Bore x Stroke

[scumdog]

Hi-tech talk.

Long stroke = more torque but less revs

Bigger bore = more rev capability.


I think.

[Forest]

So when I whap the throttle at 1500rpm - it immediately wheelstands.

Sounds like you want an electric bike.

An electric motor develops its maximum torque at zero rpm.

[Forest]

As a real-world example of an under-square engine, look at the 1,800cc twin used in the Yamaha MT-01.

Bore 97mm and stroke 113mm.

[scumdog]

Short rod - high torque.

Excessive side-loads on cylinders too...

[Motu]

So now after 5 pages of conflicting theories,we can conclude that engine characteristics has nothing whatsoever to do with bore/stroke relationships,2 or 4 stroke,valve or camshaft configuration,or cylinder layout or numbers.What really matters is what the designer has in mind.....and that means all the little things are more important than saying ''my bike has more torque because it's a V twin.'' Personally I still consider that the flywheel is the most influential component of an engine.

[Titanium]

Ah stuff it ...... go forced induction! chuck on a turbo or supercharger! or even both!

[cooneyr]

Perfect.

Sounds simple when you say it quick don't it.

Been a few variable intake devices over the years.

After that post was standing in the shower (good place for thinking that) and realised that Vtec is a perfect example of variable intake 'restrictions' using cam timing (opening durations).

Landing? Ah, now we are talking about thrust. There is a relationship can be derived between torque and thrust . That introduces some new concepts into our discussion. Coefficient of friction, hysteresis, lots of nice interesting stuff.And quite a bit of pure physics. We'll need some mathematicians as well as engineers. This could be a worthwhile discussion.

Keep it going - I'm interested. Where is there hysteresis in an engine system?

Ah stuff it ...... go forced induction! chuck on a turbo or supercharger! or even both!

That just cheating Just read the Press article about the VW 1.4TSi engine this morning - super charger and turbo (yes I know they are both just different types of super chargers).

Cheers R

[Ocean1]

So now after 5 pages of conflicting theories,we can conclude that engine characteristics has nothing whatsoever to do with bore/stroke relationships,2 or 4 stroke,valve or camshaft configuration,or cylinder layout or numbers.What really matters is what the designer has in mind.....and that means all the little things are more important than saying ''my bike has more torque because it's a V twin.'' Personally I still consider that the flywheel is the most influential component of an engine.

I gave up playing with engines way back when you could no longer take a typical standard offering and easily double it's performance without spending a hell of a lot. Things have got more sophisticated. Even so, the short list of simple design variables in those 5 pages add up to a huge range of possible engine designs, each one with quite different power delivery characteristics.

Murphy usually gets a say in any build too, some engines just seem to work better than they should, some never seem to quite work as you expect, in spite of numerous tweaks or rebuilds. Even with the vastly improved design and analysis software used now it's still more art than science.

After that post was standing in the shower (good place for thinking that) and realised that Vtec is a perfect example of variable intake 'restrictions' using cam timing (opening durations).

Yeah. That's more to do with fill rates than port velocity though.

There's an ideal air speed for fuel mixing, the only way to maintain that over any change in rev's is to change the cross sectional area of the port.

There's been variable volume intake tract systems that do work, but to get the best effect you want the csa to change smoothly, over most of the port length. Difficult. If you manage the mechanical bit ideally you'd use another channel on the ecu to control it...

[rwh]

I would suggest making the conrods more than half the stroke.

Richard

[Trouser]

What effect does differing rod/stroke length relationships have?

[Ocean1]

What effect does differing rod/stroke length relationships have?

It changes the velocity profile of the piston.

That means you can optomise the rod angle to the crank to best suit the duration of the pressure profile for the rev's you want and the fuel you're using.

[Ocean1]

Here y'go, best description I can find...

Effects of Long Rods
Pro:
Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.
The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.
For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Con:
They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.
Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.
To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.
Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.

Effects of Short Rods
Pro:
Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.
The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.
Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Con:
Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.
Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

[Big Dave]

Conversely, pros connoting the pros and cons of conrods has pros and cons.

[Zealot]

Ah, engineering. Where everything is a comprise between opposing factors to achieve a desired result.

[Ocean1]

Ah, engineering. Where everything is a comprise between opposing factors to achieve a desired result.

Except for size.

There ain't no substitute for cubic inches.