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Measuring and adjusting compression ratios on two strokes is not as simple as just reading a compression gauge. There are
two ways of stating compression; corrected, which is measured when the exhaust port closes and equates to what you would see
on a gauge, and uncorrected, which is a measurement using the full piston stroke, like on a four stroke. It is important to
understand what each one means, and the differences between them. Published numbers over 10:1 will almost certainly be uncorrected,
and numbers in the range of 6:1 to 9.5:1 are corrected. Typically, Japanese motors use corrected numbers, where European motors
use uncorrected.
A common mistake is to set compression after just taking a reading on a compression gauge. This is not recommended, for
a variety of reasons. The only accurate way is to get a measurement of the volume of the combustion chamber with the piston
at TDC, and then calculate both the corrected and uncorrected numbers from measurement of bore, stroke, and exhaust port height.
Squish clearance should also be measured.
Imagine two engines with the same bore, stroke and head volume, but one has a higher exhaust port.Both will have exactly
the same uncorrected compression, but the one with the higher exhaust port will have a lower corrected compression and lower
gauge reading. If one were to raise the compression to make them both read the same on a gauge, the uncorrected number could
rise to a value that is unsafe. The motor cares most about the uncorrected number, as it is this value that determines fuel
octane required and is used for engineering calculations of horsepower and torque. The reason for this is that with a tuned
pipe, the returning pressure wave can push the cylinder pressure to well above atmospheric before the exhaust port closes,
which means the running compression is higher than the corrected number or gauge reading would suggest.
People often to want to raise their compression based on what their gauge reads. This can be a big (and costly) mistake
! I have seen many instances where someone reads 160psi on their gauge, and wants to raise it to 200psi. But in measuring
the actual volume, it has compression that should show 210psi without any change. If the compression were raised from this
point, it would almost certainly blow up ! Gauges can give inaccurate readings, rings may be sealing poorly, cylinder walls
may be damaged, and cylinders with powervalves can give erratic readings.
Squish clearance is another important factor in head design. This is the clearance between the piston and the squish band
of the head when the piston is at TDC. It has a strong influence on turbulence in the combustion chamber, which directly affects
how fast the mixture burns. The old way of setting squish was to just make sure the piston didn't hit the head when running,
but this is not the way it should be done. Like most engineering criteria, there is a correct range that needs to be targeted.
Too much clearance, and the burning is slow and power is low; too little, and burning is too fast and puts extreme loads on
the motor, similar to what detonation does. A correct value of maximum squish velocity, or MSV, will ensure good power output
within safe limits. I use a computer program to calculate these values and ensure they are in the correct range. The factors
that influence the MSV are RPMs, stroke, rod length, squish band width, squish clearance, and compression ratios. Only when
all these factors are considered and adjusted can a head be considered well designed.
Compression is also important to consider when designing ports. Years ago, porting was targeted just at timing numbers
for a desired RPM; if you wanted 10,000rpm, you needed 190 degrees of exhaust duration. Then a more accurate method became
known, using port time-area values. These calculations suggested a target area range for ports to feed the motor at the desired
RPMs, and was a big improvement. Further developments, and what is used now, uses the time-area values, but targets desired
power output levels based on BMEP, or brake mean effective pressure. What this means, is that the motor needs more port if
the compression is higher, even if the RPM's are not changed. So if you raise the compression on an engine and don't consider
the port design, you may actually lose RPM capability. Conversely, if you have an engine with a lot of exhaust, it may respond
well to a higher compression...as long as all the rest of the variables remain safe.
When I plan an engine modification program, I start with selecting a compression target that will be appropriate for the
desired output of the engine, the RPM range, the fuel to be used, the type of use, and the quality of parts used. When this
is established, I can then design the ports to support these same criteria. In this way, all variables of engine performance
are designed to work together, at the desired level. This produces engines which combine optimum power, broad powerband, ease
of tuning, and reliability.
RD/RZ/Banshee case assembly
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As always, you should refer to your manual for specifics of your model, this information is suggested as a supplement to a
full manual. Parts diagrams are available at Yamaha's website, don't hestitate to look up your model to make sure you have
all the parts required in position.
The cases shown are worn Banshee parts used for demonstration.
Prior to assembly, thoroughly clean cases of any oil, dirt, or grease, and scrape the mating surfaces and gasket surfaces
with a razor blade or gasket scraper. A wire brush in a drill works well to remove case sealer and gasket particles. Inspect
all threaded holes and studs, and make sure both front and rear locating dowels are in place, as well as bearing locating
clips.
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Place a few drops of 2-stroke oil on the seal lips, and place them on the crank and transmission shafts. The drive side crank
seal should have the castellated nibs facing the bearing, the ignition side seal may have "outside" marked on it,
but if it does not it can go on either way. The output and clutch pushrod seals should have the flat surfaces facing out.
The crank bearings should be left dry at this point to keep the case sealing surfaces clean, they will be lubed after assembly.
Coat the lower case half with case sealer. I prefer 3-Bond 1211 white silicone, Yamabond or 3-Bond 1104 is also acceptable.
Place 1 or 2 drops of red or green Locktite or equivalent on the bearing surface of the lower case for each crank bearing
and the countershaft bearing, then spread it evenly across the bearing seating surface.
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Install the transmission in the case, making sure to align the shift forks with the corresponding gears. Observe the locating
pins on the crankshaft bearings, and position them all in a line. Lay the crank in the case half, with the locating pins in
their corresponding indentations in the case, and the seal ribs in the grooves in the case. Tap the crank down tight with
a plastic hammer or wood block, checking to make sure the pins are properly located and the bearings and seal seated.
Place the top case half on the lower half, and install the upper case bolts. Tighten them just enough to pull the case
halves together, then stand the case up on it's rear end and install the lower nuts. Tighten them in sequence, from inside
to outside, in 2 or 3 steps until you reach a final torque of 18ft/lbs. Then tighten the top bolts to 9 ft/lbs or hand tight.
Make sure the crank and transmission shafts turn freely, and the trans shafts should have about 1mm of end play until the
clutch and counter sprockets are installed.
Just prior to installing the pistons and cylinders, place a couple drops of 2-stroke oil on the rod oil slots and thrust
washers, and a few drops down each main bearing lube hole in the transfer cavity. Also oil piston pins and top end bearings
prior to installation. I put just a drop of oil on the piston skirts and wipe it over the surface.
Use Locktite on screws for the clutch bearing retainer. Install kick idler gear and kickstarter, making sure the spring
tab is inserted into it's slot in the case. I use a very thin film of 1211 silicone on the flat surface of the primary drive
gear, to make sure it seals against the crank bearing. Install the clutch washer, sleeve, basket, inner washer, and hub, followed
by the clutch plates. On steel plates with a cutout on the outside, stagger them so they are more or less evenly spaced around
the clutch assembly so the clutch remains balanced. (The cutouts are to allow the plates to sling out on the heavy side so
they don't vibrate against the hub). With all the plates installed, and Locktite on the threads, it is easy to hold the clutch
assembly by hand so you can tighten the crank and clutch nuts with an air or electric impact. Be sure to index the clutch
pressure plate so it engages the splines of the hub, and use only hand tight on the spring screws, never an impact.
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© Copyright 2005, Scott Clough
See also; Study on squish effect, by Neels van Niekerk
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