Short rods

[pancho]

 Folowing up on the discussion on con-rod length {which I cant find} I have just been reading about the new Subaru.
 Longer stroke, smaller bore, MORE horsepower!
 Other changers have been made, but interesting.
 cheers pancho.

[Ktm181]

if you drove it you might think it had more!  It does produce what it has lower in the revs than the previous motor, it also has more comp and is a LOT better on fuel with the auto (cvt) now being MORE economic  than the manual, all actual testing not "claimed".

Kt

[Mike52]

I have been involved in putting together a early 87 WR 250 Husky motor [ no exhaust port valve ]  in a 86 WR frame.
Much anticipation as it has a 10mm longer stroke and a smaller bore than earlier models.
I thought that it would rev lower and be a bit torkier.
Got to ride it at Murphy's Creek Vinduro on the weekend and could not believe the difference between it and a 86 WR 240.
It has no vibration , revs like a 125 and pulls up hills as well.
Stunning to ride and very unexpected.

[Lozza]

Folowing up on the discussion on con-rod length {which I cant find} I have just been reading about the new Subaru.
 Longer stroke, smaller bore, MORE horsepower!
 Other changers have been made, but interesting.
 cheers pancho.

Not so surprising, longer stroke is more torque, HP is a function of torque.

[lukeb1961]

Folowing up on the discussion on con-rod length {which I cant find} I have just been reading about the new Subaru.
 Longer stroke, smaller bore, MORE horsepower!
 Other changers have been made, but interesting.
 cheers pancho.

Not so surprising, longer stroke is more torque, HP is a function of torque.

Herr Stolk, you are carefully omitting any discussion in regard to revs (the other component of HP), heat transfer, piston side loads, peak piston speed, torque peak range, etc.
So while I often ponder the pros and cons of a long stroke engine, it is not an automatic assumption that a longer stroke and smaller bore equals higher horsepower. I would like to read the thinking behind moving the Subaru to a longer stroke. Was it a packaging question, for example, rather than a technical issue? Was it fuel-burn, was it... something else?

[Nathan S]

I'm confused by the discussion so far: Stroke and rod length are pretty much independent of each other.

Rod:stroke ratio is an important consideration.
Assuming everything else remains constant:
The longer the rod, the lower the side-loading on the piston/bore, the lower the peak acceleration of the piston, and the greater the dwell time of the piston near TDC.
The downside is that it makes the motor taller/wider, and increases the peak piston speed. There is a theory that says a too-long rod can make engines with 4"+ bore more prone to detonation, but that doesn't seem to hold much sway with modern fuels and ignitions.

Broadly speaking, a performance 4T motor will benefit from a larger bore and smaller stroke (for any given cylinder capacity). The larger bore gives more room for valves and the shorter stroke reduces the loads at any given engine speed.
You can see this with any modern 4T bike motor.

Also broadly speaking, 2T performance motors benefit from longer strokes and smaller bores. You can see the way this has evolved in all of the 2T MX motors - they've all moved toward longer strokes over the years - even the YZ & KTM85s are undersquare nowdays.
The 144/150s with the 58x54 dimensions are the exception, but that's largely because they're an adaptation of a 54x54 125 motor - TM and KTM have both made dedicated 144 motors that are undersquare.

[Mike52]

Guys
Don't confuse longer conrods with longer stroke.
There is a trick in chev's [ drag racing ] to get more hp.
They use a longer conrod and a piston with the gudgeon moved up towards the head to compensate.
They sometimes end up with the oil rings crossing the gudgeon.
The end result is more leverage on the c/shaft per bang which equals more hp.
The stroke remains the same and the deck height remains the same.
Simplified version.
Cheers

[Lozza]

Don't think anybody's confused and longer rod doesn't equal more HP it equals more torque(times RPM thanks Luke) from dwell at TDC and reduced side thrust. Leverage is from crank pin offset, not from the rod.

MX 250's went way undersquare, 125's stayed square and 500's were just over square, engines like the special 250cc kart Gas Gas and the NSR500V are both 68X68 as is the 99 YZ 250 favoured by kart racers.Where they spend most time at high rpm the reduced stroke is better for crank/rod life What is really odd is modern trials bikes all have big bore short stroke engines, which goes against everything

+12mm rod 
http://www.youtube.com/watch?v=S2k8rry1XWQ

[vmx42]

What is really odd is modern trials bikes all have big bore short stroke engines, which goes against everything

Not really Lozza...

Modern trials bikes need instant throttle response from down low, but they don't need to rev very high at all [their powerbands are quite narrow]. I think the oversquare configuration just makes for improved acceleration which is what they really need.

Their power needs are unique, as are their engine configurations. Back to you...

VMX42

[Lozza]

that's what I mean Jeff , one would assume that would be ideal for a long stroke engine, but all the modern trials bikes have 'old school' dimensions of around 72 X60mm for 250's

[vmx42]

that's what I mean Jeff , one would assume that would be ideal for a long stroke engine, but all the modern trials bikes have 'old school' dimensions of around 72 X60mm for 250's

But the old long stroke engines don't have the immediate burst of acceleration required from just above idle. It's not about the numbers, it is how it feels and responds that is important...

The old school trials engines were all about smooth power and the ability to respond cleanly from very low revs... they didn't need that explosive response that the modern bikes have [and need]. I certainly know which type is more fun... 

[pancho]

 It appears that the primary motivation was packaging in the new car, however other benifits recognised was less detonation problems with the new narrower combustion chamber.
 There is more leverage on the rotating crank from the rod being shorter especially on the early part of the power stroke where the gas expansion is very short lived. (Constant pressure theory.)
 The shorter rod alters the velocity ratio to the extent that the piston travells faster and further down the stroke (than the longer rod) probably utilizing the mean effective pressure to greater advantage.
 
 Interesting!
 cheers pancho.

[Mike52]

I have pages and pages of this stuff.
The author is well respected in the USA.
Note that the author runs tests over at least 4 engines one of which remains standard as a control .  Now that's a scientific approach.

[pancho]

 That looks really interesting Mike! I'd like to read the article.
 
 Comments in my last post were from me, the only bit that came from the report I read was the point that they were looking at package size.
 cheers pancho.

[Mike52]

That looks really interesting Mike! I'd like to read the article.
 cheers pancho.

Here you go Pancho










I had all this info for a reason , won its class 2 years in a row and was getting quicker as it wore out.