compression increase via piston

[Deleted member 12676]

There is this piston from www.treatland.tv that is 1mm taller from the wrist pin than the standard 48cc piston. That is probably the perfect distance to bring the compression to around 130 or 140 psi and allow you to keep the head gasket. And it is cheaper than buying a high compression head. That added height would change the port timing though which would run terrible unless you used a rotary tool to change the port heights first. Here are my recommendations:
Exhaust port
26.3mm from top of cylinder (for 107 degrees ATDC which gives the best all around power with the standard exhaust pipe)
Transfer ports
30.7mm from top of cylinder (for 123 degrees ATDC which gives 1 degree more blowdown time than stock)
Intake port
56.7mm from top of cylinder to bottom of port (which gives a perfect 62 degrees BTDC timing)
Piston skirt
45.1mm from top of piston edge to bottom middle of intake skirt. (the exhaust side skirt needs to be reduced to 46mm to keep it from hitting the crank wheels)

from treatland.tv
(FOR MORE COMPRESSION)
honda hobbit stock 40mm piston $30
50mm from piston edge to bottom of piston
24mm from wrist pin to bottom of piston
26mm from wrist pin to top edge of piston
10mm wrist pin diameter
1.5mm ring vertical thickness

 

[Fabian]

I like your technical information Jaguar.

Other members may find it very useful and a method to shortcut their own development time.

 

[Deleted member 12676]

That is the whole idea. No need to reinvent the wheel. Testing and developing has been my hobby the past year and I just hope others take advantage of the tricks of the trade that I have presented.

 

[Deleted member 12676]

Also, an additional head gasket will have to be added, or you can use a dremel to remove metal at the head where the top edge of the higher piston would otherwise hit.

 

[happycheapskate]

If you add a gasket, or remove metal from the head, will the piston swap still increase compression?

 

[Fabian]

Jaguar, do you know of any alternative cylinder and piston combinations in a larger bore size than 47mm which will fit on the crankcase of the Chinese 2-stroke bicycle engine.
A genuine 80cc of swept cylinder volume would be desirable.
A 50mm or better still a 52mm piston is a tempting thought.

 

[Deleted member 12676]

Normal compression is about 100 and I think it increases about 25psi with every .5mm vertical space decrease, so adding a .6mm gasket should give about 120psi with the 1mm higher piston. Using a rotary tool on the outer edges of the head, to make space for the piston, would probably allow around 145psi which would be my preference. Just make sure you buy a new wrist pin bearing because the original chinese one can't take the increased pressure, especially if you still have a standard CDI which fires too early at high rpm which causes too much combustion pressure and causes failure of that bearing.
I am sure there are other top ends that can fit but you'll have to start a thread to see what others have used and how much of a match/mismatch it is for the cylinder studs and transfer ports. I've only worked with 48cc and 55cc but JNMotors is sending me a 66cc engine to modify for racing. One problem with thinking about using a larger diameter piston/cylinder is that the engine is already a bit out of proportion from the ideal square design which would be equal stroke and bore. It has 38mm stroke and 47mm bore. Also the connecting rod is longer than the ideal 2x stroke which reduces torque. But I guarantee you this 66cc can be made into a fire-breathing monster without enlarging the bore. I will do the same mods to it that I've done to my 55cc which is almost strong enough to do wheelies without pulling on the bars.

 

[Fabian]

My requirements are different in that i want significantly more low rpm torque.

Do you know of any alternative cylinder and piston combinations in a larger bore size than 47mm which will fit on the crankcase of the Chinese 2-stroke bicycle engine?

 

[Deleted member 12676]

My question to you would be to find out what you have done to maximize the power of the engine you already have.
There's so much you can do to surpass the power you'd get from having 2mm more bore.
Best things for low rpm torque: higher compression, better carb (Dellorto), intake and exhaust extension, torque pipe (which you can approximate by adding a baffle extension to one of the aftermarket expansion chambers), and using JBWeld and a rotary tool to redirect the transfer flow more rearward (which makes a big difference over the sucky stock arrangement).

 

[Fabian]

Do you know of any alternative cylinders with a 52mm bore size and matching piston with a 10mm wrist pin bore size to suit the standard connecting rod?