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.

Calculations

D=(180-cos^-1((T^2+R^2-L^2)/(2*R*T)))*2
T=R+L+C-E
  • 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
28 158.6
28.5 155.8
29 152.9
Millimeters Degrees
29.5 150.0
30 147.0
30.5 144.0
31 140.9
31.5 137.8
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
37 98.7
(Courtesy of Puchforum.com)

Other Tools

There are also tools you can use, like Two Stroke Timing Moped Tuning - A website and Android App to help you tune your moped, calculate port size and timing, with focus on Puch Maxi.

This calculator is also very useful for calculating duration vs distance.

Another one is 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)
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
Place the radio button at the variable that's unknown. (The variable you want to calculate)

For one speed two speed Automatics and variomatic

For one / two speed and automatic moped, usable timings are:


  • For the intake timing, calculate the blowdown.

The trick to tuning in blowdown is in the way the pipe hits. wide power pipes don't hit as hard as a hi RPM narrower peaky powerband pipes for the most part. Centetbleed and twin bleed pipes hit harder and wider than 95% of the standard stinger pipes. Thar are two things we don't want to happen. Problem #1 Is at around 35% to 40% of your pipes powerband power peak RPM the pipes hit is colliding with exiting exhaust as the EX-port is in the blowdown timing stage. This slows the presser drop in the combustion-chamber and this will stall or back-flow the transfer ports. And this will make it a bitch to tune & jet with staling and back flowing transfers. To fix this you need to increase the area and or timing of the blowdowns flow out the EX-port/s. So we need to have are combustion-chamber presser mach or be lower than the transfer ports presser just as the transfers crack open. The lower the presser the faster mix gets flowing and more transfer port flow = more power.

Problem #2 You get power greedy and now have so much blowdown that the pipes presser and hits wave cannot hold or push the hit in on the up stroke of the blow down timing. Now you have so much fresh transfer mix going out the EX-port on the up strok you will have 50% less power in your midrang with just 1/2deg to much of blowdown. Just the chilling of the mix in the pipe will cost you power. Having a pinch to much blowdown Is ok on Pug's, Moby's, with clutch pulleys and other 7,000+RPM clutches'. And if you go a 1/2deg more you can have a doorstop.

So go very slow or do a old trick with a spar piston. The trick is you cut the top of the piston down on the exhaust side 0.5mm=0.020". With this piston installed you just bump the EX-port up 0.25mm=0.010" at a time on single EX-port kits, and test& tune till you start lousing power. kits with two or more EX-ports bump up 0.2mm=0.008" at a time. When you start to loose power Replace the piston and your golden.

  • Blowdown is best set up using a % of your transfer port timing as a base guide.

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 shod not exceed ether 50% the combined with of the transfer ports or exceed 55% of the EX-port with. This will be close to 1/3rd=33% of the bore if you have good wide porting.

  • blowdown tuning starting points for single EX-port kits.
    • 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.

Now 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 tuning starting points for split/twin and three port exhaust port kits.
    • 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.
  • 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-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.
  • Old single EX-port timings chart/notes.
    • 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 ;)

  • Piston Port timing:
  • 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.
  • It's best to have a wide piston port than wide with big timing for street use.
    • Piston port timing will start at 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.......
    • When you 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.



Good luck and have fun. Crazy Wayne out