Bore x Stroke

[Ixion]

What somewhat confuses this argument is that bore and stroke can seldom be considered in isolation.

The old long stroke singles, f'instance, ALSO had very small valve areas. mainly for the sake of fuel economy. And because a small bore means a small head surface area (as noted, good for emissions) , but hard to fit big valves in. that was one reason for the move to four valve heads.And becuse noone cared so much about max power figures. If you wanted more power, you got a bike with a bigger motor. Sort of like crusiers nowdays.

Which is one reaosn why the Beezer had more grunt. It could pump more air.

But I don't think the hydraulic analogy is valid. Sure, there is a bigger poiston area. But the VOLUME of gas is the same (all other things being equal, which they never are), so the bigger piston area implies a lower pressure per square inch .

[TLDV8]

Did i miss the bit about conrod ratio's ?

[Motu]

Not a lot of air flow at low speeds,rather the opposite.One of the problems with the Triumph was the valves and ports were too large - you will notice that by the '60's the exhausts were stepped down to a smaller size to increase the speed of gas flow.The A65 head was a later design,and had better low speed gas flow....faster,not slower.The XS650 had the opposite problem in that the head couldn't flow at high speed and that always held them back.Kenny Roberts had some special heads made by Yamaha to get his 750's as fast as a Triumph.

[Big Dave]

Some of the high torque attributes I favour about the Harley thunderstorm engine I'm told are due to the shape of the combustion chamber.

It can be more steeply domed because the cams are still down below.

[Motu]

Did i miss the bit about altered conrod ratio's ?

Short rod - high torque.

[Titanium]

square = (piston) bore and (distance between TDC and BDC) stroke are the same measurement.

I am thinking that one of the most famous "square" engines was the Z900 kawasaki with a 66 mm bore and 66 mm stroke. This was one of the reasons for its superior performance in its day.

Check out wiki.

http://en.wikipedia.org/wiki/Oversquare

[Big Dave]

So Give list of engine parameters to maximise torque?

(and I assume it will be the opposite for power)

[Titanium]

Short rod - high torque.

short rod = high revs. Develops power and torque at revs.

oh, and there is no replacement for displacement! when it comes to torque!

[Ixion]

Power IS torque. More torque, more power. You mean to flatten the torque curve. Move the peak torque down the rev scale.

[Titanium]

So Give list of engine parameters to maximise torque?

(and I assume it will be the opposite for power)

displacement and long stroke.

look at big ships, trucks, diesel engines .... all big displacment and low revs, big torque and high compression (diesel engines that is)

Power is high compresion, perfect afr's, bang on fuel / timing / spark ratio and moving a whole lot of fuel / air at the right mixture through an engine (intake, combustion and exhaust)

Torque = http://en.wikipedia.org/wiki/Torque

Stroker kit = http://http://en.wikipedia.org/wiki/Stroker_Kit

[Big Dave]

Power IS torque. More torque, more power. You mean to flatten the torque curve. Move the peak torque down the rev scale.

Design an engine optimised for low end torque.

The Theorixion 1000.

[Big Dave]

displacement and long stroke.

look at big ships, trucks, diesel engines .... all big displacment and low revs, big torque and high compression (diesel engines that is)

Power is high compresion, perfect afr's, bang on fuel / timing / spark ratio and moving a whole lot of fuel / air at the right mixture through an engine (intake, combustion and exhaust)

Do I want heavy pistons and a short conrod? Or a heavy flywheel.

edit - oops I see long stroke.

[Ixion]

Long stroke often sort of presupposes a heavy flywheel , cos you need some inertial momentum to spin the motor all the way down and all the way up again.

[Edbear]

Just to add to the erudite comments, "longer stroke" motors have higher piston speeds and put more stress on componentry like con rods and pistons. Most of this stress occurs as the piston/rod decelerates and accelerates around top and bottom dead centre. The limiting mean piston speed from memory in the days when the British twins reigned supreme was about 4000 ft/min. This will have improved a little with forged components, better alloys etc, but not hugely so; hence the move to shorter strokes to reduce piston speed whilst increasing power.

A really good example was when I drag raced a supercharged 350cc Triumph twin. The supercharger allowed it to breathe more freely so it would rev more easily. The higher revs added significant extra stress and I was forever having the piston crowns detach at the oil control ring. I'd reached the strength limits of the standard stroke motor, so I machined a short stroke crank and married it to a modified 500cc (bigger bore) barrel to give a very over-square 350. This dropped the piston speed but I also lost torque - the essential ingredient of drag racing. Months followed of testing different flywheel weights, cam timing, boost and upping the percentage of nitro in the methanol until it was competitive again.

So there you are.... a practical demo of engine design 101

Nice story! Bet you had some fun! Unfortunately I can't bling you again yet!

It's hip to be square.

It used to be square to be hip... Has it come back around again?

[Titanium]

Do I want heavy pistons and a short conrod? Or a heavy flywheel.

edit - oops I see long stroke.

Flywheels ..... that is another thread!

Typically a light small dia flywheel will offer up a very fast, free, high reving engine. But have a low inertia contribution. A light flywheel also helps in cornering due to a reduced gyroscopic effect.

Heavy flywheels can still be spun very quickly and will assist with momentum but will take longer to "spool" up.

Flywheels also act as a dampner in the drive line to protect the runnig gear from the mechanical "noise" created during the combustion process.