Porting differences of different engine sizes

[Deleted member 12676]

I finally got a hold of a new 69cc (ie: 80cc in the minds of drunk Chinese) cylinder and piston and crank.
Looking at the port timings between the 48cc, 60cc, and 69cc and considering the blowdown* the most important factor for ability to rev high, it looks like the best is the 60cc followed by the 48cc and then the 69cc. Here's what they have:
48cc 15.2degrees
60cc 17.6 degrees
69cc 13.5 degrees
To get the 69cc equal to the 48cc you'd have to remove 1.2mm from the top of the exhaust port. I know 1.2mm is a very small amount but with these small engines a small port height change can make a big port timing change.
The good points of the 69cc is that the intake timing is almost perfect at 118 degrees, and the intake area at the base of the transfers is larger than the other two cylinders which is important for top rpm power.

Intake port duration on a non-reed valve intake also has a big effect on peak rpm available. I found that 120 degrees is the max it should be for ability to rev higher without losing low rpm power. To get 120 on the 60cc you need to remove .5mm from the bottom of the piston skirt. On the 48cc you need to remove 1.1mm from the intake skirt. On the 69cc remove .4mm.

*Blowdown is the degrees from exhaust port opening to transfer port opening. It has to be enough to provide enough time for the exhaust to "leave" so that the cylinder pressure is low enough for the intake charge to enter through the transfers. Limiting the blowdown limits the peak rpm.

 

[Fabian]

Hi Jaguar

Can you please give a full set of degree figures for the 48cc, 60cc and 69cc timing specs. It will be interesting to see the differences between the port configurations.

*intake port opening and closing specs* ===== *exhaust port opening and closing specs* ===== *transfer port opening and closing specs*

 

[Deleted member 12676]

Assuming a .7mm deck height (distance from top edge of piston to top of cylinder at TDC) here are the port durations:
engine - exhaust - transfers - intake - blowdown
48cc - 140.7 - 110.3 - 113 - 15.2
60cc - 135.7 - 100.6 - 116.8 - 17.6
69cc - 141.6 - 114.7 - 117.6 - 13.5

 

[Fabian]

Assuming a .7mm deck height (distance from top edge of piston to top of cylinder at TDC) here are the port durations:
engine - exhaust - transfers - intake - blowdown
48cc - 140.7 - 110.3 - 113 - 15.2
60cc - 135.7 - 100.6 - 116.8 - 17.6
69cc - 141.6 - 114.7 - 117.6 - 13.5

Jaguar, you are the man. Thank you very much for supplying this port duration information.

Do you have the port opening points from TDC for the exhaust - transfers - intake on all 3 engines?

 

[Deleted member 12676]

engine - exhaust - transfers - intake* - piston skirt
48cc - 28mm - 32mm - 55.5mm - 44.8mm
60cc - 28.7mm - 33.1mm - 56.3mm - 45mm
69cc - 29.3mm - 33.1mm - 57mm - 44.7mm

* bottom of intake port

engine - stroke - con rod
48cc - 38mm - 85mm
60cc - 38mm - 85mm
69cc - 40mm - 89mm

 

[Fabian]

You are a treasure trove of good information.

Thanks

 

[Deleted member 12676]

Intake port duration on a non-reed valve intake also has a big effect on peak rpm available. I found that 120 degrees is the max it should be for ability to rev higher without losing low rpm power. To get 120 on the 60cc you need to remove .5mm from the bottom of the piston skirt. On the 48cc you need to remove 1.1mm from the intake skirt. On the 69cc remove .4mm.

If your engines "deck" is not .7mm (maybe due to varying thicknesses of base gaskets) then add the difference to the removal distances listed above to get 120 degrees intake duration. For example; your 48cc engine has 1mm deck which is .3mm more than .7mm I measured on mine. Add that .3mm to the listed 1.1mm removal distance for a total of 1.4mm. Remove 1.4mm from the piston skirt (or bottom edge of intake port).