Calculating port timing

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This is a short article about calculating port timing. This is copied and pasted off the Technical board, and it's about advancing timing. It also includes a helpful tool.

Advancing the timing is pretty simple. You just need to calculate the current timing.




  • R is stroke divided by 2
  • L is the connecting rod length (90mm in a Puch, 85mm in a V1)
  • C is the distance of the crown of the piston to the top of the cylinder in TDC (Deckheight)
  • E is the distance between the port and the top edge of the cylinder

Everything is measured in millimeters.

And then fill in the desired timing and reverse-calculate it.

For Puch's

Here's the tools out for people with the Puch E50 or ZA50 engine.

  • You can use this degree tape to help you figure out your port timing if you have a Puch or Tomos-sized flywheel.
  • You can also use this chart by measuring from the top of the cylinder, using milimeters:
E50 port height (mm) in relation to timing (degrees)
Millimeters Degrees
21.5 193.7
22 191.1
22.5 188.5
23 185.9
23.5 183.2
24 180.6
24.5 177.9
25 175.2
25.5 172.5
26 169.8
26.5 167
27 164.3
27.5 161.5
Millimeters Degrees
28 158.6
28.5 155.8
32 134.7
32.5 131.4
33 128.2
33.5 124.8
34 121.4
34.5 117.9
35 114.3
35.5 110.5
36 106.7
36.5 102.8
(Courtesy of

Other Tools

There are also tools you can use, like Jaap's Puch Calculator. Quick translation of the program: (See also Foreign language moped dictionary and Dutch-English moped dictionary for your other translation needs.) There is also an applet that will help you convert piston travel in mm to crank rotation in degrees. A post here talks about how to make an initial port map and how to transfer your newly-calculated measurements back into the cylinder.

  • Spoel-/uitlaattiming = Transfer-/Exhaust timing
Timing (In degrees)
Port height (in mm)
Deck height (in mm)
Stroke (43mm standard)
Con. rod length (90mm standard)
  • Inlaattiming = intake timing
Timing (in degrees)
Intake height (in mm)
Piston length (At the longest point, in mm)
Deck height (in mm)
Stroke (43mm standard)
Con. rod length (90mm standard)
  • Poortbreedte = Port width
True width (in mm)
Port map width (in mm)
Bore (in mm)
  • Snelheid, toeren = Speed, RPM
Speed (In KM/H)
Front sprocket (In number of teeth)
Rear sprocket (in number of teeth)
Internal gearing (Leave it the way it is for Puch)
Tire radius (in meters -- the length that the tire rolls for one revolution, if you catch my drift)
  • Informatie = Information
Place the radio button at the variable that's unknown. (The variable you want to calculate)

For Automatics

For an automatic, usable timings are:

  • Exhaust timing:
    • 138-150: Stock on most Mopeds to keep you under 30 MPH.
    • 151-161: Low RPM powerband timing.Good for low & mid rang torque power, up to 7K-8K RPM rang.
    • 162-168: Very usable timing. Good mid rang torque & power up to 8K-9K RPM range.
    • 169-175: Best all around power band timing. Good upper mid rang 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-12.5 RPM range.
    • 188-198:Very limited use. Almost no torque with a narrow high high RPM power band at Pro use racing high revs.
  • For the intake timing, calculate the blowdown.
    • 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 13K 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 27 isn't for everyday use on an automatic. Don't go higher than 25 if you don't know what you're doing ;)

Good luck and have fun. Crazy Wayne out