing of the modified cylinder porting specs for a RM250.
6) The computer programs require precise measurements of engine components. Here
an inside divider is used to measure the chordal width of the exhaust port.
7) This is an example of a measurement of the transfer ports.
8) A burette tube is used to measure the volume of the cylinder head for use in
designing the cylinder head.
9) Jesse Williams of MOTOWERX in America demonstrates how a pipe is fabricated and
fitted to the bike. Jesse offers a custom pipe building service using a
combination of TSR SOFTWARE Programs and an inertia dynamometer.
NOTE: TSR Software is continually being improved so check with TSR for latest Software.
Eric Gorr --- E-mail:...info@eric-gorr.com
Sorry for being so long. The article as you can see has a lot to do with TSR software, but I hope it helps. :D
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G-Money, I don't think you have your facts right about the V-Force, Moto Tassinari is not from Italy, their shop is in Lebanon, NH, right on the Vermont line, and from what I have heard, they are great people!! I love my V-Force 3's in my YZ250!!!
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V force 2 works best for high rpm all thoseGP 125's can't be wrong.
TSR software is OK but it doesn't look at the engine dynamicaly, Port 2000 is a good place to start, but by no means has all the answer.For instance it doesn't take into account the ignition and what changes in the curve do.
To answer the question, there is no short answer, no quick fix and no short cut.Everything has to work in harmony. There are a few things you coud try first would be to retard the ignition slightly and then chop 10mm from the header of the pipe.
Yes the mighty disc valve is the go although reed technology has caught up with discs lately
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What I've done to my 01,02 and 05 RM's was machine 0.5 mm off the head and use 2 base gaskets which changes the port timing and compression ratio for better top end. Best thing is it's cheap and easy to do.