Porting the Chinese 48cc HT engineI use Jaaps Puch calculator (download here), after measuring port heights, to know the port open durations in degrees. (At the bottom of this page is the translation of that site into English.) But anyone can use a degree wheel to figure out the port timings in degrees by observing when the piston edge aligns with the port edge. Click here to go to a good on-line port duration calculator. (Use 38 for stroke, 85 for conrod, .8 for "deck").
There's two ways to describe the port timing of each port. One, the most common, is to describe the total amount of crank degrees the port is open during one crank rotation. The second way is to tell at what crank degrees, in relation to zero degrees (piston top dead center), the port begins to open. From top dead center to bottom dead center, one half rotation, is 180 degrees. So if the exhaust port opens at 110 ATDC (after top dead center) then to get the total open port duration you'd subtract 110 from 180 and multiply by two. So 180-110=70. 70x2=140. 110°ATDC is 140° duration. If the intake port of a piston port engine (like the Grubee) opens around 60°BTDC (before top dead center) then you can figure the duration just by multiplying by two. That would equal 120° duration. For changing any port, more duration is for higher rpm power and less duration befits lower rpm power.
Standard 48cc : 141° exhaust duration (28mm from top of cylinder), 111° transfer duration (32mm), 112° intake duration (55.5mm), 44.8mm piston length at intake side, .8mm deck height, 38mm stroke.
Except for the intake timing this engine is ported good for a mild low rpm power. An ideal intake timing for the standard engine is 60° BTDC (120° dur.). To get 60° just lower the bottom of the intake port to 56.7mm from the top of the cylinder (or remove 1.2mm from the bottom of the intake side of the piston skirt). Also widen the port by 4mm on each side. (See explanation later on page.) If you need more speed you can change the rear sprocket to one with less teeth or raise the exhaust port. A 10% decrease in number of teeth will result in a 10% increase in speed. But this change reduces your ability to accelerate and climb streets.
Here's how you can change the ports of the engine if you want the best all around power from it. If you don´t make .8mm piston ramps for even more low rpm power then make the distance to the transfers 31.2mm. These dimensions have been scientifically calculated to be the best for a top rpm of 7000.
Don't port for higher rpm's unless you plan to also balance the crank. Otherwise the engine will be annoyingly vibrating above 5000 rpm and will destroy the con-rod and crank main bearings quickly. This is especially important for the larger 66/80cc engines which use a similar crank with a heavier piston that imbalances it even more.
Here is porting for the best low rpm power. It's basically the same as stock except for more intake duration and wider ports. Raise the exhaust port for more top speed if desired.
Porting can be done at home with a rotary tool that uses the common 1/8" diameter shank bits. WalMart sells a good one with 3 speeds and 1" diameter cutting wheels which can be trimmed down to smaller diameters as need be. Just be sure to not have your eyes in the path of metal bits being thrown outward away from the wheel. Accurate measurement of port heights can be done with a digital caliper, also from WalMart.
Solid Carbide Burr with Ball End (not for iron/steel) for making boost ports if you install a reed valve. Rotary Cutting Wheels Dremel 402 1/8" Mandrels
Exhaust port enlarging:
Transfer ports timing:
Lowering intake port:
Expansion chambers: A pipe designed for use with this engine should have a broad powerband and increase top rpm power. Unfortunately most all pipes sold for the Grubee engine are made for pocketbike racers and have a narrow racing powerband (as evidenced by the 12 degree or more baffle angle whereas motocross pipes only have a 10 degree angle). If you want it for having fun then that is fine but for street use there are no pipes available that are suitable other than making your own torque pipe. Click here to read about it. Of course you can buy a racing pipe and make it into a decent pipe by cutting the baffle in half and installing a 2" long cylinder to reduce and lengthen the baffle return wave. Most people think that the distance of the header pipe is just a matter of taste but I want you to know that you shouldn't buy an expansion chamber unless you are willing to go to the trouble to test different lengths until you find the right one to give you the best top speed.
Using a better carb will enhance power all thru the rpm range if it is jetted right. Don't buy a carb larger than 16mm unless you port for high rpm's. If you stick with the stock carb then be sure to throw away the trashy HT intake filter and put a good performance filter on it for less intake restriction and more engine protection. I tested the 12mm Dellorto against the 14mm stock NT carb and got more low end grunt and top speed using the Dellorto (because of better mixing) even though it was smaller.
Using better piston/rings is advised because the standard piston has rings that allow too much ring end gap. The following listed piston/rings are available from www.treatland.tv and the Honda piston is 1mm taller than stock from the wrist pin so that it gives a strong compression boost without having to mill the head and discard the head gasket. With the Honda piston you may need to dremel the edges of the heads combustion area to allow .8mm distance between it and the piston at TDC.
Honda Hobbit 40mm piston $30
A great read: Go to Micro Car Project and click onto Port_Timing_Alteration and Other_Solutions on the left hand sidebar.
Piston diagnostic photos: http://www.smellofdeath.com/lloydy/piston_diag_guide.htm
Calculate 2 stroke engine displacement: http://www.everything2stroke.com/resource/displace.php
Below are some examples of port timing from various minis:
Metrakit 65cc (43.5mm stroke) torquey. PORT MAP: http://i136.photobucket.com/albums/q194/flip27foto/PuchMaxi/naamloos.jpg
Eurocilindro/Athena 70cc (45mm) reed-valved fast+torquey
RGD/TCCD 70cc (45mm)
Malossi 60cc (42mm)
Polini 65cc (43,5mm) torquey reed-valve intake engine
Airsal 70cc (45mm) "perfect timed cylinder" http://img144.imageshack.us/img144/3323/airsal70cc2vf7.jpg
Jaaps Puch calculator
Spoel-/uitlaattiming = Transfer or Exhaust timing
Timing: (total degrees port is open. Divide by 2 and subtract from 180 to get degrees ATDC)
Inlaattiming = Intake timing
Timing: (total degrees port is open. Divide by 2 for degrees BTDC)
Snelheid: speed in km/hr (convert to mph at http://www.sciencemadesimple.net/speed.php )