Firstly .Ive enjoyed reading through this thread ,and have gained more insight into the 'Pros' and 'Cons' of various common methods of modifying by doing so.
Id'e like to ad something that I discovered by accident,and that I haven't seen mentioned anywhere .
The con rod in the JS vox 21 and Innova engines when fitted reversed ,ie,oil hole towards back plate,transforms the engine completely ! For the better!

The engine becomes easier to tune,longer run time,more linear power band ,and a screaming top end,with a little less bottom hit ,and gets longer life.

The rod is offset to one side at the piston end ,and in the original orientation sits closer to one side of the piston (exhaust side) ,and wear can be seen where it rubs against the piston pin journal inside.the rod also contacts the backing plate and wears rapidly,ie ,tries to walk off the crank pin.One side of the rod journal is milled curved and the other flat and larger(like many other rods) but in the JS they put the flat side facing the crank and the dome towards the back cover ! At top of stroke the rod is almost clear of the back plate where its milled to clear the piston skirt , and because of the smaller curved side of the rod is facing back ,it catches.
We just drill a 1.1 mm hole from the curved side to intersect the original oil hole and its done.(I / WE think the factory might of made a mistake and drilled the oil hole the wrong way around ) Ive done this mod to many of the early and late JS's and so have others over here (many from new) and every one loves them .:lol:

Noddy, can you add some pics?

can u post some photo,tq

JS engines
 

Im'e working on it ! But it will take some time , I will try to find some video of the engine running also .;)

sorry I haven't had the opportunity to strip a motor for pics yet ,but to explain further .
If you look at the con-rod side on (the knife-edge) you can see that one side
of the crank-pin journal is larger than the other and sticks out more (the side facing the crank ) same at the piston pin journal end .
when viewed from above ,with the liner removed ,and the con rod held gently against the crank ,you will see that the piston is not central in the bore,and if carefully moved for and aft to take up play / allowable movement,it will,nearly touch the exhaust side ,but plenty clearance intake side .In other words ,when assembled the con rod is not in the middle of the bore !,the knife edge is closer to the back cover,the larger side is toward the crank and is machined flat,the smaller dia side is milled slightly curved and faces the back cover.:tire:

I did a crankshaft timing mod. in a few days ago. Advance open by 6, delay close by 2. I then ran the engine just once. Not exactly know how it perform. Found some difference in characteristic. I run rc boat. So pipe is narrow band, engine work at max. power rpm only. I lengthen the pipe by 5mm.

The engine is 199X's REX 350 Super Competition on road engine. Total exhaust duration 175 degree. I believe this value is regarded as buggy timing nowadays.

Hope to hear any idea.

Thanks,


Leo

Are you trying to pick up a little extra acceleration by opening the rotary valve 6 deg earlier?
Is that why you added 5mm to the pipe?

What rpm are you trying to develop peak HP at? Or do you know what rpm your prop allows the
engine to rev up to. It would help if you posted a picture of the pipe and its length.

Did you figure out your transfer port timing? It would be helpful to know your BDT (blow down time).
The BDT will determine how effective your pipe is. If it is too mild, it will effect the pipe's performance.


Bill M.

Hello Bill,

My mind was to give up some acceleration, rc boat seldom accelerate, and want some more high end torque, rc boat mostly run at high rpm. After the mod. I believe that delay inlet close must accompanied with change in higher max. power rpm. I actually want more high end torque but not higher rpm.

Refer to article on HOW TO TIME A FRV(CRANKSHAFT TIMING):
http://www.modelenginenews.org/techn...me.html‎

The inlet should open at BDC. But most nitro engine do open around 30 degree ABDC.

Refer to 2 stroke engine animation:
http://www.btorcboat.com/images/two-stroke_c_3.gif
http://www.btorcboat.com/images/two-stroke_c_3.gif

Side port gasoline engine, with inlet reed valve, start to draw fresh charge very soon after BDC, frame 19 of the animation. This is by the negative pressure generated by the pipe divergent cone. But this cannot happen in most nitro engine at the same angle. While the inlet not yet opened, the negative pressure establish a low pressure inside the crankcase. As the pulse reflect and inlet open, crankcase low pressure disappear. Don't konw the flow direction.

Refer to www.modelenginenews.org. With half of the crankshaft structure removed, a weak crankshaft. I believe this O.S. Speed R2101 work very close to a side port gasoline engine: http://www.osengines.com/engines-car...043/index.html

With sight of inlet late open. I mod. my engine to advance open by 6, hope to make it to work like side port gasoline engine. Delay close by 2, want more high end torque.

Boat pipes are narrow band. Long pipe give better low end, short pipe give better high end. None work like wide band car pipe. I tried the old pipe length but not work. Lengthen 5mm work.

I need some more testing before I knew that whether my aims were achieved.

Hope to hear any idea.

Best,


Leo

I have been building and designing pipes for a very long time, and I have
built pipes for RC Boat racers too.

Some of what you say is true, but much of it is also myth.




Bill M.

I found something interesting by accident and thought I would share.

PORTING DESIGN CHECK ON TWO-STROKE ENGINES

This small web site has an excel spread sheet download that calculates
port openings in degrees, time areas based on both Blair and Jennings, and
expected power output in Bar.

One way to measure ports that hasn't been mentioned is to use a magnetic
stand and a dial indicator to measure distance from BDC to port close. Although
you need to know the Bore, Stroke, and Rod Length.



We should make something similar and circulate it on RCTech.


Bill M.