Long stroke vs. short stroke

[Jalopyshoppe]

A longer rod dwells longer at TDC & BDT. That's why engines with them have better torque.

I actually use that to my advantage in one of my race car engines. I run 351w rods in my 302, with special pistons. The decreased skirt length does have accelerated wear, but the engine makes 300ft/lb at 2000 rpm. I'm getting into similar territory building a 100ci Harley engine. It'll be a square bore/stroke but run long rods.

 

[SFMCjohn]

It should be noted that this animation is an archaic piston port 2 stroke. Most modern 2 strokes are reed valve engines which as you probably know feed the intake for pretty much a full revolution - well over 300 degrees of crankshaft rev. Intake timing is pretty much a non issue with reeds. Gordon Jennings mostly wrote about piston port and disc valve engines.

... that’s interesting...

... found this gif, but it does not
show transfer ports or how the exhaust
may assist with providing pressure to
the crankcase in a reed valve engine ...

... do modern reed valve 2T engines
rely more on the pumping action
of the piston (compared to piston-port
engines) to provide pressure for the intake charge ...?

That would be the bore/stroke of a 400EXC. Many of the 450 KTM's use 72 mm stroke - like many 2 stroke 250's. Some earlier race bike 450's used a shorter stroke like that but the bore was 95 mm's.

... I’ll have to go see
if I can find any KTM 450s
with a 72mm stroke ...
so far the OP’s XR and the DR350
are the only bikes I could find
with a stroke anywhere near 70mm ...?

‘90-‘01 Suzuki DR350; 79mm / 72.1 = 1.096
‘96 Honda XR 400; 85mm / 70mm = 1.214

2012 KTM 450 Rally: 97mm / 60.9 = 1.593
2018 Honda CRF450 RALLY: 97mm / 60.8 = 1.595
2018 KTM 450 Rally: 95.0mm / 63.4mm = 1.498

... looks like the latest 2018 KTM 450 Rally has
a longer stroke than the 2012 ...

edit, found 72mm stroke KTMs, 2008 and earlier:

The 450EXC and 525EXC crank and rod are the same. (72mm stroke)
The 400EXC has a short stroke crank with a long rod. (64mm stroke)
The 450SX RFS has a short stroke crank and a short rod. (63.4 stroke to get under 450cc)
576 is 78mm stroke from memory.

pre-‘08 KTM 450 EXC: 89mm / 72mm = 1.236

edit: apparently KTM and the aftermarket were/are
sources for a KTM RSF with 99mm bore / 78mm stroke = 1.269, 600cc displacement ... ATV guys seem mostly interested in the
stroked RSF engine ...?

Lyndon Poskitt supplies a piston & cylinder
kit to build an LC4 732cc engine:

105 mm cylinder and piston optimized for use with LPR high lift camshafts. To achieve 732 cc this must be used in conjunction with 84.5 mm stroke crank. This is specifically for the LC4 engine and is available in a lower compression ratio if required (for low grade fuel locations etc).

... 105mm / 84.5mm = 1.242:1
(interesting to see the differences in the
aftermarket crank in the top pic vs the KTM 690 crank, above)

2016 KTM 690 Enduro R: 102mm / 84.4mm = 1.207:1; 67bhp @ 8000rpm ...

 

[Wrycque]

The suction wave from the pipe will pull the mixture through the cases. Don’t need much blowdown just a great pipe.

Pretty much every KTM 450/525 EXC or XC-W has a 72 mm stroke going back about a dozen years. The current 500 is 95 x 72 mm. Hard to improve on a great combination.

They go up to 613 cc on the RFS AFAIK.

LC4 engine is a paint shaker...

 

[Blankpage]

Two stroke doesn't look all that efficient. Seems there would be a lot of unburned fuel going out that exhaust. I've owned a couple of two strokes in my teens and didn't really focus on gas mileage.

 

[Wrycque]

The problem is the 2 stroke does not fire every revolution like it should. The mixture only comes close to stoichiometry every once in awhile so it fires when it can but usually most of the unburned gasses go out the E port. The problem is worst at idle and gets better as the revs build. Then the engine comes on the pipe it's firing well and making its best power - for a few thousand rpm at best. All you have to do is listen to a 2 stroke idle to hear what I'm talking about.

The new injected 2 stroke KTM's are coming to America. They pass Euro4 but for reasons I know nothing about are only going to be red sticker in CA.