Boring / Stroking engines

[whitecrook]

Was thinking earlier about this and wondered:


basically theres a few engines where the 'standard' is a medium/short stroke and 'standard' bore.

Sometimes these engines are bored out to bigger pistons, giving a larger cubic capacity and more power..?

And sometimes these engines have the con rods changed (whatever) to give a longer stroke on the piston, effectively giving a larger cubic capacity as well.

And sometimes both these things are done.

But what is the real world difference between giving an other wise identical engine a longer stroke versus a bigger bore?

 

[whitecrook]

Longer stroke more torque, bigger bore more rpms

Longer rods do not change the stroke they change the compression ratio, only the crank can change the stroke

Yeah , I thought about that after I posted. In my example the crank shaft would be changed.

 

[whitecrook]

The stroke length to bore size must be a ratio and increasing both in line with each other produces a bigger more powerful engine because of more torque and rpm ability..Up to a certain size, I guess RPM goes down when the engine starts to get 'massive'. There must e a sweet spot or a sweet zone somewhere?

 

[whitecrook]

VW aircooled engines are commonly uprated by increasing the bore and crank.

Increasing the bore usually will have a drawback. Making the cylinder walls thinner can weaken the engine, or decrease heat dissipation which can lead to distortion.

Increasing the stroke is a better way of increasing the power. I'd much rather have more torque low dow, but its generally more expensive as the engine has to have more work done to allow the bigger crank.

tbh it was these engines I had in mind when thinking about it, it's a very common way to increase performance/power with these things.

But as mentioned above, from the factory there are loads of engines that come bored/stroked. e.g. the 4.2 litres V8s in the range rovers are bored (and stroked?)