This mornign I was discussing with my friend about pistons. He was saying how a longer piston rod can increase torque and max rpm of the engine. I can see how the decreased traveled distance of the piston (on each stroke) would allow an increase in max rpm and also allow rpms to climb faster, but I don't understand how it could increase torque. He was trying to explain how each stroke will be more powerful...but I didn't quite understand it... Doesn't a longer piston rod decrease the stroke, hence DEcreasing the toruqe outputted?

I don't know much about internals, so I might be making things up out of my head.....any insights about this??

 

This might help...

http://science.howstuffworks.com/fpte4.htm

 

The K24A2 engine we have in our car is essentially a stroked version of a K20. The extra 0.4L of displacement comes from the increased stroke length. Typically you can't just change the rods without changing the crankshaft.

Without getting too technical, a longer stroke means the piston has more leverage on the crankshaft which gives you more torque.

Edit: JTso wins

 

But doesn't it decreases the max rpm of the engine, instead of increasing it?
That's why the K20A has a higher redline than the K24A?

 

Everything else being equal, yes.

The longer stroke increases piston speeds so at any given RPM it's travelling faster than the shorter stroked engine. Revving a K24 to 8000rpm is like revving a K20 to 9400rpm.

 

I understand that a longer stroke usually would yield more torque, which is what I'm trying to get at, because my friend was telling me that a longer piston rod would give more torque...

BUT for a longer piston to work in the same engine, the connecting point of the piston rod to the crankshaft would have to be closer to the center of rotation of the shaft, otherwise the pistons will hit the valves, (and since the connecting point of the piston rod is now closer to the center of rotation of the crankshaft, the engine can run at higher rpms.....) which essentially decreases the length of the stroke, which in theory should decrease torque....

that's where I'm confused at

 

You're right, you need to change the crankshaft to be able to use longer rods.

 

I think I have found the answer to the question....can we have our motorheads summarize this?

http://e30m3performance.com/tech_art...atio/index.htm

 

What you're getting at here is correct. The STROKE can only be changed by changing the crankshaft radius. This has effects on max RPM, etc because piston speeds depend on the stroke.

Changing the connecting rod length alone only has effects on compression ratio because it changes the size of the clearance volume. Stroke is unchanged. And you're right that if you make it too long, then the clearance volume can become too small, and the piston crown will interfere with the valves (= dead engine). There's another issue of the con rod interfering with the actual block/crankcase if it's too long (i.e. it makes too big of an angle at a 90-degree crank throw), but the compression ratio problems usually happen first so it's not something to worry about most times (unless you have a very long stroke).

This is probably more detail than you wanted.