Two stroke timing moped tuning

This website and app will help you tune your moped or any two stroke engine.

The Android app can be downloaded here.

Input fields are white, calculated fields pink.

See instructions below.

Please send me an email at richardax@hotmail.com if you have questions or comments.

This app was developed in order to tune a Puch Maxi stock cylinder.

The Puch Maxi has one gear and centrifugal clutch which means it has to have a full range rpm.

Therefore the prepopulated values are quite conservative.

Note: Old values will be stored in cache the next time you open the app. The values are stored in browser cache and will be erased when clearing cache in your phone.

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 rpm even if that means you will loose some low rpm?

You basically want the ports to be as large as possible, without going too high on it’s duration.

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

Exhaust: You want as much of the exhaust gases to leave the cylinder as possible in order for the pressure to be reduced when the transfes open. THis means you would want the exhaust to open early, i.e. a long duration. However, you also want the gas to produce as much power as possible, i.e. you want the cycle to be as long as possble and not push power out the exhaust, this is why a too long duration will actually decrease power. If an expansion chamber is used you can increase the duration and the resonanse in the chamber will push some of the gases back at the closing of the exhaust port. If you don’t use an expansion chamber, you should stay below 175° duration.

Transfer: For the transfers you need to first determine the exhaust duration and then look at the blowdown you need. If you want to keep low RPM torque you want a blowdown of 20-25° or 17-19% and as large port area as possible, i.e. “most useful RPM” can be much higher than the target RPM. Note that the blowdown will be determined by both the transfer duration and the exhaust duration, so if you increase exhaust duration, the blowdown will increase even if you don’t change the transfer ports. Note that shape of transfer ports are very important and usually very hard to grind, so be careful with the transfers.

Intake: You want to get as much air and gas in to the cylinder, which means you want the area to be as large as possible, but if you still want some low RPM torque you should stay below 140° duration, unless you are using reed or disc valves.

Once you know the duration you want for each port, enter them in the formulas above and you will see what you need to do to the port areas in order to reach the target RPM.

You want the Intake and the Exhaust “most useful RPM” both to be at your target RPM and the Transfer area to be as large as possible given the duration and blowdown that fits the characteristics you are aiming for.

Use the info below to help you on the way.


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 range 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