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Porting the Chinese 48cc HT engine

I 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.


Accesories:
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:
According to a free pipe design program, for low rpm power with an expansion chamber the exhaust port area needs to be no more than an equivalent 17.8mm circular diameter to match the 20mm diameter of the exhaust pipe at a 1/1.125 ratio. That means the port should be no more than 12mm high and 24mm wide (measuring with something straight from left to right edge). Raising the port ceiling 1mm from stock and widening it slightly will result in this dimension. Top to bottom port height is equal to 38.7 minus the # of millimeters the port is from the top of the cylinder. That will result in less than the measured amount because the piston doesn't quite go down to the bottom of the port. Mine, at 12x24mm, is equal to that 17.8mm diameter. Experts say the port should not exceed 70 degrees. That equates to 24.5mm for a 48cc, and 28.1mm for a 66cc. (Those widths are not straight line, but rather the port width on paper impressed with the port edges.) Click here to read more about port shape and port timing.

Transfer ports timing:
Stock is 124.5° ATDC (111
° duration) which shouldn´t be increased unless the exhaust port timing is also increased. Here is some info if you want to change its height but don´t have a digital caliper: The transfer port distance to top of cylinder is hard to read using a ruler since it's kinda in the middle of the cylinder. So I would mark the piston with the transfer distance I wanted by measuring from the bottom of the piston. Then I inserted the piston bottom end first into the cylinder from the top. Holding the piston so that the mark is equal to the top of the cylinder I could see the difference between the port height and the bottom edge of the piston skirt in order to know how much metal I needed to dremel off to raise the transfers to where I wanted them. The area to be dremeled off can be marked with a black felt tip pen.

Lowering intake port:
Stock intake distance from top of cylinder to bottom of intake port on the 48cc HT is 55.5mm (56.1° BTDC). I first lowered my intake to 57mm (60.9° BTDC) and it had good grunt (relatively speaking) and started and idled good. I then lowered my HT's intake port to 58.5mm (65.5° BTDC) and squared it off more so that it had less of a curve to it. The bike did not want to start and had no low rpm power at all. It was embarrassing having to pedal so much to get it going. It did better at the highest rpms though and gained 1mph top speed. So unless you are porting for a screamer then I don't advise going lower than 57mm. I fixed it by putting JB Weld at the bottom of the intake port to bring it back up to 56.3mm (59°BTDC) which returned the needed low-rpm torque to the engine. Look at the graph below showing the air/fuel delivery ratio change with intake timing change on a 150cc engine. A period of 118° (59° BTDC) and 125° (62.5° BTDC) gave the best results (and anything between these two).

 

Higher Compression
(This section has its own page now. click here)

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
51mm from crown to bottom of piston
26mm from wrist pin center to top edge of piston
10mm wrist pin diameter
2mm ring vertical thickness

A great read: Go to Micro Car Project and click onto Port_Timing_Alteration and Other_Solutions on the left hand sidebar.

from http://www.mopedarmy.com/wiki/Puch_cylinder_kit_summary
(Make booster port equal to or 7° before transfer port opening (2mm higher).
Maximum exhaust port width is 70-72% of cylinder diameter.)

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
2 Transfer Ports 123° ATDC (114
° duration)
Exhaust port 103.5° ATDC (153° duration)
Blowdown: 19.5°

Eurocilindro/Athena 70cc (45mm)  reed-valved fast+torquey
# Intake port (variable): Raised intake as booster-port
# Transfer Ports 118.6° (122.75° duration): 4 ports + 1 wide booster
# Exhaust port 97.5° (165° duration): Oval
# Blowdown: 21°

RGD/TCCD 70cc (45mm)
http://img201.imageshack.us/img201/1639/rgd70ccfi4.jpg
# Intake port 63.5° BTDC: oval
# Transfer Ports 119.5° ATDC: 4
# Boost ports 123°: 2
# Exhaust port 98° : oval
# Blowdown: 21.5°

Malossi 60cc (42mm)
# Intake port 61° BTDC (122° open): oval
# Transfer Ports 125° ATDC (110° open): 2
# Boost ports 135° (90° open): 2
# Exhaust port 100.5° (159° open): oval
# Diameter exhaust port: 25mm
# Blowdown: 24.5°

Polini 65cc (43,5mm) torquey reed-valve intake engine
# Tranfer Ports 123° (114° open): 4 ports
# Exhaust port 89° (164°): oval
# Blowdown: 25°

Airsal 70cc (45mm) "perfect timed cylinder" http://img144.imageshack.us/img144/3323/airsal70cc2vf7.jpg
# Intake port 68° BTDC (138° open): oval
# Transfer Ports 123° ATDC (114° open): 2
# Boost ports 123° (114° open): 2
# Exhaust port 86° (170° open): oval
# Blowdown: 28°

Jaaps Puch calculator

Click the button at the variable you want to calculate and fill in all other fields.

    Spoel-/uitlaattiming = Transfer or Exhaust timing

    Timing: (total degrees port is open. Divide by 2 and subtract from 180 to get degrees ATDC)
    Poorthoogte: Port height (mm of port from top of cylinder)
    Deck: (mm from piston top to cylinder top at TDC. .8mm for my HT)
    Slag: Stroke (of piston=38mm for HT)
    Drijfstanglengte: Connecting rod length (85mm for HT)

    Inlaattiming = Intake timing

    Timing: (total degrees port is open. Divide by 2 for degrees BTDC)
    Inlaathoogte: Intake height (in mm from top of cylinder to bottom of intake port)
    Zuigerlengte: Piston length (at the intake side, 44.8 for HT)
    Deck: (mm from piston edge to cylinder top at TDC. .8mm for my HT)
    Slag: Stroke (38mm for HT)
    Drijfstanglengte: Connecting rod length (85mm for HT) 
    
    Snelheid, toeren = Speed, rpm

    Snelheid: speed in km/hr (convert to mph at http://www.sciencemadesimple.net/speed.php )
    Toeren: rpm
    Voortandwiel: front sprocket teeth (10 for HT)
    Achtertandwiel: rear sprocket teeth (44 for stock HT)
    Interne verhouding: primary gear reduction ratio (4.1 on the HT)
    Omtrek: outer circumference of driven wheel (2.133 for 26x2.125" tire)

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