Bore x Stroke

[cooneyr]

Except for size.

There ain't no substitute for cubic inches.

Lighter vehicle weight?

[Motu]

Lighter vehicle weight?

I've always found a big advantage in light rider weight.....lard arses kill the performance of their bikes.

[Big Dave]

I've always found a big advantage in light rider weight.....lard arses kill the performance of their bikes.

My mate Trip was bitching because Wendy forgot to bring the ramp on the trailer when his Norton broke down - again.

So I picked up said Norton and placed it gently on the trailer.

:-P

[Blackbird]

My mate Trip was bitching because Wendy forgot to bring the ramp on the trailer when his Norton broke down - again.

So I picked up said Norton and placed it gently on the trailer.

:-P

Hahaha - good one. A mate of mine in the UK has one of the early Triumph Tridents with Lucas Prince of Darkness electrics. He reckoned that his wife having to turn out with a trailer to surrounding counties most weekends rather strained their relationship. He also said that he was on first name terms with every AA patrol officer within 100 miles!

[Motu]

So I picked up said Norton and placed it gently on the trailer.

Isn't human nature such a wonderful thing? Tough guys are so keen to show off their strength - I just step back and watch them do it....job done! Brains beats brawn every time....so do fast legs.

[Big Dave]

Isn't human nature such a wonderful thing? Tough guys are so keen to show off their strength - I just step back and watch them do it....job done! Brains beats brawn every time....so do fast legs.

Like I EVER show off.......

[Ixion]

Except for size.

There ain't no substitute for cubic inches.

Aerodynamics. And weight.

[Ocean1]

Lighter vehicle weight?

Aerodynamics. And weight.

For sure. But we were discussing engines...

[Motu]

I've got a few engines under the bench - but they don't do a damn thing until I fit them into something.....

[Mikkel]

I've got a few engines under the bench - but they don't do a damn thing until I fit them into something.....

They can still make noise can't they?

I mean - with speed limits being what they are noise seems to be the most important aspect of any engine these days.

[Mr.Pete]

Except for size.

There ain't no substitute for cubic inches.

In a way,you're right.An increase in cubes will always make more torque at low rpm.If you change nothing else,don't expect to make more power;but expect to make the same power at lower rpm.

A long stroke 490cc cylinder could be 79mm bore X 100mm stroke.It might make max power at 6000 rpm.
A square 490 cc cylinder would be 85.54 bore and stroke.It could make max power at 6491 rpm with approx the same inertia stress level.It would take more intake /exhaust flow to achieve that,but less cam duration because of the lower piston speed. 8.18% more power.
A short stroke 490cc cylinder could be 111.7mm bore X 50mm stroke.It could make max power at 8485 rpm with about the same inertia stress as the longer stroke cylinders.It would require even more intake/exhaust flow and slightly less cam duration.It could make 41.4% more power than the long stroke engine and 30.7% more than the square cylinder.It's piston speed would be 70.7% of the long stroke engine.

You don't need much more flywheel inertia for a long stroke.If you still push 490cc to 8.5:1 compression,it takes the same amount of energy for a long or short stroke engine.The conrod will have slightly more weight,because it needs to be longer.The smaller piston will have much less weight.Piston weight is about 3 times the weight of the rod small-end.There's not much kinetic energy in a piston and conrod at idle,even with the slightly higher piston speed of a long stroke.
As the piston approaches TDC the piston's kinetic energy is absorbed by the crank,and is available to accelerate the piston on the following down-stroke.You don't need any extra flywheel inertia from added flywheel mass to achieve this.The kinetic energy is not lost or wasted;it's merely transfered from piston to flywheel,then back to the piston.You might lose a very small amount in friction at the conrod bearings.
Flywheel inertia is mostly a function of your intended idle speed.If you're happy to double the idle rpm you could reduce flywheel inertia to 25%.

A given cam duration will suit a certain piston speed (as measured in feet/minute or metres/minute).The long stroke engine will reach that piston speed at lower rpm and,if all else is correctly proportioned,you get maximum torque.This leads to the wives' tale "long stroke engines make better torque at low rpm".
A short stroke engine can make the same torque at the same low rpm,it just requires less cam duration than the long stroke engine.Give it the same intake and exhaust flow,and it will make the same power curve as the long stroke engine.

The long stroke engine does have a small advantage in fuel efficiency and economy.There is less piston and cylinder head area absorbing precious heat and energy from the combustion process.There is also less piston ring circumference to allow blowby past the rings.

[DEATH_INC.]

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

You want a big arsed compound turbo two stroke diesel then. Possibly with nitrous to help it spool up quicker.