Basic two-stroke tuning

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BASIC TWO-STROKE TUNING
Changing the power band ofyour dirt bike engine is simple when you know the basics. A myriad of differentaftermarket accessories is available for you to custom tune your bike to bettersuit your needs. The most common mistake is to choose the wrong combination ofengine components, making the engine run worse than stock. Use this as a guideto inform yourself on how changes in engine components can alter the powerbandof bike's engine. Use the guide at the end of the chapter to map out yourstrategy for changing engine components to create the perfect power band.

TWO-STROKE PRINCIPLES
Although a two-strokeengine has less moving parts than a four-stroke engine, a two-stroke is acomplex engine because it relies on gas dynamics. There are different phasestaking place in the crankcase and in the cylinder bore at the same time. Thatis how a two-stroke engine completes a power cycle in only 360 degrees ofcrankshaft rotation compared to a four-stroke engine which requires 720 degreesof crankshaft rotation to complete one power cycle. These four drawings give anexplanation of how a two-stroke engine works.
1) Starting with thepiston at top dead center (TDC 0 degrees) ignition has occurred and the gassesin the combustion chamber are expanding and pushing down the piston. Thispressurizes the crankcase causing the reed valve to close. At about 90 degreesafter TDC the exhaust port opens ending the power stroke. A pressure wave ofhot expanding gasses flows down the exhaust pipe. The blow-down phase hasstarted and will end when the transfer ports open. The pressure in the cylindermust blow-down to below the pressure in the crankcase in order for the unburnedmixture gasses to flow out the transfer ports during the scavenging phase.
2) Now the transfer portsare uncovered at about 120 degrees after TDC. The scavenging phase has begun.Meaning that the unburned mixture gasses are flowing out of the transfers andmerging together to form a loop. The gasses travel up the back side of the cylinderand loops around in the cylinder head to scavenge out the burnt mixture gassesfrom the previous power stroke. It is critical that the burnt gasses arescavenged from the combustion chamber, in order to make room for as muchunburned gasses as possible. That is the key to making more power in atwo-stroke engine. The more unburned gasses you can squeeze into the combustionchamber, the more the engine will produce. Now the loop of unburned mixturegasses have traveled into the exhaust pipe's header section. The gasses aren'tlost because a compression pressure wave has reflected from the end of theexhaust pipe, to pack the unburned gasses back into the cylinder before thepiston closes off the port. This is the unique super-charging effect of two-strokeengines. The main advantage of two-stroke engines is that they can combust morevolume of fuel/air mixture than the swept volume of the engine. Example: A125cc four-stroke engine combusts about 110cc of F/A gasses but a 125cctwo-stroke engine combusts about 180cc of F/A gasses.
3) Now the crankshaft hasrotated past bottom dead center (BDC 180 degrees) and the piston is on theupstroke. The compression wave reflected from the exhaust pipe is packing theunburned gasses back in through the exhaust port as the piston closes off theport the start the compression phase. In the crankcase the pressure is belowatmospheric producing a vacuum and a fresh charge of unburned mixture gasses isflowing through the reed valve into the crankcase.
4) The unburned mixturegasses are compresses and just before the piston reaches TDC, the ignitionsystem discharges a spark causing the gasses to ignite and start the processall over again.

CYLINDER PORTING
The cylinder ports are designed to produce acertain power characteristic over a fairly narrow rpm band. Porting or tuningis a metal machining process performed to the cylinder ports (exhaust &transfers) that alters the timing, area size, and angles of the ports in order toadjust the power band to better suit the rider's demands. For example, aveteran trail rider riding an RM250 in the Rocky mountain region of the USAwill need to adjust the power band for more low end power because of the steephill climbs and the lower air density of higher altitudes. The only way todetermine what changes will be needed to the engine is by measuring andcalculating the stock engine's specifications. The most critical measurement istermed port-time-area. This term is a calculation of a port's size area andtiming in relation to the displacement of the engine and the rpm. Experiencedtuners know what the port-time-area values of the exhaust and transfer portsshould be for an engine used for a particular purpose. In general, if a tunerwants to adjust the engine's power band for more low to mid range he would dothe following things. Turn down the cylinder base on a lathe to increase theeffective stroke (distance from TDC to exhaust port opening). This also retardsthe exhaust port timing and shortens the duration and increases the compressionratio. Next the transfer ports should be narrowed and re-angled with epoxy toreduce the port-time-area for an rpm peak of 7,000 rpm. The rear transfer portsneed to be re-angled so they oppose each other rather than pointing forward tothe exhaust port. This changes the loop scavenging flow pattern of the transferports to improve scavenging efficiency at low to mid rpm (2,000 to 5,000 rpm).An expert rider racing mx in England would want to adjust the power band of anRM250 for more mid to top end power. The cylinder would need to be tunedradically different than for trail riding.
Here is an example. Theexhaust port would have to be raised and widened to change the port-time-areapeak for a higher rpm (9,000 rpm). For either of these cylinder modificationsto be effective, other engine components would also need to be changed to getthe desired tuning effect.

CYLINDER HEAD
Cylinder heads can bereshaped to change the power band. Generally speaking, a cylinder head with asmall diameter and deep combustion chamber, and a wide squish band (60% of thebore area). Combined with a compression ratio of 9 to 1 is ideally suited forlow to mid range power. A cylinder head with a wide shallow chamber and anarrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, isideally suited for high rpm power.
There are many reasons whya particular head design works for certain types of racing. For example; a headwith a wide squish band and a high compression ratio will generate highturbulence in the combustion chamber. This turbulence is termed Maximum SquishVelocity, MSV is rated in meters per second (m/s). A cylinder head designed forsupercross should have an MSV rating of 28m/s. Computer design software is usedto calculate the MSV for head designs. In the model tuning tips chapters ofthis book, all the head specs quoted have MSV ratings designed for the intendedpower band changes.

CRANKSHAFT
There are two popular modshop-up companies are doing to crankshafts; stroking and turbo-vaning. Strokingmeans to increase the distance from the crank center to the big end pin center.There are two techniques for stroking crankshafts; weld old hole and re-drill anew big end pin hole, or by installing an off-set big end pin. The method ofweld and re-drilling is labor intensive. The off-set pin system is cheap,non-permanent, and can be changed quickly. In general, increasing the stroke ofa crankshaft boosts the mid range power but decreases the engine's rpm peak.
The term"Turbo-Crank" refers to a modification to the crankshaft of atwo-stroke engine, whereby scoops are fastened to the crank in order to improvethe volumetric efficiency of the engine. Every decade some hop-up shop revivesthis old idea and gives it a trendy name with product promises that it can'tlive up to. These crank modifications cause oil to be directed away from theconnecting rod and often times the vanes will detach from the crank at highrpm, causing catastrophic engine damage. My advice, don't waste the $750!

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CARBURETOR
In general a smalldiameter carburetor will have high velocity and a good flow characteristic fora low to mid rpm power band. A large diameter carburetor works better for highrpm power bands. For 125 cc engines a 34mm carburetor works well for supercrossand enduro and a 36 or 338 mm carburetor works best for fast mx tracks. For 250cc engines a 36 mm carburetor works best for low to mid power bands and a 39.5mm carburetor works best for top end power bands. Recently there has been atrend in the use of air-foils and rifle-boring for carbs. These innovations aredesigned to improve air flow at low throttle openings. Some companies sell carbinserts, to change the diameter of a carb. Typically a set of inserts is soldwith a service of over boring the carb. For example; a carb for a 250cc bike(38mm) will be bored to 39.5mm and two inserts will be supplied. The carb canthen be restricted to a diameter of 36 or 38mm.

REED VALVE
Think of a reed valve likea carburetor, bigger valves with large flow-areas work best for high rpm powerbands. In general, reed valves with six or more petals are used for high rpmengines. Reed valves with four petals are used for dirt bikes that need stronglow end and mid range power. There are three other factors to consider whenchoosing a reed valve. The angle of the reed valve, the type of reed material,and the petal thickness. The two common reed valve angles are 30 and 45degrees. A 30-degree valve is designed for low to mid rpm and a 45 degree valveis designed for high rpm. There are two types of reed petal materials commonlyused, carbon fiber and fiberglass. Carbon fiber reeds are lightweight butrelatively stiff (spring tension) and designed to resist fluttering at highrpm. Fiberglass reeds have relatively low spring tension so they instantlyrespond to pressure that changes in the crankcase, however the low springtension makes them flutter at high rpm thereby limiting the amount of power.Fiberglass reed petals are good for low to mid power bands and carbon fiber reedsare better for high rpm engines.
Boyesen Dual Stage reedshave a large thick base reed with a smaller thinner reed mounted on top. Thissetup widens the rpm range where the reed valve flows best. The thin reedsrespond to low rpm and low frequency pressure pulses. The thick reeds respondto higher-pressure pulses and resist fluttering at high rpm. A Boyesen RADvalve is different than a traditional reed valve. Bikes with single rear shockshave off-set carbs. The RAD valve is designed to redistribute the gas flow tothe crankcases evenly. A RAD valve will give an overall improvement to thepower band. Polini of Italy makes a reed valve called the Supervalve. Itfeatures several mini sets of reeds positioned vertically instead ofhorizontally like conventional reed valves. These valves are excellent forenduro riding because of improved throttle response. In tests on an inertiachassis dyno show the Supervalve to be superior when power shifting. Howeverthese valves don't generate greater peak power than conventional reed valves.Supervalves are imported to America and sold by Moto Italia in Maine.

EXHAUST PIPE
The exhaust pipe of atwo-stroke engine attempts to harness the energy of the pressure waves fromcombustion. The diameter and length of the five main sections of a pipe, arecritical to producing the desired power band. The five sections of the pipe arethe head pipe, diffuser cone, dwell, baffle cone, and the stinger. In general,after market exhaust pipes shift the power band up the rpm scale. Most pipesare designed for original cylinders not tuned cylinders. Companies likeMOTOWERKS custom computer design and fabricate pipes based on the cylinderspecifications and the type of power band targeted.

SILENCER
Silencers come in allsorts of shapes and sizes. A long silencer with a small diameter enhance thelow to mid power because it increases the bleed-down pressure in the pipe. Asilencer with a short length and a large core diameter provides the bestbleed-down pressure for a high rpm engine. Too much pressure in the pipe athigh rpm will radically increase the temperature of the piston crown and couldcause the piston to seize in the cylinder.

FLYWHEEL WEIGHTS
The flywheel is weightedto improve the engine's tractability at low to mid rpms. There are twodifferent types of flywheel weights, weld-on and thread-on. A-Loop performs theweld-on flywheel weight service. Steahly makes thread-on flywheel weights. Thisproduct threads onto the fine left-hand threads that are on the center hub ofmost Japanese magneto rotors. normally the threads are used for the flywheelremover tool. Thread-on flywheel weights can only be used if the threads on theflywheel are in perfect condition. The advantage to weld-on weights is theycan't possibly come off.
External rotor flywheels have a larger diameterthan internal rotor flywheels so they have greater flywheel inertia. Internalrotor flywheels give quicker throttle response

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AFFECTS OF THE IGNITIONTIMING
Here is how changes in thestatic ignition timing affects the power band of a Japanese dirt bike.Advancing the timing will make the power band hit harder in the mid range butfall flat on top end. Advancing the timing gives the flame front in thecombustion chamber, adequate time to travel across the chamber to form a greatpressure rise. The rapid pressure rise contributes to a power band's"Hit". In some cases the pressure rise can be so great that it causesan audible pinging noise from the engine. As the engine rpm increases, thepressure in the cylinder becomes so great that pumping losses occur to thepiston. That is why engines with too much spark advance or too high of acompression ratio, run flat at high rpm.
Retarding the timing willmake the power band smoother in the mid-range and give more top end over rev.When the spark fires closer to TDC, the pressure rise in the cylinder isn't asgreat. The emphasis is on gaining more degrees of retard at high rpm. Thiscauses a shift of the heat from the cylinder to the pipe. This can prevent thepiston from melting at high rpm, but the biggest benefit is how the heataffects the tuning in the pipe. When the temperature rises, the velocity of thewaves in the pipe increases. At high rpm this can cause a closersynchronization between the returning compression wave and the piston speed.This effectively extends the rpm peak of the pipe.

HOW TO ADJUST THE TIMING
Rotating the stator platerelative to the crankcases changes the timing. Most manufacturers stamp thestator plate with three marks, near the plate's mounting holes. The center markis the standard timing. If you loosen the plate mounting bolts and rotate thestator plate clockwise to the flywheel's rotation, that will advance theignition timing. If you rotate the stator plate counterclockwise to theflywheel's rotation, that will retard the ignition timing. Never rotate thestator plate more than .028in/.7mm past the original standard timing mark.Kawasaki and Yamaha stator plates are marked. Honda stators have a sheet metalplate riveted to one of the mount holes. This plate insures that the stator canonly be installed in one position. If you want to adjust the ignition timing ona Honda CR, you'll have to file the sheet metal plate, with a 1/4in rat-tailfile.

AFTERMARKET IGNITIONS
The latest innovation inignition systems is an internal rotor with bolt-on discs that function asflywheel weights. PVL of Germany makes these ignitions for modern Japanese dirtbikes. Another advantage to the PVL ignition is that they make a variety ofdisc weights so you can tune the flywheel inertia to suit racetrack conditions.
MSD is an aftermarket ignition componentmanufacturer. They are making ignition systems for CR and RM 125 and 250. MSD'signition system features the ability to control the number of degrees ofadvance and retard. These aftermarket ignition systems sell for less than theOEM equivalent

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TIPS FOR BIG BORINGCYLINDERS
In the mid nineties,European electro-plating companies started service centers in America. Thismade it possible to over bore cylinders and electro-plate them to precisetolerances. This process is used by tuners to push an engine's displacement tothe limit of the racing class rules, or make the engine legal for a differentclass.
When you change thedisplacement of the cylinder, there are so many factors to consider. Factorslike; port-time-area, compression ratio, exhaust valves, carb jetting,silencer, and ignition timing. Here is an explanation of what you need to dowhen planning to over bore a cylinder.
Port-Time-Area - This isthe size and opening timing of the exhaust and intake ports, versus the size ofthe cylinder and the rpm. When increasing the displacement of the cylinder, thecylinder has to be bored to a larger diameter. The ports enter the cylinder atangles of approximately 15 degrees. When the cylinder is bore is made larger,the transfer ports drop in height and retard the timing and duration of thoseports. The exhaust port gets narrower. If you just over bored and plated acylinder, it would have much more low end power than stock. Normally tunershave to adjust the ports to suit the demands of the larger engine displacement.Those exact dimension changes can be determined with TSR's Time-Area computerprogram.
Cylinder Head - The head'sdimensions must be changed to suit the larger piston. The bore must be enlargedto the finished bore size. Then the squish band deck height must be set to theproper installed squish clearance. The larger bore size will increase thesquish turbulence so the head's squish band may have to be narrowed. The volumeof the head must be increased to suit the change in cylinder displacement.Otherwise the engine will run flat at high rpm or ping in the mid range fromdetonation.
Exhaust Valves - When thebore size is increased, the exhaust valve to piston clearance must be checkedand adjusted. This pertains to the types of exhaust valves that operate withinclose proximity of the piston. If the exhaust valves aren't modified, thepiston could strike the valves and cause serious engine damage.
Carb - The piston diameterand carb bore diameter are closely related. The larger the ratio between thepiston size and the carb size, the higher the intake velocity. That makes thejetting richer. Figure on leaning the jetting after an engine is over bored.
Ignition Timing - Thetiming can be retarded to improve the over rev. Normally over bored enginestend to run flat on top end.
Pipe and Silencer -Because only the bore size is changed, you won't need a longer pipe only onewith a larger center section. FMF's line of Fatty pipes work great on engineswith larger displacement. Some riders use silencers that are shorter withlarger outlets to adjust the back-pressure in the pipe for the larger enginedisplacement.

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Compression Ratio.
Most people like to see the compression ratio pushed up ashigh as possible. High compression has always equated with high horsepower. Iagree that the compression ratio should be made as high as practical, but oftenthe manufacturer has already found the limit and built the engine accordingly.All you can do in this instance is check that production tolerances have notlowered the ratio significantly below that which the manufacturer intended.
Something you must always remember when dealing with2-stroke engines is that increasing the compression ratio will not give a powergame equivalent to that which you would pick up in a 4-stroke engine. Heat isthe enemy of 2-stroke engines and stitching the compression ratio to give a 10%power increase will possibly results in only a 3 percent rise at the most; therest will be lost in heat energy and pumping losses. However, at lower enginespeeds the cylinder will not be completely with fuel / air mixture and thepower may jump by 5-6% because there is not such a heat loss. This is, in fact,the real benefit of raising the compression ratio, not to increase maximumpower but to pick up mid-range power and possibly widen the power band.
Because so much confusion exists in the motorcycle industryrelating to compression ratio we need to define exactly what is meant bydetermine compression ratio. compression ratio is defined as the ratio of thevacuum of the cylinder with the piston at TDC to the volume after the cylinderwith the piston at BDC.
CR = Compression Ratio
CV = Cylinder Volume
CCV = Combustion Chamber Volume
CR = CV + CCV
..............CCV
CV is found with the formula:
Pi.D(2) X S 4000
where Pi = 3.1416
D = bore diameter in mm
S = Stroke
Example of a CV: bore = 51.5mm
stroke = 60mm
CR = 14:1
Thus: CV = Pi.(D)2 X S = 124.98cc
......................4000
Measured with a burette CCV is found to be 9.8cc
Thus CR = CV + CCV = 124.98 + 9.8 = 13.75:1
......................CCV.................9.8
Therefore the engine has a compression ratio just a bitlower than that specified.
If we wanted a 15:1 ratio we would need to adjust thechamber volume. To find the required chamber volume that we would need , we usethe formula:
CCV = CV___
............ CR - 1
= 124.98 = 124.98 = 8.93cc
.....15 - 1 ........14
Thus the volume must be reduced from 9.8cc to 8.93 = 0.97ccreduction. We skim the head to reduce the volume and find the displacementcalculation of:
S = CV X 4000
............Pi.(D)2
= 0.87 X 4000 = 0.42mm
....Pi X (51.5)2