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)
  • 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 E50's

Here's the easy way out for people with the Puch E50 engine.

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:

  • 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:
    • 150-160: Low exhaust timing. Torque, not high revs.
    • 160-170: Very usable timing. Good torque and some power in the mid to high range.
    • 170-180: High exhaust timing. Very limited use. Almost no torque with a narrow power band at high revs.
  • For the intake timing, calculate the blowdown.
    • Blowdown of 25: Low blowdown. Wide power band with little peak. Rides well for everyday use.
    • Blowdown of 30: Pretty narrow power band. Low power at low revs, higher power at high revs. You need a well tuned clutch to be able to ride with a blowdown as high as this, or it will stall out.
    • Blowdown of 35+: Way too high for an automatic. This is only ridable on a well tuned variomatic or shifter engine.

As a rule of thumb, a blowdown higher then 28 isn't for everyday use on an automatic. Don't go higher than 28 if you don't know what you're doing ;)

Good luck and have fun.

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