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?

 

[Dr Who]

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

 

[gurusan]

bigger bore doesn't mean more RPMs as you will have bigger and heavier pistons. However longer stroke will result in crazy piston accelerations as you rev higher, so often there's a sacrifice, but you also gain torque with longer stroke.

Usually it's easier and cheaper to just bore out the engine as you then only have to pay for machining costs and new pistons...whereas to stroke an engine you need to change out the crankshaft as well as the rods most of the time.
The problem is that you can only bore out the cylinder walls so much depending on the engine....however there are ways to strengthen the cylinders but these all depend on the engine design (open vs closed deck, etc)

 

[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.

 

[Clarkey]

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

That theory falls apart when you look at a Honda B18C though

 

[burns]

Usually it's easier and cheaper to just bore out the engine as you then only have to pay for machining costs and new pistons...whereas to stroke an engine you need to change out the crankshaft and most of the time rods.

What's a time rod?

 

[gurusan]

poor sentence construction. fixed

 

[Jonnycoupe]

That theory falls apart when you look at a Honda B18C though

The stroked B16B?

Its more about rod ratio for the revability. With the bore and stuff influencing valve area. So when your stuck with the standard head and the huge range of variations there are few rules.

Either way increasing capacity will increase torque. Poor head breathing will just prevent you then seeing a significant PEAK power figure increase.

 

[Bluelion]

I'm interested on the effect of stroke length and fuel economy. I run an oldish LandRover with a 2.25 4-cylinder petrol engine, which has (I understand) a pretty long stroke for low down torque and develops from the factory a whopping 73hp according to wikipedia.

Now the engine will idle down to around 340rpm before getting too lumpy which again I attribute to the stroke. But would you expect a long-stroke engine like this to be more or less fuel efficient than a similar sized short-stroke engine?

 

[Dr Who]

Yes the 2 don't always follow.

Generally the shorter stroke engines rev better than long stroke, so for the same capacity, larger bore revs more than longer stroke.

So to clarify, if you build a 2.2 engine from a 2.0 by increasing the bore, it will rev better but make less bottom end torque than a 2.2 made from the same engine by increasing the stroke.

Leverage wins over punch

 

[Spuderoony]

Engine stroking:

Phwoar!

 

[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?

 

[BigglesPiP]

It is a tendency that an engine with a large bore compared to stroke (over-square) will be more revy, and that a "stroker" (under-square) will be a lazier motor with a characteristic of good torque. There are plenty of exceptions to this tendency though.

As for which is easier, it depends on availability of tuning parts. If you can buy a crank and set of rods; stroking an engine is but the work of an afternoon. If not, boring it out and fitting larger pistons is often easier.

Sometimes you see unexpected choices though; when Alfa were preparing the 155 for the BTCC, and the 8000 odd RPM rev limiter, rather than using the 2.0L 16V twin Spark, they used the older 8 valve 1.8L twin spark and stroked it to 2 litres, no boring.

 

[Dogbreath]

It's an old wives tale that a long stroke engine gives more torque than a short stroke one of the same capacity. All else being equal, torque will be the same. However, an over-square engine allows larger valves relative to it's capacity, so torque at lower RPM can suffer a little. Also the cam timings used for over-square engines often take advantage of their ability to rev higher, which (without variable timing) always sacrifices torque at lower RPM.

Con rod length can vary compression ratio, again all else being equal, but the rod length has other effects so modifying rod length to achieve a given CR is not usually a good idea. The rod ratio (rod length/stroke) is important as it determines the motion of the piston. A short rod in comparison to stroke will create a more non-linear piston motion which increases secondary vibration and increases the side thrust on the piston; all undesirable traits.

 

[Jonnycoupe]

Then you have to consider the effect of offset cranks to provide more torque to the crank and the varying compromises that occur when you play with engine geometry leaves more/less room for error with the timing at TDC.

 

[Dr Who]

It's an old wives tale that a long stroke engine gives more torque than a short stroke one of the same capacity.

It is an old wives tale that long stroke engine produce more bottom end torque?

Citation needed.

I think people are just arguing semantics rather than answer the question...

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

All these exceptions people keep mentioning are not "identical engines" as other things can bring about changes in this characteristic such as the use of Variable Valve timing. They can make a short stroke engine more torquey low down the revs, but retain revability.

The question that remains is what are the affects on the same engine by varying either the bore or stroke.

Increasing the stroke will increase the low end torque (and all torque) and make the motor more flexible at the expense of peak rpm.

Increasing the bore will retain the rpm capability (assuming you dont whack in heavier pistons) increase torque, but at a higher rpm than the stroked motor.

These changes for a given engine assume similar breathing and cylinder pressure, so all things being equal.

 

[gurusan]

I don't see how it's an old wives tale... force x distance = moment. By increasing the distance you get a larger moment about the crank axis :/

 

[Dr Who]

I don't see how it's an old wives tale... force x distance = moment. By increasing the distance you get a larger moment about the crank axis :/

That's the point its basic physics, however, a shorter stroke manages similar torque higher up the rev range (more force applied through more combustions).

 

[Dogbreath]

That's the point its basic physics, however, a shorter stroke manages similar torque higher up the rev range (more force applied through more combustions).

You need to brush up on your basic physics. A shorter stroke engine of the same capacity has a larger bore. Force=pressure * area. Torque = force * distance. Do the math as they say in the US.

As I have already stated, all else being equal a long stroke and short stroke engine of the same capacity will produce the same torque. The point is that all else is usually not equal as I have also explained.

 

[Fuzz]

If you have the same peak pressure on the same size piston crown, increasing the stroke will increase the torque but decrease the max attained safe RPM.