Rod length also affects "port timing" of 4-stroke engines. If the intake valve closes at say 45 degrees after BDC, the engine with the shorter rod will have the piston lower in the cylinder than the long rod engine due to increased rod angle.

Brian:

Thanks. Always appreciate your dry humor.

But getting serious now, the best way to stroke a model engine is first, be sure it's cool, and then do it just like you stroke a cat. Gently. From the head to the foot. Stroke it the wrong way or get rough and it will either bite or claw you.

Haw.

Bill.

Brian,


In full size engines, stroking is performed by grinding the crank-pins off-center (further out from the crankshaft C/L)and using smaller i/d rod big-end bearings.

If the crank-pin is ground 1 mm smaller, tangentially with its outer perimeter, the stroke would increase by 1 mm, since the center-to-center distance (crank-pin to crankshaft) would increase by 0.5 mm.

...But the crank-pin would also be weakened considerably...

Dar:

It is true that stroking a crank involves grinding the crank pins on centers with a larger radius, but before grinding the crank pin is welded up on the outer periphery and the sides, then when it is ground it is finished at the original diameter, standard size bearings are used, and if welded properly it is no weaker than it was originally.

When I had my machine shop one of my larger machines was an automatic submerged arc welder just for doing this kind of welding.

Bill.

True, Bill.

In full size engines, with a pin diameter of 30-40 mm, grinding 1 mm smaller will not weaken the pin to a very great extent. In a model engine, grinding a 7 mm crank-pin down to 6 mm, however...

This is too costly to make it worthwhile, in a $100 model engine...

Just buying a larger engine would cost much less.

"How can an engine have more stroke without increasing the distance from the crank pin center to the center of the crankshaft? I have two engines that have the same crankshaft yet one is supposed to have 1.6mm more stroke"
Ok this CAN be true , if they are talking about effect compression stroke which not the actual crankshaft stroke , but the stroke of the piston after the ports are closed( this term is only used when talking about 2st's)
Stewart

Stewart,


When most people seek a way to 'stroke' an engine, they are looking for more swept displacement and more power.

When you 'drop the sleeve' and increase the effective stroke, you are also making the timing numbers more modest; less intake and exhaust duration, and the engine will be easier to operate, but with less output.
It may have more low-end torque, however.

I believe this is not what Brian had in mind, when raising this question.

Actually it is what he was asking , he has 2 motors with the same crank but different stroke. therefor the 2 motors must be ported differently to have different effective stroke.
Stewart

Take a Fox .60 and a Fox .74, the .60 has a bore of .905 and and a stroke of .937, the .74 has a bore of 1 inch and a stroke of .937, no matter what you do to those engines other than change one of those demensions, they're still going to be a .74 and a .60. You can use terms like effective or corrected but they do not change that, the swept volume will always be the same. You can improve the volumetric effeciency somewhat by altering the port or cam timing, utilizing tuned pipes etc. but usually at the expense of tractability.

I already said that's howyou stroke an engine way back at the beginning of this thread.