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

D=(180-cos^-1((T^2+R^2-L^2)/(2*R*T)))*2

T=R+L+C-E

R is stroke devided by 2 L is the conrod 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.

Or just take the easy way out...

Portheight in relation to timing in a Puch engine. All in MM and 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 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 Puchforum.net)

There are also tools you can use. Like Jaap's Puch Calculator.

<http://home.planet.nl/~moelk037/Jaap's%20Puch%20Calculator.exe>

Quick translation of the program:

Spoel-/uitlaattiming = Transfer-/Exhausttiming

Timing (In degrees) Portheight (in mm) Deckheight (in mm) Stroke (43mm standard) Con. rod length (90mm standard)


Inlaattiming = intake timing

Timing (in degrees) Intakeheight (in mm) Pistonlength (At the longest point, in mm) Deck (in mm) Stroke (43mm standard) Con. rod length (90mm standard)


Poortbreedte = Port width

True width (in mm) Portmap width (in mm) Bore (in mm)


Snelheid, toeren = Speed, RPM

Speed (In KM/H) RPM 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


Oh and for an automatic, useable timings are:

Exhaust timing:

150-160: Low exhaust timing. Torque, not high revs. 160-170: Very useable 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 powerband at high revs.

For the intake timing, calculate the blowdown.

Blowdown of 25: Low blowdown. Wide powerband with little peak. Rides well for everyday use. 30: Pretty narrow powerband. 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. 35+: Way too high for an automatic. This is only rideable 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 then 28 if you don't know what you're doing ;)

Good luck and have fun.