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Displacement cm3
49
RPM to analyze
7000
Exhaust duration °
150
Ideal 150-175° (lower gives more range, higher gives more peak)
Exhaust height mm
14
Note: Only the height uncovered at BDC. Not always the same as port height
Exhaust width mm
25
Stay below 70% of cylinder diameter
Exhaust square to mean ratio
0.785
Rectangle = 1, Oval = 0.785
Exhaust mean area cm2
2.75
Exhaust half open area multiple
0.70
Part of area when port is half open (in degrees from open to BDC).
Exhaust half open area cm2
1.92
Exhaust Angle area
5.88
Exhaust TAm
0.13994
Needs to be > 0.145
Port open time (ms)
3.57
Exhaust TAm restriction @RPM
0.97
TAm @ rpm / 0.145 (needs to be >1 in order to not restrict at current rpm)
Exhaust most useful RPM (low)
6533
Exhaust most useful RPM (high)
7000
Transfer duration °
105
Ideal 115-120°
Blow down °
22.5
Ideal 20-25° (lower gives more range, higher gives more peak)
Blow down %
21.43
Ideal 17-19%
Transfer height mm
8
Transfer width mm
55
Transfer square to mean multiple
0.95
Rectangle = 1, Oval = 0.785
Transfer area cm2
4.18
Transfer half open area multiple
0.75
Part of area when port is half open (in degrees from open to BDC).
Transfer half open area cm2
3.13
Transfer Angle area
6.71
Transfer TAm
0.15969
Needs to be > 0.09
Transfer TAm restriction @RPM
1.77
TAm @ rpm / 0.15 (needs to be >1 in order to not restrict at current rpm)
Transfer most useful RPM (low)
11183
Transfer most useful RPM (high)
13979
Intake durations °
138
Ideal 130-150 (>140 will decrease torque under 4,000rpm)
Intake height mm
13
Intake width mm
27
Intake square to mean multiple
0.9
Rectangle = 1, Oval = 0.785
Intake mean area cm2
3.16
Intake half open area multiple
0.65
Area to be used is when port is half open (in degrees from open to TDC).
Intake half open area cm2
2.05
Intake Angle area
5.77
Intake TAm
0.13746
Needs to be > 0.15 for piston port intake
Intake TAm restriction @RPM
0.92
TAm @ rpm / 0.15 (needs to be >1 in order to not restrict at current rpm)
Intake most useful RPM (low)
6010
Intake most useful RPM (high)
6869
Carburetor min size @RPM (mm)
13.89
Min size of carburetor in order to produce power at RPM analyzed.
Rim size (inches)
16
Tire height (inches)
2.25
Gear ratio (crank turns per 1 sprocket turn)
5.05
Front sprocket
17
Back sprocket
45
Speed (km/h)
51.39
Speed (mph)
31.94
Glossary:

RPM: Revolutions Per Minute, i.e. the number of turns the engine will do in a minute.
Duration: Number of degrees which a port is open, i.e. if a port opens at 100° (after TDC) and close at 260° the duration is 160° (for intake you will have to use BDC as reference)
TDC/BDC: Top Dead Center (piston top position), Bottom Dead Center (piston bottom position)
Displacement: The “CC” (cm3) of your engine, (½bore * PI * stroke)
Mean half open area: The mean area of a port (in cm2) when it is “half open” (in °, not in mm, i.e. if a port opens at 100° it will be half open at 140° (between open and 180° (~70% for Exhaust, ~75% for transfer, ~65% for intake)
Angle Area: The area of a port in relation to the displacement and duaration (Mean half open area / displacement* duration)
Time Area (TAm): (TAm = Z / N / 6 * A / D (TAm is Time Area in ms-cm2/cm3, Z is port open duration in degrees, N is crankshaft speed in RPM/1000, A is Mean half open area, D is displacement in cm3)
Con Rod length: the length of the con rod (the rod that connects the piston to the crank)
Tuning

The first thing you need to do when tuning a two stroke is to determine its characteristics.

Do you want it to be full range, still strong at low rpm or do you want it for higher power at rpmeven if that means you will loose some low rpm?

You will start by determine the duration for each port, and then try to get as much area as possible without exceeding that duration.

Use the info below to find the input you need.

Exhaust, Transfer and intake duration, simplified (Dragonfly75)

This is a simplified table of durations based on target RPM.

Peak power RPM Exhaust Transfer Blowdown Intake
6,000 158 120 19 125
7,000 167 122 22.5 130
8,000 175 124 25.5 135
9,000 183 126 28.5 140
10,000 192 128 32 145
Exhaust duration:

138-150: Stock on most Mopeds to keep you under 30 MPH.
151-161: Low RPM powerband timing.Good for low & mid range torque power, up to 7K-8K RPM range.
162-168: Very usable timing. Good mid range torque & power up to 8K-9K RPM range.
169-175: Best all around power band timing. Good upper mid range torque with hi RPM power up to a 9K-10K RPM range.
176-182: Gives weak power under 5K, good power at 7K, and will rip from 8K up peaking from 9K up to a 10.2K-11.2K RPM range.
183-187: Gives no power under 5.5K very weak upper mid ranger, some power at 7K, good at 8K, starts to rip from 9+K, pulls to the 11.5K-13.5 RPM range.
188-198:Very limited use. Almost no torque with a narrow high high RPM power band at Pro use racing high revs.
Exhaust port shape and size:

For single EX-port kits you want the top of your EX-port to be flat in the center of the top of the port. This flat area with should not exceed either 50% the combined width of the transfer ports or exceed 55% of the EX-port width. This will be close to 1/3rd=33% of the bore if you have good wide porting.

You do not want to exceed a width of 70% of the bore for a single port in order to protect the piston rings. Talking about piston ring, you will need to make sure the piston ring gap will not end up in a port opening, it will destroy the ring.
For the split/twin and three port exhaust with these you can have the flat area of the top of the EX-ports that is two times wider in the flat zone. So now it’s close to 66%=2/3rds of the bore instead of one 1/3rd with the single port exhaust that’s 100% more area. With 100% more flow area with we need less blowdown timing to mach the wider area cross section.

Blowdown
Blowdown is the duration of which the exhaust is open until the transfers open. It can be useful to measure this both in degrees and in % (blowdown ° / transfer opening °)

Blowdown tuning starting points for single EX-port kits (Crazy Wayne)

16% to 17.5% of transfer timing with stock or weak hitting pipes up to 8,500RPMs with 154deg to 165deg exhaust.
17.5% to 18.5% Good timing for strong pipes working in the 7,000RPM’s to 9,200RPM rang with 164deg to 170degs exhaust.
18.% to 19.% Hard hitting pipes working in the 7,500RPM to 10,500RPM rang with 168deg to 174degs exhaust.
18.5% to 19.5% This is near the max for most one speeds with clutches engaging in the 4,000 to 5,000RPM rang,
With hard hitting pipes working in the 8,500RPM to 11,000RPM rang with 172deg to 178degs exhaust.
This is the end of the road for 90% of the one speeds, now entering the variomatic zone.
19.5% to 20% Is for big pipes working the 9,000RPM to 12,500 with EX-port timings of 176deg to 180deg.
20% to 20.5% If you don’t know what your doing just stop, this is for 178deg to 187 EX-port timings Spinning 10,000RPM to 13,500+RPM.
Blowdown tuning starting points for split/twin and three port exhaust port kits (Crazy Wayne)

14.0% to 15.0% of transfer timing with stock or weak hitting pipes up to 8,500RPMs with 156deg to 165deg exhaust.
15.0% to 16.0% Good timing for strong pipes working in the 7,000RPM’s to 9,200RPM rang with 164deg to 170degs exhaust.
15.5% to 16.5% Hard hitting pipes working in the 7,500RPM to 10,500RPM rang with 168deg to 174degs exhaust.
16.0% to 17.0% This is near the max for most one speeds with clutches engaging in the 4,000 to 5,000RPM rang,
With hard hitting pipes working in the 8,500RPM to 11,000RPM rang with 172deg to 178degs exhaust.
This is the end of the road for 90% of the one speeds, now entering the variomatic zone.
17.0% to 17.5% Is for big pipes working the 9,000RPM to 12,500rpm rang with EX-port timings of 176deg to 180deg.
17.5% to 18.0% If you don’t know what your doing just stop, this is for 178deg to 187 EX-port timings Spinning 10,000RPM to 13,500+RPM.
Old single EX-port timings chart/notes (Crazy Wayne)

Blowdown of 16 to 20: Stock on most Mopeds to keep you under 30 MPH with the best MPG.
Blowdown of 18 to 22: Low RPM powerband timing.Good for low & mid rang torque power, up to 8K-9.5K RPM rang.
Blowdown of 20 to 24: Very usable timing. Works best with weaker hitting pipes. Good mid rang torque & power up to 9K-10.5K RPM range.
Blowdown of 23 to 26: very good all around Wide power band timing. Will work with most pipes. Strong upper mid rang torque with hi RPM power up to a 9.5K-11K RPM range.
Blowdown of 25 to 27: A good pipe is a must. Best all around power band timing. Riper upper mid rang torque with hi RPM power Wide power band with a big power hit. Best power for everyday use up to 10K-11.5 RPM
Blowdown of 26 to 28: Low power under 5K, Good mid range. works best with pipes that peak hard in the 8K to 11.5K range fading/topping out at 12K This the max for most street set ups.
Blowdown of 28 to 30: Works best with a tuned variomatic or shifter engine, Very low power under 6K. Weak mid rang under 7.5K. The power band starts to get narrower and more peaky. needs a HIGH RPM pipe. gives the best power in the 9k to 12+K rang, will pull past 13.5K with some set ups.
Blowdown of 30: At this point 99.% of you will just have a door stop. Very narrow power band. No power at RPM’s under 8K, higher power at high high revs. You will fry most one speed well tuned clutch’s to be able to ride with a blowdown as high as this, You need a well tuned variomatic or shifter engine that can spin over 14K.
Blowdown of 32+: Way too high for any 1 speed or 2 speed automatic. This is only ride able at the race track on the very very best tuned variomatic shifter engine.
As a rule of thumb, a blowdown higher then 25deg isn’t for everyday use on an automatic with full clutch engagement under 5,000RPM. Don’t go higher than 22deg if you don’t know what you’re doing 😉

Transfer Port duration (Jenning)

Note: The transfer duration refers to the open period of the main transfer ports in particular. The secondary transfers and the boost port may beneficially use durations longer than shown.
Peak power RPM: Transfer duration (°)
6,500: 120-124
8,000 124-128
9,000 126-130
10,000 128-132
11,000 130-134
12,000 132-136
13,000 134-140
14,000 136-142
Transfer shape and size (taken from different sources)

Smooth (as non sharp turns as possible)
Angled towards back (away from exhaust)
Top angle: flat = higher peak at high rpm, steep = wider rpm and higher torque
E.g. main transfer ~3-5°, rear transfer ~5-10°, boost ~50-60°
Some say Main transfers should open first, then Secondary, then boost.
Jenning sais boost and secondary should open first…
Piston port duration (Jenning)

Lowest useful RPM: Inlet duration (°)
4,000: 140
7,000: 150-155
8,000: 155-160
9,500: 165-170
11,000: 185-190
12,000: 195-200
Piston port duration (Crazy Wane Moped Army)

120deg-130degs to flow up to 8k-10K.
135deg’s to 138deg’s is good for street tuning.
137degs to 140deg’s on a street tune variomatic, Moby, Derbi…….
Go over 140deg’s and you will get lot of carb blow back under 4K rpm’s.
If your ripping a powerband in the 9K to 11K range 138 to 142degs for one and two speeds.
If your slipping the clutch in at 6K+ and or variomatic hitting 11K+ 143degs to 147deg’s is fun.
One or two speeds with clutch slipping in at or over 7K 147deg’s to 150degs is a bought the max you want to go for off track use.
For the track with power bands starting at 10K clutch slipping in at 8.5K or higher one, two speed or variomatic spinning to 12.5K 152 deg’s to 154deg’s.
Spinning to 14K 155degs to 158deg’s
The max i go is 160degs or it will get boggy when i blip the throttle and be hard to start.
It’s best to have a wide piston port than wide with big timing for street use.

If you go big on the PP timing you will get big power at hi rpm’s but it will be harder to tune low rpm jetting with all the extra blow back.

Useful links:

http://www.mopedarmy.com/wiki/Puch_cylinder_kit_summary#Airsal_70cc_.2845mm.29

https://play.google.com/store/apps/details?id=com.sahapon.zmod

http://www.dragonfly75.com/motorbike/

http://www.dragonfly75.com/motorbike/portdurations.html

http://www.dragonfly75.com/motorbike/formulas.html

http://www.macdizzy.com/cyl_primer.htm

http://www.mopedarmy.com/wiki/Calculating_port_timing

Motorcycle tuning

Jenning full: http://www.amrca.com/tech/tuners.pdf

Jenning ch 3: http://edj.net/2stroke/BellPerformanceTuning/Bell%20Chapter%203.pdf

Deg to mm: http://concubidated.com/ignition.php